Patent Publication Number: US-9429114-B2

Title: System and methods for evaporative emissions leak testing

Description:
BACKGROUND AND SUMMARY 
     Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations, and then purge the stored vapors during a subsequent engine operation. In an effort to meet stringent federal emissions regulations, emission control systems may need to be intermittently diagnosed for the presence of leaks that could release fuel vapors to the atmosphere. 
     A typical method of testing for the presence of leaks in an emission control system includes applying a vacuum to a fuel system that is otherwise sealed. An intact fuel system may be indicated if a threshold vacuum is met. In some examples, the fuel system may be sealed while containing a vacuum, and an intact fuel system may be indicated if the rate of vacuum bleed-up is less than a threshold. Failure to meet these criteria may indicate degradation in the fuel system. In some examples, an intake manifold vacuum may be used as the vacuum source applied to the emissions control system. However, hybrid-electric vehicles (HEVs) have limited engine run time, and may thus have limited opportunities to perform such a test. Further, in order to improve fuel efficiency, vehicles may be configured to operate with a low manifold vacuum, and may thus have limited opportunities with sufficient vacuum to perform a leak test. 
     In order to meet emissions regulations, such vehicles are required to include an on-board vacuum pump, which may be included in an evaporative leak check module (ELCM). The ELCM may be coupled to the evaporative emissions system, within a canister vent line, for example. The ELCM may thus supply the vacuum for appropriate leak tests. However, installing an ELCM in a vehicle is a relatively expensive manufacturing cost, which increases with a correlation to evaporative emissions system and fuel tank volume. Further, in applying a vacuum to the fuel tank, the ELCM draws fuel vapor into the fuel vapor canister. This may require an increase in canister size and/or the addition of an additional bleed canister in order to prevent bleed emissions in hybrid vehicles, which have limited opportunities to purge the canister for the same reasons an ELCM is required in the first place. 
     The inventors herein recognized the above stated problems and issues and have developed systems and methods in order to at least partially address them. In one example, a method for a fuel system is provided, comprising: during a first engine-off condition, coupling a fuel tank to a fuel vapor canister, and indicating degradation based on a change in pressure at the fuel vapor canister, and during a second engine-off condition, coupling the fuel vapor canister to an intake of an engine, and indicating degradation based on a change in pressure at the fuel vapor canister. The first engine-off condition may include an absolute fuel tank pressure greater than a threshold, while the second engine-off condition may include an engine spinning unfueled. In this way, a vacuum or pressure may be applied to the fuel vapor canister during an engine-off condition without requiring a dedicated vacuum pump coupled to the fuel vapor canister. 
     In another example, a method for a fuel system is provided, comprising: during a first condition, responsive to a first absolute fuel tank pressure being less than a threshold, maintaining a fuel tank sealed for a threshold duration; indicating degradation of the fuel tank responsive to a second absolute fuel tank pressure being less than the threshold; responsive to the first absolute fuel tank pressure being greater than the threshold, coupling a fuel tank to a fuel vapor canister; and indicating degradation based on a change in pressure at the fuel vapor canister. The method allows for passive testing of the fuel tank and fuel vapor canister using fuel tank pressure accumulated over a diurnal cycle. In this way, the canister side of an emissions control system may be tested without saturating the fuel vapor canister with hydrocarbons, thereby decreasing bleed emissions. 
     In yet another example, a system for a hybrid-electric vehicle is provided, comprising: a fuel tank coupled to a fuel vapor canister via a fuel tank isolation valve; an engine intake coupled to the fuel vapor canister via a canister purge valve; a canister vent coupling the fuel vapor canister to atmosphere via a canister vent valve; a fuel tank pressure sensor coupled to the fuel tank; a canister vent pressure sensor coupled within the canister vent; and a controller configured with instructions stored in non-transitory memory, that when executed, cause the controller to: during a first engine-off condition, open the fuel tank isolation valve while maintaining the canister purge valve and canister vent valve closed, and indicate degradation based on a change in pressure at the canister vent pressure sensor; and during a second engine-off condition, open the canister purge valve while maintaining the canister vent valve and fuel tank isolation valve closed, and indicate degradation based on a change in pressure at the fuel vapor canister. By coupling separate pressure sensors to the fuel tank side and the canister side of the emissions control system, canister side leaks may be tested independently of the fuel tank pressure sensor. In this way, the fuel tank may remained sealed during some canister-side degradation tests, thereby maintaining fuel vapor isolated and reducing the transfer of fuel vapor to the fuel vapor canister. 
     The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings. 
     It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         FIG. 1  schematically shows an example vehicle propulsion system. 
         FIG. 2  schematically shows a fuel system and emissions system for a vehicle engine. 
         FIG. 3  shows an example flow chart for a high-level method for performing an evaporative emissions leak test in a hybrid-electric vehicle. 
         FIG. 4  shows an example flow chart for a high-level method for performing an evaporative emissions leak test in a hybrid-electric vehicle while the vehicle is on. 
         FIG. 5  shows an example flow chart for a high-level method for performing an evaporative emissions leak test in a hybrid-electric vehicle while the vehicle engine is on. 
     
    
    
     DETAILED DESCRIPTION 
     This detailed description relates to systems and methods for evaporative emissions leak testing. More specifically, the description relates to systems and methods for leak testing in hybrid-electric vehicles that do not require a dedicated vacuum pump, and that reduce the transfer of fuel vapor to the fuel vapor canister during testing. The method may be applied to a hybrid vehicle propulsion system, such as the system depicted in  FIG. 1 . The propulsion system may include an engine system, fuel system, and emissions control system, as depicted in  FIG. 2 . A method for performing an evaporative emissions leak test in a hybrid-electric vehicle is depicted in  FIG. 3 . A method for performing evaporative emissions leak tests during a vehicle-on condition is depicted in  FIG. 4 , while a method for performing evaporative emissions leak tests during an engine-on condition is depicted in  FIG. 5 . 
       FIG. 1  illustrates an example vehicle propulsion system  100 . Vehicle propulsion system  100  includes a fuel burning engine  110  and a motor  120 . As a non-limiting example, engine  110  comprises an internal combustion engine and motor  120  comprises an electric motor. Motor  120  may be configured to utilize or consume a different energy source than engine  110 . For example, engine  110  may consume a liquid fuel (e.g., gasoline) to produce an engine output while motor  120  may consume electrical energy to produce a motor output. As such, a vehicle with propulsion system  100  may be referred to as a hybrid electric vehicle (HEV). 
     Vehicle propulsion system  100  may utilize a variety of different operational modes depending on operating conditions encountered by the vehicle propulsion system. Some of these modes may enable engine  110  to be maintained in an off state (i.e. set to a deactivated state) where combustion of fuel at the engine is discontinued. For example, under select operating conditions, motor  120  may propel the vehicle via drive wheel  130  as indicated by arrow  122  while engine  110  is deactivated. 
     During other operating conditions, engine  110  may be set to a deactivated state (as described above) while motor  120  may be operated to charge energy storage device  150 . For example, motor  120  may receive wheel torque from drive wheel  130  as indicated by arrow  122  where the motor may convert the kinetic energy of the vehicle to electrical energy for storage at energy storage device  150  as indicated by arrow  124 . This operation may be referred to as regenerative braking of the vehicle. Thus, motor  120  can provide a generator function in some embodiments. However, in other embodiments, generator  160  may instead receive wheel torque from drive wheel  130 , where the generator may convert the kinetic energy of the vehicle to electrical energy for storage at energy storage device  150  as indicated by arrow  162 . 
     During still other operating conditions, engine  110  may be operated by combusting fuel received from fuel system  140  as indicated by arrow  142 . For example, engine  110  may be operated to propel the vehicle via drive wheel  130  as indicated by arrow  112  while motor  120  is deactivated. During other operating conditions, both engine  110  and motor  120  may each be operated to propel the vehicle via drive wheel  130  as indicated by arrows  112  and  122 , respectively. A configuration where both the engine and the motor may selectively propel the vehicle may be referred to as a parallel type vehicle propulsion system. Note that in some embodiments, motor  120  may propel the vehicle via a first set of drive wheels and engine  110  may propel the vehicle via a second set of drive wheels. 
     Engine  110  may be started with an engine starting system that includes motor  120  driven by energy from energy storage device  150 . In another example, the starter may be a powertrain drive motor, such as a hybrid powerplant connected to the engine by way of a coupling device. The coupling device may include a transmission, one or more gears, and/or any other suitable coupling device. The starter may be configured to support engine restart at or below a predetermined near zero threshold speed (e.g., below 50 or 100 rpm). In other words, by operating motor  120 , engine  110  may be spun. During some conditions, such as during a key-on condition when engine operation is desired for vehicle motion, the engine may be started (e.g., using motor assistance) and spun fueled (that is, with fuel and air being injected into engine cylinders) to enable cylinder combustion. During other conditions, as elaborated in  FIGS. 3-5 , such as during selected key-on or key-off conditions, the engine may be started with motor assistance and spun unfueled (that is, with no air or fuel injected into the engine cylinders) to generate intake vacuum. The engine may be spun until a threshold vacuum is generated after which the spinning may be stopped. The generated vacuum may be subsequently applied to fuel system  140  for leak detection diagnostics. 
     In other embodiments, vehicle propulsion system  100  may be configured as a series type vehicle propulsion system, whereby the engine does not directly propel the drive wheels. Rather, engine  110  may be operated to power motor  120 , which may in turn propel the vehicle via drive wheel  130  as indicated by arrow  122 . For example, during select operating conditions, engine  110  may drive generator  160 , which may in turn supply electrical energy to one or more of motor  120  as indicated by arrow  114  or energy storage device  150  as indicated by arrow  162 . As another example, engine  110  may be operated to drive motor  120  which may in turn provide a generator function to convert the engine output to electrical energy, where the electrical energy may be stored at energy storage device  150  for later use by the motor. 
     Fuel system  140  may include one or more fuel storage tanks  144  for storing fuel on-board the vehicle. For example, fuel tank  144  may store one or more liquid fuels, including but not limited to: gasoline, diesel, and alcohol fuels. In some examples, the fuel may be stored on-board the vehicle as a blend of two or more different fuels. For example, fuel tank  144  may be configured to store a blend of gasoline and ethanol (e.g., E10, E85, etc.) or a blend of gasoline and methanol (e.g., M10, M85, etc.), whereby these fuels or fuel blends may be delivered to engine  110  as indicated by arrow  142 . Still other suitable fuels or fuel blends may be supplied to engine  110 , where they may be combusted at the engine to produce an engine output. The engine output may be utilized to propel the vehicle as indicated by arrow  112  or to recharge energy storage device  150  via motor  120  or generator  160 . 
     In some embodiments, energy storage device  150  may be configured to store electrical energy that may be supplied to other electrical loads residing on-board the vehicle (other than the motor), including cabin heating and air conditioning, engine starting, headlights, cabin audio and video systems, etc. As a non-limiting example, energy storage device  150  may include one or more batteries and/or capacitors. 
     Control system  190  may communicate with one or more of engine  110 , motor  120 , fuel system  140 , energy storage device  150 , and generator  160 . As will be described by the process flows of  FIGS. 3, 4, and 5 , control system  190  may receive sensory feedback information from one or more of engine  110 , motor  120 , fuel system  140 , energy storage device  150 , and generator  160 . Further, control system  190  may send control signals to one or more of engine  110 , motor  120 , fuel system  140 , energy storage device  150 , and generator  160  responsive to this sensory feedback. Control system  190  may receive an indication of an operator requested output of the vehicle propulsion system from a vehicle operator  102 . For example, control system  190  may receive sensory feedback from pedal position sensor  194  which communicates with pedal  192 . Pedal  192  may refer schematically to a brake pedal and/or an accelerator pedal. 
     Energy storage device  150  may periodically receive electrical energy from a power source  180  residing external to the vehicle (e.g., not part of the vehicle) as indicated by arrow  184 . As a non-limiting example, vehicle propulsion system  100  may be configured as a plug-in hybrid electric vehicle (HEV), whereby electrical energy may be supplied to energy storage device  150  from power source  180  via an electrical energy transmission cable  182 . During a recharging operation of energy storage device  150  from power source  180 , electrical transmission cable  182  may electrically couple energy storage device  150  and power source  180 . While the vehicle propulsion system is operated to propel the vehicle, electrical transmission cable  182  may disconnected between power source  180  and energy storage device  150 . Control system  190  may identify and/or control the amount of electrical energy stored at the energy storage device, which may be referred to as the state of charge (SOC). 
     In other embodiments, electrical transmission cable  182  may be omitted, where electrical energy may be received wirelessly at energy storage device  150  from power source  180 . For example, energy storage device  150  may receive electrical energy from power source  180  via one or more of electromagnetic induction, radio waves, and electromagnetic resonance. As such, it should be appreciated that any suitable approach may be used for recharging energy storage device  150  from a power source that does not comprise part of the vehicle. In this way, motor  120  may propel the vehicle by utilizing an energy source other than the fuel utilized by engine  110 . 
     Fuel system  140  may periodically receive fuel from a fuel source residing external to the vehicle. As a non-limiting example, vehicle propulsion system  100  may be refueled by receiving fuel via a fuel dispensing device  170  as indicated by arrow  172 . In some embodiments, fuel tank  144  may be configured to store the fuel received from fuel dispensing device  170  until it is supplied to engine  110  for combustion. In some embodiments, control system  190  may receive an indication of the level of fuel stored at fuel tank  144  via a fuel level sensor. The level of fuel stored at fuel tank  144  (e.g., as identified by the fuel level sensor) may be communicated to the vehicle operator, for example, via a fuel gauge or indication in a vehicle instrument panel  196 . 
     The vehicle propulsion system  100  may also include an ambient temperature/humidity sensor  198 , and a roll stability control sensor, such as a lateral and/or longitudinal and/or yaw rate sensor(s)  199 . The vehicle instrument panel  196  may include indicator light(s) and/or a text-based display in which messages are displayed to an operator. The vehicle instrument panel  196  may also include various input portions for receiving an operator input, such as buttons, touch screens, voice input/recognition, etc. For example, the vehicle instrument panel  196  may include a refueling button  197  which may be manually actuated or pressed by a vehicle operator to initiate refueling. For example, as described in more detail below, in response to the vehicle operator actuating refueling button  197 , a fuel tank in the vehicle may be depressurized so that refueling may be performed. 
     In an alternative embodiment, the vehicle instrument panel  196  may communicate audio messages to the operator without display. Further, the sensor(s)  199  may include a vertical accelerometer to indicate road roughness. These devices may be connected to control system  190 . In one example, the control system may adjust engine output and/or the wheel brakes to increase vehicle stability in response to sensor(s)  199 . 
       FIG. 2  shows a schematic depiction of a vehicle system  206 . The vehicle system  206  includes an engine system  208  coupled to an emissions control system  251  and a fuel system  218 . Emission control system  251  includes a fuel vapor container or canister  222  which may be used to capture and store fuel vapors. In some examples, vehicle system  206  may be a hybrid electric vehicle system, and may include components as described for vehicle propulsion system  100 . 
     The engine system  208  may include an engine  210  having a plurality of cylinders  230 . The engine  210  includes an engine intake  223  and an engine exhaust  225 . The engine intake  223  includes a throttle  262  fluidly coupled to the engine intake manifold  244  via an intake passage  242 . The engine exhaust  225  includes an exhaust manifold  248  leading to an exhaust passage  235  that routes exhaust gas to the atmosphere. The engine exhaust  225  may include one or more emission control devices  270 , which may be mounted in a close-coupled position in the exhaust. One or more emission control devices may include a three-way catalyst, lean NOx trap, diesel particulate filter, oxidation catalyst, etc. It will be appreciated that other components may be included in the engine such as a variety of valves and sensors. 
     An air intake system hydrocarbon trap (AIS HC)  224  may be placed in the intake manifold of engine  210  to adsorb fuel vapors emanating from unburned fuel in the intake manifold, puddled fuel from leaky injectors and/or fuel vapors in crankcase ventilation emissions during engine-off periods. The AIS HC may include a stack of consecutively layered polymeric sheets impregnated with HC vapor adsorption/desorption material. Alternately, the adsorption/desorption material may be filled in the area between the layers of polymeric sheets. The adsorption/desorption material may include one or more of carbon, activated carbon, zeolites, or any other HC adsorbing/desorbing materials. When the engine is operational causing an intake manifold vacuum and a resulting airflow across the AIS HC, the trapped vapors are passively desorbed from the AIS HC and combusted in the engine. Thus, during engine operation, intake fuel vapors are stored and desorbed from AIS HC  224 . In addition, fuel vapors stored during an engine shutdown can also be desorbed from the AIS HC during engine operation. In this way, AIS HC  224  may be continually loaded and purged, and the trap may reduce evaporative emissions from the intake passage even when engine  210  is shut down. 
     Fuel system  218  may include a fuel tank  220  coupled to a fuel pump system  221 . The fuel pump system  221  may include one or more pumps for pressurizing fuel delivered to the injectors of engine  210 , such as the example injector  266  shown. While only a single injector  266  is shown, additional injectors are provided for each cylinder. It will be appreciated that fuel system  218  may be a return-less fuel system, a return fuel system, or various other types of fuel system. Fuel tank  220  may hold a plurality of fuel blends, including fuel with a range of alcohol concentrations, such as various gasoline-ethanol blends, including E10, E85, gasoline, etc., and combinations thereof. A fuel level sensor  234  located in fuel tank  220  may provide an indication of the fuel level (“Fuel Level Input”) to controller  212 . As depicted, fuel level sensor  234  may comprise a float connected to a variable resistor. Alternatively, other types of fuel level sensors may be used. 
     Vapors generated in fuel system  218  may be routed to an evaporative emissions control system  251  which includes a fuel vapor canister  222  via vapor recovery line  231 , before being purged to the engine intake  223 . Vapor recovery line  231  may be coupled to fuel tank  220  via one or more conduits and may include one or more valves for isolating the fuel tank during certain conditions. For example, vapor recovery line  231  may be coupled to fuel tank  220  via one or more or a combination of conduits  271 ,  273 , and  275 . 
     Further, in some examples, one or more fuel tank vent valves in conduits  271 ,  273 , or  275 . Among other functions, fuel tank vent valves may allow a fuel vapor canister of the emissions control system to be maintained at a low pressure or vacuum without increasing the fuel evaporation rate from the tank (which would otherwise occur if the fuel tank pressure were lowered). For example, conduit  271  may include a grade vent valve (GVV)  287 , conduit  273  may include a fill limit venting valve (FLVV)  285 , and conduit  275  may include a grade vent valve (GVV)  283 . Further, in some examples, recovery line  231  may be coupled to a fuel filler system  219 . In some examples, fuel filler system may include a fuel cap  205  for sealing off the fuel filler system from the atmosphere. Refueling system  219  is coupled to fuel tank  220  via a fuel filler pipe or neck  211 . 
     Further, refueling system  219  may include refueling lock  245 . In some embodiments, refueling lock  245  may be a fuel cap locking mechanism. The fuel cap locking mechanism may be configured to automatically lock the fuel cap in a closed position so that the fuel cap cannot be opened. For example, the fuel cap  205  may remain locked via refueling lock  245  while pressure or vacuum in the fuel tank is greater than a threshold. In response to a refuel request, e.g., a vehicle operator initiated request, the fuel tank may be depressurized and the fuel cap unlocked after the pressure or vacuum in the fuel tank falls below a threshold. A fuel cap locking mechanism may be a latch or clutch, which, when engaged, prevents the removal of the fuel cap. The latch or clutch may be electrically locked, for example, by a solenoid, or may be mechanically locked, for example, by a pressure diaphragm. 
     In some embodiments, refueling lock  245  may be a filler pipe valve located at a mouth of fuel filler pipe  211 . In such embodiments, refueling lock  245  may not prevent the removal of fuel cap  205 . Rather, refueling lock  245  may prevent the insertion of a refueling pump into fuel filler pipe  211 . The filler pipe valve may be electrically locked, for example by a solenoid, or mechanically locked, for example by a pressure diaphragm. 
     In some embodiments, refueling lock  245  may be a refueling door lock, such as a latch or a clutch which locks a refueling door located in a body panel of the vehicle. The refueling door lock may be electrically locked, for example by a solenoid, or mechanically locked, for example by a pressure diaphragm. 
     In embodiments where refueling lock  245  is locked using an electrical mechanism, refueling lock  245  may be unlocked by commands from controller  212 , for example, when a fuel tank pressure decreases below a pressure threshold. In embodiments where refueling lock  245  is locked using a mechanical mechanism, refueling lock  245  may be unlocked via a pressure gradient, for example, when a fuel tank pressure decreases to atmospheric pressure. 
     Emissions control system  251  may include one or more emissions control devices, such as one or more fuel vapor canisters  222  filled with an appropriate adsorbent, the canisters are configured to temporarily trap fuel vapors (including vaporized hydrocarbons) during fuel tank refilling operations and “running loss” (that is, fuel vaporized during vehicle operation). In one example, the adsorbent used is activated charcoal. Emissions control system  251  may further include a canister ventilation path or vent line  227  which may route gases out of the canister  222  to the atmosphere when storing, or trapping, fuel vapors from fuel system  218 . 
     Canister  222  may include a buffer  222   a  (or buffer region), each of the canister and the buffer comprising the adsorbent. As shown, the volume of buffer  222   a  may be smaller than (e.g., a fraction of) the volume of canister  222 . The adsorbent in the buffer  222   a  may be same as, or different from, the adsorbent in the canister (e.g., both may include charcoal). Buffer  222   a  may be positioned within canister  222  such that during canister loading, fuel tank vapors are first adsorbed within the buffer, and then when the buffer is saturated, further fuel tank vapors are adsorbed in the canister. In comparison, during canister purging, fuel vapors are first desorbed from the canister (e.g., to a threshold amount) before being desorbed from the buffer. In other words, loading and unloading of the buffer is not linear with the loading and unloading of the canister. As such, the effect of the canister buffer is to dampen any fuel vapor spikes flowing from the fuel tank to the canister, thereby reducing the possibility of any fuel vapor spikes going to the engine. 
     Vent line  227  may also allow fresh air to be drawn into canister  222  when purging stored fuel vapors from fuel system  218  to engine intake  223  via purge line  228  and purge valve  261 . For example, purge valve  261  may be normally closed but may be opened during certain conditions so that vacuum from engine intake manifold  244  is provided to the fuel vapor canister for purging. In some examples, vent line  227  may include an air filter  259  disposed therein upstream of a canister  222 . 
     In some examples, the flow of air and vapors between canister  222  and the atmosphere may be regulated by a canister vent valve coupled within vent line  227 . When included, the canister vent valve may be a normally open valve, so that fuel tank isolation valve  252  (FTIV) may control venting of fuel tank  220  with the atmosphere. FTIV  252  may be positioned between the fuel tank and the fuel vapor canister within conduit  278 . FTIV  252  may be a normally closed valve, that when opened, allows for the venting of fuel vapors from fuel tank  220  to canister  222 . Fuel vapors may then be vented to atmosphere, or purged to engine intake system  223  via canister purge valve  261 . 
     Fuel system  218  may be operated by controller  212  in a plurality of modes by selective adjustment of the various valves and solenoids. For example, the fuel system may be operated in a fuel vapor storage mode (e.g., during a fuel tank refueling operation and with the engine not running), wherein the controller  212  may open isolation valve  252  while closing canister purge valve (CPV)  261  to direct refueling vapors into canister  222  while preventing fuel vapors from being directed into the intake manifold. 
     As another example, the fuel system may be operated in a refueling mode (e.g., when fuel tank refueling is requested by a vehicle operator), wherein the controller  212  may open isolation valve  252 , while maintaining canister purge valve  261  closed, to depressurize the fuel tank before allowing enabling fuel to be added therein. As such, isolation valve  252  may be kept open during the refueling operation to allow refueling vapors to be stored in the canister. After refueling is completed, the isolation valve may be closed. 
     As yet another example, the fuel system may be operated in a canister purging mode (e.g., after an emission control device light-off temperature has been attained and with the engine running), wherein the controller  212  may open canister purge valve  261  while closing isolation valve  252 . Herein, the vacuum generated by the intake manifold of the operating engine may be used to draw fresh air through vent  27  and through fuel vapor canister  22  to purge the stored fuel vapors into intake manifold  44 . In this mode, the purged fuel vapors from the canister are combusted in the engine. The purging may be continued until the stored fuel vapor amount in the canister is below a threshold. 
     Controller  212  may comprise a portion of a control system  214 . Control system  214  is shown receiving information from a plurality of sensors  216  (various examples of which are described herein) and sending control signals to a plurality of actuators  281  (various examples of which are described herein). As one example, sensors  216  may include universal exhaust gas oxygen (UEGO) sensor  237  located upstream of the emission control device, temperature sensor  233 , pressure sensor  291 , and canister temperature sensor  243 . Other sensors such as pressure, temperature, air/fuel ratio, and composition sensors may be coupled to various locations in the vehicle system  206 . As another example, the actuators may include fuel injector  266 , throttle  262 , fuel tank isolation valve  253 , pump  292 , and refueling lock  245 . The control system  214  may include a controller  212 . The controller may be shifted between sleep and wake-up modes for additional energy efficiency. During a sleep mode the controller may save energy by shutting down on-board sensors, actuators, auxiliary components, diagnostics, etc. Essential functions, such as clocks and controller and battery maintenance operations may be maintained on during the sleep mode, but may be operated in a reduced power mode. During the sleep mode, the controller will expend less current/voltage/power than during a wake-up mode. During the wake-up mode, the controller may be operated at full power, and components operated by the controller may be operated as dictated by operating conditions. The controller may receive input data from the various sensors, process the input data, and trigger the actuators in response to the processed input data based on instruction or code programmed therein corresponding to one or more routines. Example control routines are described herein with regard to  FIGS. 3-5 . 
     In some configurations, a canister vent valve (CVV)  297  may be coupled within vent line  227 . CVV  297  may function to adjust a flow of air and vapors between canister  222  and the atmosphere. The CVV may also be used for diagnostic routines. When included, the CVV may be opened during fuel vapor storing operations (for example, during fuel tank refueling and while the engine is not running) so that air, stripped of fuel vapor after having passed through the canister, can be pushed out to the atmosphere. Likewise, during purging operations (for example, during canister regeneration and while the engine is running), the CVV may be opened to allow a flow of fresh air to strip the fuel vapors stored in the canister. In some examples, CVV  297  may be a solenoid valve wherein opening or closing of the valve is performed via actuation of a canister vent solenoid. In particular, the canister vent valve may be an open that is closed upon actuation of the canister vent solenoid. In some examples, CVV  297  may be configured as a latchable solenoid valve. In other words, when the valve is placed in a closed configuration, it latches closed without requiring additional current or voltage. For example, the valve may be closed with a 100 ms pulse, then opened at a later time point with another 100 ms pulse. In this way, the amount of battery power required to maintain the CVV closed is reduced. In particular, the CVV may be closed while the vehicle is off, thus maintaining battery power while maintaining the fuel emissions control system sealed from atmosphere. A vent line pressure transducer (VLPT)  232  may be disposed within vent line  227  between canister  222  and CVV  297 . 
     Leak detection routines may be intermittently performed by controller  212  on fuel system  218  and emissions control system  251  to confirm that the systems are not degraded. As such, leak detection routines may be performed while the engine is off (engine-off leak test) using engine-off natural vacuum (EONV) generated due to a change in temperature and pressure at the fuel tank following engine shutdown and/or with vacuum supplemented from a vacuum pump. Alternatively, leak detection routines may be performed while the engine is running by operating a vacuum pump and/or using engine intake manifold vacuum. However, installing a vacuum pump in a vehicle is a relatively expensive manufacturing cost, which increases with a correlation to evaporative emissions system and fuel tank volume. Further, in applying a vacuum to the fuel tank, the vacuum pump draws fuel vapor into the fuel vapor canister. This may require an increase in canister size and/or the addition of an additional bleed canister in order to prevent bleed emissions in hybrid vehicles, which have limited opportunities to purge the canister for the same reasons a vacuum pump is required in the first place. 
       FIG. 3  shows an example flow chart for a high-level method  300  for performing an evaporative emissions leak test in a hybrid-electric vehicle. More specifically, method  300  describes a method for performing an evaporative emissions leak test without the use of a vacuum pump, and without loading a fuel vapor canister above a threshold. Method  300  will be described in reference to the systems described in  FIGS. 1-2 , though it should be understood that method  300  may be applied to other systems without departing from the scope of this disclosure. Method  300  may be carried out by a controller, such as controller  212 , and may be stored as executable instructions in non-transitory memory. 
     Method  300  begins at  305  by evaluating operating conditions. Operating conditions may be estimated, measured, and/or inferred, and may include ambient conditions, such as temperature, humidity, barometric pressure, etc., engine conditions, such as engine operating status, engine speed, engine load, etc., as well as fuel system conditions, such as fuel level, fuel tank pressure, fuel vapor canister load status, etc. Continuing at  310 , method  300  may include determining whether the vehicle is in a vehicle-on state. If the vehicle is in a vehicle-on state, method  300  may proceed to  312 . At  312 , method  300  may include entering a vehicle-on leak test. An example vehicle-on leak test is described further herein and with regards to  FIGS. 4 and 5 . Method  300  may then end. 
     If the vehicle is not in a vehicle-on state, method  300  may proceed to  315 . At  315 , method  300  may include determining whether the vehicle soak duration is greater than a threshold. The vehicle soak duration may comprise the length of time elapsed from the most recent vehicle-off event. The vehicle soak duration threshold may be predetermined (e.g., 4-6 hours) or may be based on operating conditions. For example, the vehicle soak duration may be based on the ambient temperature, a change in ambient temperature during the vehicle soak duration, an expected change in ambient temperature during the vehicle soak based on the time of day, an amount of heat rejected to the fuel tank during the previous vehicle-on condition, which may in turn be based on engine operating conditions during the previous vehicle-on condition, etc. For vehicles with an isolated fuel tank, the vehicle soak duration threshold may be based on an expected amount of time necessary for the fuel tank to undergo a threshold change in temperature, and thus develop either a positive pressure or a vacuum there within. If the vehicle soak duration is less than the threshold, method  300  may proceed to  317 . At  317 , method  300  may include maintaining the fuel tank isolated for the threshold duration. The fuel tank may be isolated by maintaining a fuel tank isolation valve closed. The vehicle controller may be put to sleep and re-awoken while maintaining the fuel tank isolation valve closed. 
     When the vehicle soak duration has increased above the threshold, method  300  may proceed to  320 . At  320 , method  300  may include determining whether the absolute fuel tank pressure is greater than a threshold. Absolute fuel tank pressure may be estimated, inferred, or measured, for example by FTPT  291 . The absolute fuel tank pressure threshold may be based on operating conditions, such as ambient barometric pressure, fuel fill level and fuel composition. The absolute fuel tank pressure threshold may be based on a pressure/vacuum that would be indicative of an intact fuel tank. In other words, if the fuel tank included a leak of a threshold size, the threshold pressure/vacuum would be unlikely to be reached. 
     However, an absolute fuel tank pressure below the threshold may not necessarily be indicative of degradation. Rather, the fuel tank may be at a zero-crossing point of the diurnal cycle. As the ambient temperature increases and decreases throughout the diurnal cycle, there may be two or more instances over a 24 hour cycle where an intact fuel tank has a fuel tank pressure that is equal to the ambient barometric pressure. As such, if the absolute fuel tank pressure is not greater than the pressure threshold, method  300  may proceed to  322 . At  322 , method  300  may include sleeping the controller for a duration, and then re-awakening the controller. The sleeping duration may be predetermined (e.g., 3 hours) or may be based on ambient conditions, such as ambient temperature and time of day. The sleeping duration may be based on a length of time over which a change in fuel tank pressure would be expected for an intact fuel tank. Continuing at  325 , method  300  may include determining whether the absolute fuel tank pressure is greater than a threshold. The absolute fuel tank pressure threshold may be based the same as the threshold described at  320 , or may be adjusted based on updated current operating conditions, such as ambient temperature and barometric pressure. If the absolute fuel tank pressure is not greater than the threshold, method  300  may proceed to  327 . At  327 , method  300  may include indicating fuel tank degradation. Indicating fuel tank degradation may include setting a flag at controller  212 , and may further include indicating degradation to the vehicle user, such as via illuminating a malfunction indicator lamp (MIL). Controller  212  may take further mitigating action based on fuel tank degradation, such as preventing the vehicle from operating in an engine-only mode. Controller  212  may further adjust the evaporative emissions leak testing schedule. Method  300  may then end. 
     If the absolute fuel tank pressure is greater than the threshold, at either  320  or  325 , method  300  may proceed to  330 . At  330 , method  300  may include indicating that the fuel tank is intact. Indicating that the fuel tank is intact may include recording a passing test result at controller  212 . Continuing at  335 , method  300  may include isolating the fuel vapor canister. Isolating the fuel vapor canister may include closing a canister vent valve, such as CVV  297 . Isolating the fuel vapor canister may further include closing, or maintaining closed a canister purge valve, such as CPV  261 , as well as maintaining FTIV  252  closed. Continuing at  340 , method  300  may include determining whether the fuel tank is holding a vacuum. If the fuel tank is holding a vacuum, the fuel tank vacuum may be exploited to test the canister side of the emissions system for leaks. If the fuel tank is not holding a vacuum, method  300  may proceed to  345 . At  345 , method  300  may include determining whether the canister load is below a threshold. If the canister load is below a threshold, the positive pressure within the fuel tank may be exploited to test the canister side of the emissions system for leaks without saturating the canister and increasing the likelihood of bleed emissions. The canister load threshold may be predetermined (e.g., 10% full) or may be based on operating conditions, such as fuel tank pressure, fuel composition, fuel fill level, ambient temperature, and/or other conditions that would indicate the expected canister load following venting the fuel tank to the isolated fuel vapor canister, and the likelihood that the adsorbed hydrocarbons would result in bleed emissions, which may be based on driver operating behavior, expected vehicle soak duration, time of day, ambient temperature, etc. 
     If the fuel tank is holding a vacuum, or the fuel tank is holding a positive pressure and the fuel vapor canister load is below a threshold, method  300  may proceed to  350 . At  350 , method  300  may include coupling the fuel tank to the fuel vapor canister. For example, fuel tank isolation valve  252  may be opened. When the fuel tank pressure and fuel vapor canister pressure have equilibrated, (as determined by FTPT  291  and VLPT  232 , for example) method  300  may proceed to  355 . In some examples, the fuel tank isolation valve may be closed following equilibration. At  355 , method  300  may include determining whether the canister pressure rate of change is less than a threshold. A threshold amount of vacuum bleed-up or pressure bleed-down may be allowed for a given threshold leak size. If the canister pressure rate of change is greater than the threshold, method  300  may proceed to  357 . At  357 , method  300  may include indicating canister-side degradation. Indicating canister-side degradation may include setting a flag at controller  212 , and may further include indicating degradation to the vehicle user, such as via illuminating a malfunction indicator lamp (MIL). Controller  212  may take further mitigating action based on canister degradation, such as preventing the vehicle from venting the fuel tank, and/or adjusting the canister purge schedule. Controller  212  may further adjust the evaporative emissions leak testing schedule, for example, by indicating more specific canister side degradation testing, such as canister vent valve and/or canister purge valve integrity testing. 
     If the canister pressure rate of change is less than the threshold, method  300  may proceed to  359 . At  359 , method  300  may include indicating that the canister side is intact. Indicating that the canister side is intact may include recording the passing test at controller  212 . When the integrity of the canister side of the emissions control system has been determined, method  300  may proceed to  360 . At  360 , method  300  may include coupling the canister to atmosphere (e.g., opening CVV  297 ), and may further include closing the FTIV (if open). Method  300  may then end. 
     Returning to  345 , if the fuel tank is holding a positive pressure, and the fuel vapor canister load is above a threshold, method  300  may proceed to  365 . At  365 , method  300  may include spinning the engine unfueled. For example, the engine may be started with motor assistance and spun with no air or fuel injected into the engine cylinders to generate intake vacuum. Continuing at  370 , method  300  may include coupling the isolated fuel vapor canister to engine intake responsive to engine intake vacuum increasing above a threshold. For example, the canister purge valve may be opened. In some examples, the engine may continue to spin unfueled while the canister is coupled to intake. Any desorbed fuel vapor may be adsorbed in the intake by AIS HC  224 , preventing escape emissions. The integrity of the fuel vapor canister may be determined by comparing a resulting canister pressure to a threshold indicative of an intact canister side of the emissions control system. 
     In other examples, engine rotation may be stopped when a canister pressure reaches a threshold vacuum, and the canister purge valve closed to isolate the canister side of the emissions control system. Continuing at  375 , method  300  may include may include determining whether the canister pressure rate of change (vacuum bleed-up) is less than a threshold. If the canister pressure rate of change is greater than the threshold, method  300  may proceed to  377 . At  377 , method  300  may include indicating canister-side degradation. If the canister pressure rate of change is less than the threshold, method  300  may proceed to  379 . At  379 , method  300  may include indicating that the canister side is intact. When the integrity of the canister side of the emissions control system has been determined, method  300  may proceed to  380 . At  380 , method  300  may include closing the CPV (if open), stopping engine rotation (if still spinning), and coupling the canister to atmosphere (e.g., opening CVV  297 ). Method  300  may then end. 
       FIG. 4  shows an example flow chart for a high-level method  400  for performing an evaporative emissions leak test in a hybrid-electric vehicle during a vehicle-on condition. Method  400  may be executed independently, or as a subroutine of another method, such as method  300 . Method  400  will be described in reference to the systems described in  FIGS. 1-2 , though it should be understood that method  400  may be applied to other systems without departing from the scope of this disclosure. Method  400  may be carried out by a controller, such as controller  212 , and may be stored as executable instructions in non-transitory memory. 
     Method  400  begins at  405  by evaluating operating conditions. Operating conditions may be estimated, measured, and/or inferred, and may include ambient conditions, such as temperature, humidity, barometric pressure, etc., engine conditions, such as engine operating status, engine speed, engine load, etc., as well as fuel system conditions, such as fuel level, fuel tank pressure, fuel vapor canister load status, etc. Continuing at  410 , method  400  may include determining whether the vehicle is in an engine-on state. If the vehicle is in an engine-on state, method  400  may proceed to  412 . At  412 , method  300  may include entering an engine-on leak test. An example engine-on leak test is described further herein and with regards to  FIG. 5 . Method  400  may then end. 
     If the engine is not on, method  400  may proceed to  415 . At  415 , method  400  may include determining whether the absolute fuel tank pressure is greater than a threshold, as described with regard to  FIG. 3 . If the absolute fuel tank pressure is greater than the threshold, method  400  may proceed to  417 . At  417 , method  400  may include indicating that the fuel tank is intact. Continuing at  419 , method  400  may include isolating the fuel vapor canister, and proceeding with a canister-side integrity test. The canister side integrity test may proceed as described with regard to  FIG. 3  from  335  onward. Method  400  may then end. 
     If the absolute fuel tank pressure is less than the threshold, method  400  may proceed to  420 . At  420 , method  400  may include spinning the engine unfueled to generate an engine intake vacuum. Continuing at  425 , method  400  may include coupling the fuel vapor canister to engine intake responsive to engine intake vacuum increasing above a threshold, as described with regard to  FIG. 3 . Continuing at  430 , method  400  may include determining whether a canister pressure rate of change is less than a threshold. If the canister pressure rate of change is greater than the threshold, method  400  may proceed to  432 , and may include indicating canister-side degradation, as described with regard to  FIG. 3 . Continuing at  434 , method  400  may include closing the CPV (if open), stopping engine rotation (if still spinning), and coupling the canister to atmosphere (e.g., opening CVV  297 ). Method  400  may further include setting a flag to follow up with a fuel tank integrity test when conditions permit. Method  400  may then end. 
     If the canister pressure rate of change is less than the threshold, method  400  may proceed to  434 , and may include indicating that the canister side is intact. Continuing at  435 , method  400  may include coupling the fuel tank to engine intake, for example, opening FTIV  252  and CPV  261 . CVV  297  may be closed or maintained closed to isolate the evaporative emissions system from atmosphere. Continuing at  440 , method  400  may include evacuating the fuel tank. This may include spinning the engine unfueled to evacuate the fuel tank. Any fuel vapor drawn through the fuel vapor canister into intake will be adsorbed by AIS HC  224 , preventing bleed emissions. When the fuel tank is evacuated to a threshold vacuum, the fuel tank isolation valve may be closed. The CPV may also be closed, and the engine rotation may be stopped. However, in some examples, the engine may be continually spun unfueled while coupled to the fuel tank until the fuel tank reaches (or fails to reach after a predetermined duration) a threshold vacuum level. 
     Continuing at  445 , method  400  may include determining whether the fuel tank pressure rate of change is less than a threshold. A threshold amount of vacuum bleed-up may be allowed for a given threshold leak size. If the fuel tank pressure rate of change is greater than the threshold, method  400  may proceed to  447 . At  447 , method  400  may include indicating fuel tank degradation, as described with regard to  FIG. 3 . If the fuel tank pressure rate of change is less than the threshold, method  400  may proceed to  449 . At  449 , method  400  may include indicating that the fuel tank is intact. When the integrity of the fuel tank has been determined and indicated, method  400  may proceed to  450 . At  450 , method  400  may include closing the FTIV (if open), CPV (if open), and stopping the rotating of the engine (if ongoing). Alternatively, method  400  may include opening the CVV and FTIV, and allowing the fuel tank to equilibrate to atmospheric pressure. The FTIV may then be closed. Method  400  may then end. 
       FIG. 5  shows an example flow chart for a high-level method  500  for performing an evaporative emissions leak test in a hybrid-electric vehicle during an engine-on condition. Method  500  may be executed independently, or as a subroutine of another method, such as methods  300  and/or  400 . Method  500  will be described in reference to the systems described in  FIGS. 1-2 , though it should be understood that method  500  may be applied to other systems without departing from the scope of this disclosure. Method  500  may be carried out by a controller, such as controller  212 , and may be stored as executable instructions in non-transitory memory. 
     Method  500  begins at  505  by evaluating operating conditions. Operating conditions may be estimated, measured, and/or inferred, and may include ambient conditions, such as temperature, humidity, barometric pressure, etc., engine conditions, such as engine operating status, engine speed, engine load, etc., as well as fuel system conditions, such as fuel level, fuel tank pressure, fuel vapor canister load status, etc. Continuing at  510 , method  500  may include determining whether an absolute fuel tank pressure is greater than a threshold. If the absolute fuel tank pressure is not greater than the threshold, method  500  may proceed to  515 . At  515 , method  500  may include determining whether the intake manifold vacuum is greater than a threshold. Intake manifold vacuum may be estimated, inferred, or measured, such as by MAP sensor  236 . The intake manifold vacuum threshold may be based on an amount of vacuum necessary to evacuate the canister side of the emissions control system, and/or the fuel tank. The intake manifold vacuum threshold may thus be based on the volumes of the canister side and/or fuel tank, and may be further based on fuel level, fuel composition, etc. If the intake manifold is not greater than the threshold, method  500  may proceed to  517 . At  517 , method  500  may include monitoring fuel tank pressure and intake manifold vacuum, and may further include setting a flag to follow up with additional leak testing when a threshold pressure gradient is present within the engine, fuel, and/or emissions control system. 
     Returning to  510 , if the absolute fuel tank pressure is greater than the threshold, method  500  may proceed to  520 . At  520 , method  500  may include indicating that the fuel tank is intact. Continuing at  522 , method  500  may include determining whether the intake manifold vacuum is greater than a threshold. If the intake manifold vacuum is greater than a threshold (as shown at either  515  or  522 ), method  500  may proceed to  525 . At  525 , method  500  may include sealing the canister from atmosphere, and may further include applying intake vacuum to the canister side of the emissions control system. Sealing the canister from atmosphere may include closing a canister vent valve, while applying intake vacuum to the canister may include opening a canister purge valve. The integrity of the fuel vapor canister may be determined by comparing a resulting canister pressure to a threshold indicative of an intact canister side of the emissions control system. 
     In other examples, the canister purge valve may be closed when a canister pressure reaches a threshold vacuum. Continuing at  530 , method  500  may include may include determining whether the canister pressure rate of change (vacuum bleed-up) is less than a threshold. If the canister pressure rate of change is less than the threshold, method  500  may proceed to  532 . At  532 , method  500  may include indicating that the canister side is intact. If the canister pressure rate of change is greater than the threshold, method  500  may proceed to  534 . At  534 , method  500  may include indicating that the canister side is intact. When the integrity of the canister side of the emissions control system has been determined, method  500  may proceed to  535 . At  535 , method  500  may include closing the CPV (if open), and coupling the canister to atmosphere (e.g., opening CVV  297 ). Method  500  may then end. 
     Returning to  522 , if the absolute fuel tank pressure is greater than a threshold and the intake manifold vacuum is less than a threshold, method  500  may proceed to  540 . At  540 , method  500  may include isolating the fuel vapor canister (e.g., closing the CPV and CVV). Continuing at  545 , method  500  may include coupling the fuel tank to the fuel vapor canister (e.g., opening the FTIV), and allowing the fuel tank pressure and fuel vapor canister pressure to equilibrate. Continuing at  550 , method  500  may include may include determining whether the canister pressure rate of change (vacuum bleed-up or pressure bleed-down) is less than a threshold. If the canister pressure rate of change is less than the threshold, method  500  may proceed to  552 . At  552 , method  500  may include indicating that the canister side is intact. If the canister pressure rate of change is greater than the threshold, method  500  may proceed to  554 . At  554 , method  500  may include indicating that the canister side is intact. When the integrity of the canister side of the emissions control system has been determined, method  500  may proceed to  555 . At  55 s 5 , method  500  may include closing the CPV (if open), and coupling the canister to atmosphere (e.g., opening CVV  297 ). Method  500  may then end. 
     The systems described herein and depicted in  FIGS. 1 and 2 , along with the methods described herein and depicted in  FIGS. 3, 4, and 5  may enable one or more systems and one or more methods. In one example, a method for a fuel system is provided, comprising: during a first engine-off condition, coupling a fuel tank to a fuel vapor canister, and indicating degradation based on a change in pressure at the fuel vapor canister, and during a second engine-off condition, coupling the fuel vapor canister to an intake of an engine, and indicating degradation based on a change in pressure at the fuel vapor canister. In such an example, the first engine-off condition may include an absolute fuel tank pressure greater than a threshold, and wherein the second engine-off condition includes an absolute fuel tank pressure that is less than a threshold. In some examples, the first engine-off condition may include a fuel tank vacuum, and the method may further comprise: maintaining coupling of the fuel tank and the fuel vapor canister until a pressure in the fuel tank is equal to a pressure in the fuel vapor canister; and indicating degradation based on a change in a canister pressure bleed-up rate that is greater than a threshold. In some examples, the first engine-off condition may include a positive fuel tank pressure and a canister load that is less than a threshold, and the method may further comprise: maintaining coupling of the fuel tank and the fuel vapor canister until a pressure in the fuel tank is equal to a pressure in the fuel vapor canister; and indicating degradation based on a change in a canister pressure bleed-down rate that is greater than a threshold. In some examples, the method may additionally or alternatively further comprise: during the second engine-off condition, spinning the engine unfueled, and coupling the fuel vapor canister to the intake of the engine responsive to an intake vacuum increasing above a threshold. The method may further comprise uncoupling the fuel vapor canister from the intake of the engine responsive to a canister vacuum increasing above a threshold, and indicating degradation based on a change in a canister pressure bleed-up rate that is greater than a threshold. In some examples, coupling the fuel tank to the fuel vapor canister may comprise opening a fuel tank isolation valve coupled between the fuel tank and the fuel vapor canister. Coupling the fuel vapor canister to the intake of the engine may include opening a canister purge valve coupled between the fuel vapor canister and the intake of the engine. The first and second engine-off conditions may include a fuel vapor canister that is isolated from atmosphere. The technical result of implementing such methods is that a vacuum or pressure may be applied to the fuel vapor canister during an engine-off condition without requiring a dedicated vacuum pump coupled to the fuel vapor canister. This may reduce manufacturing costs and system complexity, while maintaining adherence to federal emissions guidelines. 
     In another example, a method for a fuel system is provided, comprising: during a first condition, responsive to a first absolute fuel tank pressure being less than a threshold, maintaining a fuel tank sealed for a threshold duration; indicating degradation of the fuel tank responsive to a second absolute fuel tank pressure being less than the threshold; responsive to the first absolute fuel tank pressure being greater than the threshold, coupling a fuel tank to a fuel vapor canister; and indicating degradation based on a change in pressure at the fuel vapor canister. In such an example, coupling the fuel tank to the fuel vapor canister may comprise coupling the fuel tank to the fuel vapor canister responsive to a fuel tank vacuum. In some examples, coupling the fuel tank to the fuel vapor canister may comprise coupling the fuel tank to the fuel vapor canister responsive to a positive fuel tank vacuum and further responsive to a canister load being less than a threshold. The method may further comprise: responsive to a positive fuel tank vacuum and a further responsive to the canister load being greater than the threshold, spinning an engine unfueled; coupling the fuel vapor canister to engine intake responsive to an intake vacuum increasing above a threshold; and indicating degradation based on a change in pressure at the fuel vapor canister. The first condition may include a vehicle-off condition, and may further include a vehicle-off soak duration greater than a threshold. In some examples, the method may further comprise: during a second condition, the second condition including a vehicle-on condition, an engine-off condition, and an absolute fuel tank pressure less than a threshold, spinning an engine unfueled; coupling the fuel vapor canister to engine intake responsive to an intake vacuum increasing above a threshold; and indicating degradation based on a change in pressure at the fuel vapor canister. The technical result of implementing such methods is passive testing of the fuel tank and fuel vapor canister using fuel tank pressure accumulated over a diurnal cycle. In this way, the canister side of an emissions control system may be tested without saturating the fuel vapor canister with hydrocarbons, thereby decreasing bleed emissions without requiring an increase in canister size or the addition of an additional bleed canister. 
     In yet another example, a system for a hybrid-electric vehicle is provided, comprising: a fuel tank coupled to a fuel vapor canister via a fuel tank isolation valve; an engine intake coupled to the fuel vapor canister via a canister purge valve; a canister vent coupling the fuel vapor canister to atmosphere via a canister vent valve; a fuel tank pressure sensor coupled to the fuel tank; a canister vent pressure sensor coupled within the canister vent; and a controller configured with instructions stored in non-transitory memory, that when executed, cause the controller to: during a first engine-off condition, open the fuel tank isolation valve while maintaining the canister purge valve and canister vent valve closed, and indicate degradation based on a change in pressure at the canister vent pressure sensor; and during a second engine-off condition, open the canister purge valve while maintaining the canister vent valve and fuel tank isolation valve closed, and indicate degradation based on a change in pressure at the fuel vapor canister. In such an example, the system may not include a vacuum pump coupled to the fuel vapor canister. In some examples, the first engine-off condition may include an absolute fuel tank pressure greater than a threshold, and the second engine-off condition may include an absolute fuel tank pressure that is less than a threshold. The first engine-off condition may include a fuel tank vacuum, and the controller may further comprise instructions stored in non-transitory memory, that when executed, cause the controller to: maintain the fuel tank isolation valve open until a pressure in the fuel tank is equal to a pressure in the fuel vapor canister; and indicate degradation based on a change in a canister pressure bleed-up rate that is greater than a threshold. In some examples, the first engine-off condition may include a positive fuel tank pressure and a canister load that is less than a threshold, and the controller further may comprise instructions stored in non-transitory memory, that when executed, cause the controller to: maintain the fuel tank isolation valve open until a pressure in the fuel tank is equal to a pressure in the fuel vapor canister; and indicate degradation responsive to a change in a canister pressure bleed-down rate that is greater than a threshold. The technical result of implementing this system is that canister side leaks may be tested independently of the fuel tank pressure sensor, due to coupling separate pressure sensors to the fuel tank side and the canister side of the emissions control system. In this way, the fuel tank may remained sealed during some canister-side degradation tests, thereby maintaining fuel vapor isolated and reducing the transfer of fuel vapor to the fuel vapor canister, thus reducing potential bleed emissions. 
     Note that the example control and estimation routines included herein can be used with various engine and/or vehicle system configurations. The control methods and routines disclosed herein may be stored as executable instructions in non-transitory memory and may be carried out by the control system including the controller in combination with the various sensors, actuators, and other engine hardware. The specific routines described herein may represent one or more of any number of processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various actions, operations, and/or functions illustrated may be performed in the sequence illustrated, in parallel, or in some cases omitted. Likewise, the order of processing is not necessarily required to achieve the features and advantages of the example embodiments described herein, but is provided for ease of illustration and description. One or more of the illustrated actions, operations and/or functions may be repeatedly performed depending on the particular strategy being used. Further, the described actions, operations and/or functions may graphically represent code to be programmed into non-transitory memory of the computer readable storage medium in the engine control system, where the described actions are carried out by executing the instructions in a system including the various engine hardware components in combination with the electronic controller. 
     It will be appreciated that the configurations and routines disclosed herein are exemplary in nature, and that these specific embodiments are not to be considered in a limiting sense, because numerous variations are possible. For example, the above technology can be applied to V-6, I-4, I-6, V-12, opposed 4, and other engine types. The subject matter of the present disclosure includes all novel and non-obvious combinations and sub-combinations of the various systems and configurations, and other features, functions, and/or properties disclosed herein. 
     The following claims particularly point out certain combinations and sub-combinations regarded as novel and non-obvious. These claims may refer to “an” element or “a first” element or the equivalent thereof. Such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and sub-combinations of the disclosed features, functions, elements, and/or properties may be claimed through amendment of the present claims or through presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure.