Tranny causing low idle?

[White Lincoln]

As you may know, I have been dealing with a low idle issue ever since I finished my exhaust setup. The car idles at a stop, in gear around 500-550. It will idle at 750 in park / neutral. It used to idle at the average 750 while in gear. It's not the IAC, and I cannot find a visable vacuum leak, but I have a few issues with the tranny and wondered if that might be the issue. Also, its not the MAF or TSP, but if the TSP was adjustable, that could help (but not solve the problem).

Tranny still has a slight clunk when down shifting during when mild acceleration, harder downshift when jumping on the throttle. There is also a slight "lack of smoothness" when changing gears (slight drag sound when shifting). Tranny is a good tranny, but I think the 1-2, 2-3 accumulators are OEM from 94, I think they need to be updated for the issues above. Next oil change I plan to inspect the accumulators and replace with newer model.

Could the gear position sensor, if not exact cause issues like low idle?

Are there vacuum lines that would effect low idle elsewhere than the engine bay? Like going down a fender well to something else I can't see that I am missing? I have checked the canister lines, none of them have a leak.

Maybe back pressure from the new exhaust is causing the low idle?

 

[theterminator93]

Short version: I don't think so.

Longer version:

Is your RPM an estimate based on the gauge, or is it read from some other source? It's been over 10 years since I had my 94 3.8 so I can't recall for certain, but I can say the factory settings in the 97 4.6 tune are an idle of about 560 in drive and 800 in neutral/park - so your current behavior doesn't seem at all off.

In order for the trans to contribute to a low idle, it would need to be a sticky torque converter clutch causing excess drag, which is hydraulically impossible to activate when the trans isn't in 2nd, 3rd or 4th gear.

If there were clogged cats, I would expect that to manifest more as a performance issue while out on the road when a much higher rate of air needs to pass through them.

The TPS is not meant to be adjustable, but if it's not under 1V at idle, it could fool the PCM into thinking there's slight pressure on the pedal and open up the IAC a bit more as a result. It's easy to check with a DMM - if you need to adjust it, just hone out one of the mounting holes with a slightly larger drill bit to give you some room to twist it, then tighten it when it's in spec.

 

[White Lincoln]

Short version: I don't think so.

Longer version:

Is your RPM an estimate based on the gauge, or is it read from some other source? It's been over 10 years since I had my 94 3.8 so I can't recall for certain, but I can say the factory settings in the 97 4.6 tune are an idle of about 560 in drive and 800 in neutral/park - so your current behavior doesn't seem at all off.

Now that, I did not know. So, if my gauge is right, which I doubt it is, the wanting to stall would indicate a lower idle than what you have suggested. I will get out my tools and actually see what my idle is and report back.

n order for the trans to contribute to a low idle, it would need to be a sticky torque converter clutch causing excess drag, which is hydraulically impossible to activate when the trans isn't in 2nd, 3rd or 4th gear.

Thanks for putting my mind at ease, and I really doubted, but from my perspective of not knowing that much about auto tranny's, I had to wonder.

The TPS is not meant to be adjustable, but if it's not under 1V at idle, it could fool the PCM into thinking there's slight pressure on the pedal and open up the IAC a bit more as a result. It's easy to check with a DMM - if you need to adjust it, just hone out one of the mounting holes with a slightly larger drill bit to give you some room to twist it, then tighten it when it's in spec.

Yeppers, I know it is not to be adjusted, just as the throatle screw should never be changed, though I thought about it! But that is a bandaid to a problem and only masking what the issue really is. Let the computer do its job and you do your job to make sure the computer is not being fooled.

If there were clogged cats, I would expect that to manifest more as a performance issue while out on the road when a much higher rate of air needs to pass through them.

No, the car runs awesome. I was going to an appointment today and next thing I knew, I was doing 90 on the highway without any issues at all. After all the work I have done on this car, damn if it does not drive like new. I really am appreciating this car.

Be back with some numbers.... thanks Terminator.