From: bohdan@cbnewsd.cb.att.com (bohdan.l.bodnar)
Subject: Re: Help... Car MPG has become very poor....
Organization: AT&T
Distribution: usa
Date: Wed, 30 Dec 1992 21:01:44 GMT

In article <1992Dec30.155722.15084@wam.umd.edu> choudhry@wam.umd.edu (Zafar Ullah Choudhry) writes:
>I have 1985 Mitsubishi Mirage whose MPG has reduced from 30 to only 15MPG.
>Few weeks ago it failed the emissions test and I posted the problem in 
>this news group. I suggested to change the carb. and I did that. However,
>MPG problem is still there as is, although emissions have reduced.
>
>Is it possible that this problem is due to high valve clearences, because
>I haven't adjusted them for a long time.  My car has 1,20,000 miles on it.
>Engine doesn't give any smoke while running. In the morning there is some
>some for few minutes.
>
>I have also checked all the hoses for any leaks, and found non. BTW, spark
>plugs become full with carbon within two weeks. I'll appreciate if anyone
>can help me out of this problem.
>
>Thanks.
>Zafar.
>
>
> (


On the assumption that your engine is mechanically sound, that your air filter
is clean, and that your choke is correctly adjusted, here's what I would do...

1). Check the thermostat to see if it's stuck open.  An open thermostat will
cause the engine to run cold and, consequently, the computer system will run
open loop (thus resulting in a rich air/fuel mixture).  If the thermostat is
fine, check all electrical connections going to the coolant temperature sensor
(pulling fault codes out of the computer will tell you if the sensor's
connections are ok -- the sensors rarely fail, but the connections can become
oxidized).

The following test is made with the engine warm. It tests the air/fuel mixture
solenoid and the oxygen sensor...

2). Connect a high impedance analog voltmeter between the oxygen sensor's
output lead and ground (it's a T tap connection to the sensor -- be sure the
computer can still read the sensor's output).  With the engine warm and at
idle, you probably will have a low reading (lean air/fuel mixture).  Now,
"goose" the engine and see if the sensor's output jumps high (on my carbureted
Mustang, the output will jump to about 800 mv before the computer system
compensates, the engine comes "up to speed", and steady-state is achieved).  If
you don't get a high pulse, replace the sensor.  At an RPM range of roughly
2000 to 2500, the sensor's reading should be steady around 500 mv.  If it goes
high, stays for a while, drops low, sticks for a while, etc., you probably have
a sticking air/fuel mixture solenoid.  Disconnect the solenoid and see if the
sensor reports a stable air/fuel mixture (high voltage and stable for an
air-bleed type solenoid).  If this occurs, replace the solenoid and its gasket.

3). Check to see if the PCV valve is clogged shut (I'm assuming your engine has
a PCV valve -- some small engines use a closed breather tube which takes the
place of the PCV valve).  Since the valve is a calibrated air leak, if it is
clogged shut the air/fuel mixture will be slightly rich.  The usual "shake and
see if it rattles" test is useless unless the valve is completely clogged.
Open the crankcase side of the PCV valve and put your finger over the valve's
suction side -- you should feel a strong vacuum at idle.  With the engine
running, remove the valve -- the engine should either die or sound VERY "sick."
If the valve's removal has no influence, then the vacuum side of the PCV valve
has an obstruction.  If the valve is replace, be sure to replace the breather
(easier said than done on some engines -- e.g., Chrysler's fuel injected K cars
where the entire air cleaner has to be disassembled to get at the breather).

Before doing tests 2 and 3, I'd do #1.  #2 and #3 will result in poor fuel
efficiency and excessive emissions, but nowhere near the problems present with
a thermostat that's stuck open.  Other problems present with a stuck open
thermostat are excessive wear on the engine, power reduction, and a somewhat
cold passenger compartment during winter!  See?  Aren't emission control
problems fun?  Now, if you REALLY want a problem, send me e-mail and inquire
about the engine whose catalytic converter was glowing...

Happy motoring!

Bohdan Bodnar
AT&T Bell Laboratories
Naperville, IL 60566