- Joined
- May 17, 2021
- Messages
- 30
1993 Mitsubish Expo LRV, 1.8L engine, 230,000 miles. I've had it the last 50,000 or so, driven gently. Gradually developed rough idle though not a single cylinder miss. Compression down about half the allowed amount and somewhat uneven; I put on a rebuilt head. Compression fine, idle somewhat improved but not okay.
02 sensor (there is only one) replaced.
Classic air leak problem, right? I went looking, found leaks at manifold flange, throttle body flange, throttle shaft -- all confirmed by spraying with starter fluid and/or sloshing with water. All fixed in the obvious ways and those methods no longer find anything.
Car runs fine off idle: This is now only an idle problem. It's fine when cold, equally okay when just warm enough for closed-loop operation, but when full temp -- say 10 miles on a warm day -- it's rough when you pull up to a stop light.
This a 'federal' car -- 93 had very little emission stuff. I've been all through the few vacuum hoses, nothing found. PCV valve replaced -- that improved things some but still not okay. I have two spare ECMs for this car; performance is the same for all.
I can read the OBD-1 and ECM data: The internal numbers for intake air temp, barometric pressure, and air flow are correct and/or in specs when at idle. Injector pulse width and duty cycle are in specs for idle. Fuel pump, strainer, and filter are all new. Injectors were replaced with used -- 'tested and flow matched.'
Fuel trims are all +5 to +8 %. Hummm ...
Best results (though not satisfactory) are obtained by backing off the mechanical stop for the throttle valve a bit from the correct factory setting. But the ISC steps number still tends to be low. These things suggest an air leak somewhere.
If you spray some starter fluid at the air intake the idle smooths right out for a minute or so. It really is a fuel/air mixture issue. but who is doing this?
I'm obviously missing something but darned if I know what it is. Been chipping away at this for nearly a year now ...
02 sensor (there is only one) replaced.
Classic air leak problem, right? I went looking, found leaks at manifold flange, throttle body flange, throttle shaft -- all confirmed by spraying with starter fluid and/or sloshing with water. All fixed in the obvious ways and those methods no longer find anything.
Car runs fine off idle: This is now only an idle problem. It's fine when cold, equally okay when just warm enough for closed-loop operation, but when full temp -- say 10 miles on a warm day -- it's rough when you pull up to a stop light.
This a 'federal' car -- 93 had very little emission stuff. I've been all through the few vacuum hoses, nothing found. PCV valve replaced -- that improved things some but still not okay. I have two spare ECMs for this car; performance is the same for all.
I can read the OBD-1 and ECM data: The internal numbers for intake air temp, barometric pressure, and air flow are correct and/or in specs when at idle. Injector pulse width and duty cycle are in specs for idle. Fuel pump, strainer, and filter are all new. Injectors were replaced with used -- 'tested and flow matched.'
Fuel trims are all +5 to +8 %. Hummm ...
Best results (though not satisfactory) are obtained by backing off the mechanical stop for the throttle valve a bit from the correct factory setting. But the ISC steps number still tends to be low. These things suggest an air leak somewhere.
If you spray some starter fluid at the air intake the idle smooths right out for a minute or so. It really is a fuel/air mixture issue. but who is doing this?
I'm obviously missing something but darned if I know what it is. Been chipping away at this for nearly a year now ...