On-board detection of fuel line vapor

An on-board diagnostic system is provided for determining the presence of vapor in a fuel supply line of an internal combustion engine during engine operation. Sensing means are operatively mounted to the fuel supply line for sensing transient fuel pressure waves resulting from actuation of one or more fuel injectors. The sensing means generates a pressure signal corresponding to the transient fuel pressure waves at least over a selected test interval. Signal processing means receives the pressure signal and determines its resonance frequency over at least a selected frequency range. The test interval resonance frequency is compared to a stored resonance frequency corresponding to an acceptable vapor level in the fuel supply line. The signal processing means generates an output signal upon detecting a value difference greater than a preselected amount (e.g., greater than zero) between the test interval resonance frequency and the stored resonance frequency. Utilization means are provided for response to the output signal, for example, by illumination of an indicator light visible to the operator of a motor vehicle in which the engine is equipped with the on-board diagnostic system of the invention.

INTRODUCTION 
The present invention is directed to a diagnostic system for an internal 
combustion engine to detect vapor, particularly fuel vapor, in a fuel rail 
or other fuel supply line to the engine. More specifically, the invention 
is directed to an on-board diagnostic system for detecting such fuel line 
vapor during engine operation. 
BACKGROUND OF THE INVENTION 
It is becoming increasingly desirable to provide on-board diagnostic means 
for certain components of internal combustion engines, especially 
components which have a major impact on critical engine performance 
criteria. This is particularly true in the motor vehicle industry, where 
high precision in the control of fuel supply to the engine has become 
essential to various present and planned engine management features 
designed to meet increasingly strict emissions, performance, drivability, 
and maintenance objectives. It is now well known how to adjust the fuel 
flow to the cylinders of an engine to maintain desired fuel/air mixture 
ratio for meeting engine emission requirements by electronically 
controlling the actuation timing and duration of the engine's fuel 
injectors. Electronic fuel injector control may be incorporated into known 
electronic engine control (EEC) modules performing a variety of engine 
control functions. In accordance with such known systems, the timing of 
injector actuation is controlled by the timing of the corresponding 
actuation signal sent by the control module. The duration of injector 
actuation, during which fuel is passed through the injector from a fuel 
rail or like fuel supply means, is controlled by the duration of the 
actuation signal from the control module, that is, by the pulse width of 
the signal. 
Reliably controlling fuel supply to an engine by controlling fuel injector 
actuation signal timing and duration (i.e., pulse width) assumes the 
absence of various possible fuel system problems, such as vapor in the 
fuel supply line. Thus, especially in support of maintaining the efficacy 
of electronic engine management devices adapted to control air/fuel ratio 
by controlling the actuation of fuel injectors, it would be desirable to 
provide an on-board diagnostic system to periodically test for the 
presence of fuel line vapor during engine operation. It is a primary 
object of the present invention to provide such on-board diagnostic system 
for fuel line vapor. Additional objects and features of various 
embodiments of the invention will be apparent from the following 
disclosure. 
SUMMARY OF THE INVENTION 
A properly running engine, having a diagnostic system as herein described, 
will have a characteristic fuel line pressure wave pattern for a given 
segment of an engine cycle, at a given point along the fuel line, under 
given engine operating conditions. The pressure wave pattern will include, 
at various frequencies and amplitudes, fuel line pressure transients 
resulting from fuel injector actuations, fuel pump operation, noise, etc. 
The on-board diagnostic system of the present invention employs analysis 
of fuel pressure transients in the fuel supply line of an internal 
combustion engine to detect the presence of vapor in the fuel line. 
Sensing means mounted to the fuel supply line generates a pressure signal 
corresponding to the transient fuel pressure waves resulting from 
actuation of one or more fuel injectors. The pressure signal is received 
by signal processing means, which optionally is incorporated into an 
electronic engine control module. The signal processing means analyses at 
least a selected frequency range of the pressure signal over a selected 
time interval, for example one complete engine cycle. Such analysis yields 
one or more values indicative of vapor volume in the fuel line, including 
a resonance frequency of the pressure signal. The signal processing means 
preferably comprises a stand-alone chip set to perform Fast Fourier 
Transform (FFT) analysis of the pressure signal, or comparable 
functionality in an EEC module. Commercially available chip sets perform 
FFT analysis of waveforms as a series of digital values over time. The 
resulting test interval resonance frequency will shift in a generally 
predictable manner due to the presence of vapor in the fuel supply line. 
The signal processing means compares the test interval resonance frequency 
or other indicative value to a stored value corresponding to an acceptable 
level of fuel line vapor. Upon detecting a difference between the test 
interval value and the stored value, the signal processing means generates 
an output signal. Optionally, it may also generate a different output 
signal if an unacceptable amount of vapor is not detected. 
The stored resonance frequency or other value may be stored, for example, 
in ROM memory of an electronic engine control (EEC) module. If it 
corresponds to a zero vapor condition in the fuel line, the output signal 
indicating an unacceptable vapor level preferably is generated only when 
the aforesaid difference between the stored frequency and the test 
interval frequency exceeds a preselected differential value which also may 
be stored in ROM memory of an EEC module. A larger difference between the 
two frequencies than the stored differential value would indicate a 
frequency shift corresponding to an unacceptably high level of fuel line 
vapor. A smaller difference between the two frequencies would correspond 
to less fuel line vapor. Alternatively, the stored value may correspond to 
a preselected upper limit of vapor volume. In that case, a value 
difference greater than zero (in the direction of frequency shift away 
from the zero vapor resonance frequency) would correspond to an 
unacceptably high level of fuel line vapor. The stored frequency value and 
the stored differential value may be determined, for example, empirically 
or by modelling using known techniques. 
In accordance with one aspect of the invention, it has been found that 
analysis of fuel pressure transients acquired by a single pressure 
transducer mounted on the engine fuel rail can reliably detect fuel line 
vapor. The pressure signal from the pressure transducer is processed by 
signal processing means preferably comprising a stand alone chip set to 
perform fast fourier transform (FFT) analysis of the pressure signal, or 
comparable functionality in an EEC module. Commercially available chip 
sets perform FFT analysis of waveforms as a series of digital values over 
time. The pressure signal is analyzed, preferably, over a selected 
frequency range, e.g, o to 2000 Hz, more preferably 0 to 1000 Hz, and over 
a test interval, e.g., a complete engine cycle, to determine the test 
interval resonance frequency. The test interval resonance frequency then 
is compared to a stored resonance frequency as discussed above. An output 
signal is generated in response to detection of the presence of fuel line 
vapor, preferably upon detection of a frequency shift corresponding to 1% 
by volume or greater of fuel vapor in the fuel supply line. 
The output signal can actuate a warning to the operator (e.g., the driver 
of a vehicle) that the fuel in the vehicle is too volatile for the local 
environment, suggesting a change to less volatile fuel to avoid vehicle 
stalling due to vapor lock. This may be particularly advantageous for 
motor vehicles with so-called flexible fuel engines adapted to operate on 
any of a variety of liquid fuels. Alternatively (or in addition) the 
signal can actuate means for purging the fuel supply line, e.g., by 
increased rate of circulation of fuel in the supply line back to the fuel 
tank. The signal can also be used to cause an adjustment of the fuel 
control signals generated by the EEC module. 
In accordance with one aspect of the invention, an internal combustion 
engine is provided with an on-board diagnostic system as described above 
for detecting fuel line vapor during engine operation. Fuel injector 
control means actuates fuel injectors to pass fuel from a fuel rail during 
a controlled actuation period, i.e., the time period from actuation to 
de-actuation of the fuel injector at the end of its fuel injection 
interval. Pressure sensor means provided for sensing fuel pressure in the 
fuel rail will generate a variable voltage signal corresponding to fuel 
pressure as a function of time. The pressure sensor means may employ a 
pressure transducer comprising, for example, a pressure responsive 
diaphragm exposed to the fuel in the fuel rail and a signal conditioner to 
generate a continuous analog voltage output pressure signal. The pressure 
signal from the pressure sensor means will vary over time in response to 
transient fuel pressure fluctuations in the fuel rail, including those 
resulting from fuel injector actuation. Vapor in the fuel rail will be 
reflected as detectable changes in the dominant resonance frequency and 
pressure-wave speed in the fuel line. The wave speed and/or the shift in 
dominant resonance frequency or other vapor-indicative value can easily be 
measured. The elapsed time can be measured between injector actuation (or 
other event of known or correlatable time) and detection of the 
corresponding fuel pressure fluctuations, i.e., transient fuel pressure 
waves at the pressure sensor means. 
The present invention represents a significant advance in electronic engine 
control and on-board diagnosis, in part, for its recognition of the useful 
correspondence of such measurable changes in the transient fuel pressure 
waves in a fuel line, especially low-frequency pressure waves, due to the 
presence of vapor, and for its means and method of using such measurable 
changes for detecting fuel line vapor during engine operation. It should 
be understood that reference herein to pressure signal processing during 
ongoing engine operation is intended to mean not only routine on-road 
operation, but also test operation, e.g., immediately following initial 
engine or vehicle assembly. Thus, the on-board diagnosis system could be 
used, optionally while the engine is running without fuel ignition. In 
fact, a test liquid in place of gasoline or other fuel could be used, such 
as stoddard solvent, which like liquid fuel, gives a predictable fuel line 
pressure wave signal as the engine is cycled. 
Additional features and advantages of the present invention will be better 
understood in view of the following detailed description of certain 
preferred embodiments.

DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS 
While the present invention is applicable generally to any internal 
combustion engine burning liquid fuel supplied to fuel injectors via a 
fuel rail, it is particularly advantageous for four stroke multi-cylinder 
engines, especially motor vehicle engines. Accordingly, without intending 
to limit the scope of the invention, the discussion below will focus 
primarily on motor vehicle engines, for which on-board diagnostics of 
various engine performance characteristics is becoming extremely 
important. In that regard, reference in this discussion to a complete 
engine cycle is intended to mean two full revolutions of the engine. In a 
four stroke engine, each cylinder fires once during two full revolutions. 
Thus, one complete engine cycle means that each cylinder fires once. 
The present invention addresses the aforesaid diagnostic need by providing 
an on-board diagnostic system for detecting fuel rail vapor. Certain 
embodiments of the on-board diagnostic system of the invention determine 
the vapor present in a fuel rail quantitatively, while other embodiments 
determine simply that vapor is present in any amount or in an amount above 
a selected threshold value. Upon detecting fuel line vapor, the system 
initiates responsive action, such as actuation of an information signal, 
actuation of fuel rail purging and/or actuation of remedial action by 
adaptive fuel injector control means to achieve desired total fuel flow 
with each injector actuation. As to the latter, more specifically, an 
output signal from the on-board diagnostic system can serve as an input 
signal to the engine's EEC module for adaptive air/fuel ratio control, 
that is, to enable the EEC computer to adjust injector actuation duration 
and/or timing to compensate for reduced flow rate through the injectors 
resulting from the fuel rail vapor. The information signal actuated by the 
diagnostic system may be stored for subsequent access by a service 
technician and/or used to cause an audible or visible warning for the 
vehicle operator. The fuel rail purging may involve opening a purge valve, 
e.g., to an auxiliary return line to the fuel tank or to a vapor treatment 
device. Since fuel injection systems typically circulate fuel continuously 
through the fuel rail and back to the fuel tank in their ordinary 
operating mode purging may also be accomplished in some system by 
accelerating fuel flow through the fuel rail, as by increasing the rate of 
fuel pumping from the fuel tank. 
The graphs in FIGS. 1A through 1D show fuel pressure transients at a 
pressure transducer sensing fuel rail pressure. More specifically, the 
graphs plot fuel line pressure as a function of time. FIG. 1A shows fuel 
pressure transients for a system having essentially no fuel line vapor. 
FIG. 1B shows the pressure transients when there is about 1% by volume 
vapor in the fuel line. The vapor "softens" the system (comprising the 
fuel, fuel rail, etc.), changing its wave pattern or signature. FIGS. 1C 
and 1D show the further changes when vapor is increased to 2% and 5% by 
volume, respectively. The values shown in the graphs of FIGS. 1A through 
1D were generated by a computer model in accordance with known algorithms 
and techniques and are quite close to actual test results. Thus, 
comparable pulse waveforms are obtained using a pressure transducer having 
a variable voltage output signal proportional to pressure within the fuel 
rail, with zero volts preferably corresponding to substantially static 
equilibrium pressure within the fuel rail (established by a pressure 
regulator) without fuel injector actuation. Given an actuation commencing 
at time 0.00 seconds on the graph, pressure in the fuel rail is seen to 
drop at the location of the pressure transducer in response to such 
actuation after a wave propagation delay period. The output voltage of the 
pressure transducer would drop correspondingly. Pressure recovers after 
the actuation interval, that is, after the fuel injector is closed. It can 
be seen in FIGS. 1A through 1D that the pulse waveform resulting from 
actuation of a fuel injector changes with increasing fuel line vapor. The 
change in pulse waveforms has been found to correlate quite well with the 
quantitative difference in fuel line vapor. 
In particular, referring now to FIGS. 2A and 2B, the change in pressure 
signal due to fuel engine vapor is shown as a shift in resonance 
frequency. Vapor in the fuel line "softens" the system, resulting in a 
lower resonance frequency. FIG. 2A shows the frequency spectrum developed 
by FFT analysis of the pressure signals received by the signal processor 
for a test interval of one complete engine cycle. The signal processor can 
select test intervals based on signals from the engine's EEC module or 
other fuel control means. Optionally, to enhance accuracy or reliability, 
the FFT frequency spectrum can be combined, e.g., by averaging, with that 
of one or more additional such single cycle test intervals. In this way, 
the possibility is reduced of a false indication of fuel line vapor due to 
aberrant fuel pressure transients during a test interval. Similarly, the 
output signal may be generated only when two or more test intervals in a 
preselected number of consecutive test signals each separately indicates 
an unacceptable level of fuel line vapor. Thus, for example, the output 
signal may be generated by the signal processor only when at least three 
of the last five, or ten of the last fifteen or twenty test intervals 
indicates vapor. Preferably, the test interval results would be stored in 
RAM memory, with the result for each new test interval replacing the 
oldest stored result (i.e., first-in first-out). 
As stated above, the frequency graph in FIG. 2A, corresponding to no fuel 
line vapor, shows a maximum peak at about 550 Hz. The peak at 550 Hz 
identifies the natural, 1st resonance frequency of the system. In 
contrast, the graph of FIG. 2B, corresponding to fuel line vapor of about 
1% by volume, has a resonance frequency of about 350 Hz. The 200 Hz value 
difference between the two resonance frequencies is the frequency shift 
resulting from fuel line vapor. FIG. 2B also reveals that the magnitude of 
the resonance frequency is reduced. Determination of fuel line vapor may 
also be based on such reduction in magnitude and/or on the corresponding 
increase in magnitude of frequency peaks at the low frequency end of the 
spectrum. In preferred embodiments, the value difference above which an 
output signal is generated by the signal processing means preferably 
corresponds to volume in the fuel line of 1% or greater by volume, 
typically a shift of 200 Hz or greater. As discussed above, such value can 
be stored for comparison to the actual value difference found for a given 
test interval. 
Since the shift in dominant resonance frequency varies directly with the 
volume percentage of vapor in the fuel rail, it can be used by the signal 
processing means to determine either the simple presence of vapor (above a 
threshold volume of zero or more) or a quantitative value corresponding to 
the volume percentage of vapor in the fuel rail. The correspondence 
between vapor volume and frequency shift, for a series of vapor volume 
percentages can be determined for a given engine either empirically or by 
modelling, etc. Optionally, such corresponding values can be stored in 
memory (e.g., in ROM memory of the EEC) as a look-up table accessible by 
the signal processing means. The output signal of the signal processing 
means may indicate the amount of vapor in the fuel line or simply that 
vapor is present. The latter is sufficient if the output signal is used 
only to actuate a warning device to alert the operator, e.g., a vehicle 
driver, of the presence of vapor. The signal may in that case be generated 
by the signal processing means when its processing of the pressure 
transducer signal indicates the presence of vapor in an amount above zero 
volume percent or other amount selected empirically, by modelling, etc. 
In certain embodiments of the invention, the on-board diagnostic system is 
integrated with adaptive air/fuel control means. In accordance with such 
embodiments the air/fuel control means can employ the output signal from 
the signal processing means indicating the presence of vapor or, 
optionally, indicating the amount of vapor present, as an input value in 
calculating injector actuation signal timing and/or duration. 
To synchronize acquisition of pressure waveforms with the actuation of 
injectors, analyzer triggering (i.e., the point where time=0 for each 
plotted waveform) preferably is set to a fixed current shunt voltage (+80 
mv) of a selected injector at the injector controller. The delay of 
pressure response relative to the trigger event (i.e., the propagation 
delay) for the transient waveform can be used to estimate the travel time 
of the pressure disturbances from the injector nozzle to the pressure 
transducer. However, the exact propagation delay need not be precisely 
known. The time interval over which the pressure signal is analyzed should 
be sufficiently large to cover the arrival of the fuel pressure transients 
at the pressure transducer either with or without slowing due to fuel line 
vapor, etc. 
A preferred embodiment of the invention is illustrated in FIG. 3, wherein a 
six cylinder engine 10 is seen to comprise a fuel supply system for 
supplying gasoline under pressure to the combustion cylinders of the 
engine. The fuel supply system consists of high pressure electric 
Gerotor-type pump 32 delivering fuel from a storage tank 33 through an 
inline fuel filter 28 to a fuel charging manifold assembly 24 via solid 
and flexible fuel lines. The fuel charging manifold assembly, referred to 
as a fuel rail, supplies fuel to electronically actuated fuel injectors 
11-16 mounted on an air intake manifold directly above each of the 
engine's intake valves. Air entering the engine is measured by a mass 
airflow meter. Air flow information and input from other engine sensors 19 
is used by an onboard engine electronic control computer 20 to calculate 
the required fuel flow rate necessary to maintain a prescribed air/fuel 
ratio for a given engine operation. The injectors, when energized, spray a 
predetermined quantity of fuel in accordance with engine demand. The 
duration of the actuation period during which the injectors are energized, 
determined by the actuation signal pulse width, is controlled by the 
vehicle's EEC computer 20. Thus, the EEC computer serves as the fuel 
injector control means, and, typically, performs various additional engine 
control functions. 
The fuel injector is an electromechanical device that atomizes the fuel 
delivered to the engine. Injectors typically are positioned so that their 
tips direct fuel at the engine intake valves. The valve body consists of a 
solenoid actuated pintle or needle valve assembly that sits on a fixed 
size orifice. A constant pressure drop is maintained across the injector 
nozzles via a pressure regulator. An electrical signal from the EEC unit 
activates the solenoid, causing the pintle to move inward, off the seat, 
allowing fuel to flow through the orifice. 
In the embodiment of FIG. 3, fuel injector control means 20 has injector 
signal output means 22 connected to the injector drivers of the fuel 
injectors 11-16. Injector signals from fuel injector control means 20 
control the sequence and timing of fuel injector actuation, including the 
duration of the actuation period during which each fuel injector, in turn, 
is open to pass fuel from fuel rail 24 to the respective combustion 
chamber. A pressure regulator 30 is provided for regulating fuel pressure 
in fuel rail 24. Pressure regulator 30 is located proximate to fuel pump 
32. That is, it is closer to fuel pump 32 than to the fuel rail 24 and is 
upstream of the fuel filter 28. Locating the pressure regulator 30 
proximate to the fuel pump is found to provide enhanced accuracy of 
pressure readings by pressure sensor means 34 mounted on fuel rail 24. 
Suitable regulators are commercially available and will be apparent to 
those skilled in the art in view of the present disclosure. The fuel 
pressure regulator typically is a diaphragm operated relief valve with one 
side of the diaphragm sensing fuel pressure and the other side subjected 
to intake manifold pressure. The nominal fuel pressure is established by a 
spring preload applied to the diaphragm. Referencing one side of the 
diaphragm to manifold pressure aids in maintaining a constant pressure 
drop across the injectors. Fuel in excess of that used by the engine 
passes through the regulator and returns to the fuel tank 33 via shunt 
line 31. 
Suitable pressure sensor means are commercially available and include, for 
example, variable reluctance, differential pressure transducers. 
Preferably the transducer has good transient response to low frequency 
transient pressure waves, low frequency here meaning 1 KHz or lower. The 
pressure sensor means preferably also has a high output signal with low 
susceptibility to electrical noise and good durability to withstand 
vibrations and shock experienced in a motor vehicle engine environment. 
Employing pressure sensor means having a transducer diaphragm vented on 
one side to atmosphere allows gage measurement of pressure (PSIG). The 
output signal from the pressure transducer preferably is a continuous 
analog voltage out signal, where signal voltage varies directly with fuel 
pressure. Zero voltage can be set to the nominal fuel pressure established 
for the fuel rail. The pressure signal from the pressure sensor means 34 
may further comprise signal conditioning means. Thus, the pressure 
transducer may be connected by a shielded cable to a signal conditioner. 
Suitable signal conditioners for various suitable pressure transducers are 
commercially available and will be apparent to those skilled in the art in 
view of the present disclosure. In accordance with such preferred 
embodiment, the transducer signal conditioner sources the pressure 
transducer with excitation power and amplifies the transducer output. The 
resulting pressure signal, that is, analog voltage output 35 of the 
pressure sensor means 34 is, therefore, proportional to fuel rail pressure 
sensed by the pressure transducer. 
The pressure signal is input to signal processing means 37 for generating 
an output signal in response thereto. Signal processing means 37 can be, 
for example, a programmable waveform analyzer, various models of which are 
commercially available and will be readily apparent to those skilled in 
the art in view of this disclosure. Such analyzers digitize and store 
analog voltage signals. The signal processing means preferably is 
responsive to a timing signal 39 from the fuel injector control means 20 
to synchronize acquisition of pressure waveforms with the actuation of the 
individual injectors. The delay between the sending of the actuation 
signal and the arrival at the pressure sensor means of the resulting 
transient fuel pressure waveform is readily obtained empirically for any 
given application of the invention (i.e., for any given engine 
arrangement). Those skilled in the art will recognize that such 
propagation delay will vary from injector to injector, depending on such 
factors as the distance along a fuel rail between the pressure sensor 
means and the individual injector. The test interval should be 
sufficiently large to cover the arrival time of the pressure transient 
even if its travel from the injector to the pressure sensor is slowed by 
vapor in the fuel rail. 
Typically, the signal processing means will employ a test interval equal in 
length to a single full engine cycle, with signal value acquisitions every 
100 to 500 microseconds (.mu.s). Thus, at an engine operating speed of 
1000 RPM, for a six cylinder engine, the test interval would be 120 ms, 
with 240 to 1200 pressure signal value acquisitions to be processed. Those 
who are skilled in this technology will recognize that frequent sampling 
will yield more accurate or reliable results when processed, e.g., to 
produce a frequency spectrum by Fast Fourier Transform analysis as 
discussed above. 
Output signal 40 from signal processing means 37 is received by utilization 
means 41. As discussed above, utilization means 41 may comprise, for 
example, an indicator light and/or means for operating purge means, such 
as purge valve 43 to pass vapor from fuel rail 24 to fuel tank 33 or other 
vapor receiving means via conduit 44. 
The fuel injector control means 20 optionally comprises memory means 42, 
for example, a look-up table, from which it may obtain an adjustment value 
for fuel injection control based on the value of the output signal 40 from 
the signal processing means 37. In that case, utilization means 41 may 
comprise functionality within fuel injector control means 20. 
A second preferred embodiment of the invention is schematically illustrated 
in FIG. 4. The embodiment of FIG. 4 involves a more traditional fuel 
injection supply line, in that a fuel return line is provided downstream 
of the fuel rail. The system is modified, however, to deadhead the system 
during vapor testing, as now described. In addition, the pressure 
regulator is relocated to a location proximate the fuel pump, as in the 
embodiment of FIG. 3. Locating the regulator remote from the fuel rail can 
provide individual injector transients in the aggregate waveform having 
more uniform pulse-to-pulse amplitudes and signatures. Fuel pump 132 is 
mounted in fuel tank 133 in the customary manner. Fuel is supplied during 
normal engine operation via supply line 126 which passes through fuel 
filter 128 to fuel rail 124. Fuel rail 124 feeds fuel to 6 fuel injectors 
111 through 116 which are actuated by actuation signals 122 from fuel 
injector control means 120. As in the embodiment of FIG. 3, fuel injector 
control means 120 preferably is incorporated into an electronic engine 
control module or computer which performs various additional engine 
control functions. 
Engine 110 in the embodiment of FIG. 4 is adapted for normal engine 
operation, during which fuel line vapor is not analyzed. Engine 110 also 
is adapted for fuel line vapor testing operation during ongoing engine 
operation. During fuel line vapor testing, the fuel supply line is altered 
by appropriate valving, including first valve means 150 in the fuel return 
line 127 for deadheading the fuel rail. Specifically, during testing valve 
means 150 closes the fuel return line to fuel flow from the fuel rail. 
During normal engine operation valve means 150 opens the fuel return line 
127 to fuel flow from the fuel rail 124. Second valve means 155 is 
provided in fuel shunt line 131 for closing the fuel shunt line during 
normal engine operation and for opening the fuel shunt line during vapor 
testing. During normal engine operation, with first valve means 150 open 
and second valve means 155 closed, pressure in the fuel rail is regulated 
by pressure regulator 130 in fuel return line 127. During testing 
operation, with first valve means 150 closed to deadhead the fuel rail and 
second valve means 155 open, pressure is regulated by pressure regulator 
160 in shunt line 131. 
Fuel line vapor diagnosis in the embodiment of FIG. 4 is carried out 
substantially in accordance with the various techniques discussed above. 
Thus, timing signal 139 is sent by fuel injector control means 120 to 
signal processing means 137 to trigger measurement of a propagation delay 
period after which the signal processing means 137 conducts signal 
sampling of signal 135 from pressure sensor 134. 
Those skilled in the art will recognize that the subject matter disclosed 
herein can be modified and/or implemented in alternative embodiments 
without departing from the true scope and spirit of the present invention 
as defined by the following claims.