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.

 

[White Lincoln]

Using a tac-o-meter... from my timing light;
Idle in park, 800
Idle in drive 700
Timing is set to 10btdc

When the engine about dies under load, the idle speed is 550 and picks up from there. When it almost stalls after putting it in gear, the tac is 450-550 (jumps around till it settles to 750.

The inside tac at 1000 reads 1040, at 1500 = 1530; 2000 = 2030, 2500 = 2540
The tac is about 30 rpm off. But the needle at 650 does look like its a 550.

The engine runs much better using the OEM IAC I pulled from a junked 94 - 3.8l. The one I got from RockAuto, when cold will spike up to 2300, slowly drop and finally settle around 750 (in park). The low idle happens in both IAC's. Maybe both are crap?

So why the drastic drop in idle under load? I will have to play with the TPS a bit. I also wanted to see what my compression is. I did it a year or so ago and I am not convinced i did it under the best of conditions. I also want to check my fuel pressure again and test the new pump I have to see what it is pumping at. Then someday, replace the pump before I have to replace it.

 

[Grog6]

The low idle in gear can be a dragging tc clutch. The current trans/tune in lazarus does not disengage the tc clutch, except in 1st and reverse, so it dies coming to a stop sometimes, if it doesn't downshift fast enough. I think I left out the #9 ball, for some reason, and drilled a hole to increase tc flow. I have another valvebody; I need to pull the engine/ trans as one. I have an '02 trans in the garage to assemble, as well as a4V engine, lol. But I think I'll be doing Laz's 2v for now; It only needs crank,rods, bearings, and valve stem seals.

 

[White Lincoln]

I really did have to wonder if this is a tranny issue, but terminator did not think it would be. While doing the stop and go driving to an appt this morning, I got to thinking that maybe I have a vacuum leak, a small one at that, just enough to have rough idle at stop (in gear). Everything I read always points to vacuum leaks when there is rough idle (all other issues have been resolved on the car). But I also was reminded of the issue I have had with this transmission from the start, it has a harsh downshift and harsh passing gear shift. As I had mentioned, I am going to replace the 1-2 accumulator. I did see some tranny kits to help the 4r70w shift a little better (not a shift kit).

 

[Rodeo Joe]

J-Mod! ^^

Joe

 

[theterminator93]

At least on the later CDAN4 EEC-V PCMs, the computer uses a pair of tables that predefine the amount of airflow expected to maintain a given idle RPM in drive and neutral, respectively. Because the PCM has another table that correlates the IAC duty cycle required for any amount of air let through, it can cross reference these tables to determine the IAC duty cycle required to get a desired idle RPM. I doubt the EEC-IV does it that much differently, but I can't say that with any certainty.

If the idle RPM achieved with the information in the tables is off for some reason, it will adjust the airflow slightly (up or down) until it gets to the desired idle RPM. On a stock vehicle, minor variances in manufacturing would be miniscule enough to not notice a major discrepancy like what you're describing. But if there was a mechanical deviation caused by a dirty MAF, sticky IAC, leaky throttle body, unmetered air (vacuum leak) etc. it would cause idle woes like this. At idle, small vacuum leaks are amplified because the amount of air they let in is relatively fixed, but with a smaller air requirement to idle that leak becomes a greater percentage of the total and thus affects it more.

Something else the PCM does is add a little air to the idle requirements when it detects power steering activity, the A/C clutch engages, or the HSF turns on. If any of these systems is drawing more load off the engine than normal, it would require more air to compensate for that load than the PCM is adding and also throw things a little off.

 

[White Lincoln]

Awesome eval. I was thinking the same thing about a small vacuum leak causing more of an issue at idle than during driving. After putting the OEM IAC in, the car starts at cold and goes to about 1500 rpm, which is about normal and then slows down shorly after, which is what I consider to be correct. But, it is not to say that this 31 year old IAC from the junk yard won't have some issues. I would say the RockAuto IAC was crap and the junk yard one may have some issues. I keep going back to the IAC because the PCM is what controls the idle to the IAC (and other components) and since I have no codes and only have the issue when cold and in gear, I think it might be time to buy a new Motorcraft IAC and just bight the bullet on the cost. Unless someone has a better brand that they truly believe to be as reliable as the OEM version.

One other thing that I am wondering about too is if I have an injector(s) that is leaking, and the cylinder(s) are getting more fuel than the PCM is saying it needs (and the PCM would not know if its leaking) and the engine would run rich and maybe act up? I don't want to go down that road yet though, I have struts and UCA's to replace! Got a great price on Monroe strut/spring sets, left and right! And I had a pair of UAC's that I was not aware of under my work bench (I did inventory yesterday of all my new parts I have for both cars).

Thanks Terminator for the excellent write up.

 

[theterminator93]

Not sure I'd consider that super likely, at least not compared to a vacuum leak, wonky IAC or MAF sensor. If it only goes so low as to stumble in drive when cold (How cold? How long after running until it stops?) it's possibly open/closed loop related. If it IS happening in open loop only, a vacuum leak is the first place I'd start; with open loop and a cold engine the MAF is the only source of truth for the PCM to deliver fuel, and it won't know about a vacuum leak because of the lack of O2 feeback to hone in the mixture. It would be running lean...

 

[White Lincoln]

Not sure I'd consider that super likely, at least not compared to a vacuum leak, wonky IAC or MAF sensor. If it only goes so low as to stumble in drive when cold (How cold? How long after running until it stops?) it's possibly open/closed loop related. If it IS happening in open loop only, a vacuum leak is the first place I'd start; with open loop and a cold engine the MAF is the only source of truth for the PCM to deliver fuel, and it won't know about a vacuum leak because of the lack of O2 feeback to hone in the mixture. It would be running lean...

Ah, makes much more sense. And I threw my OEM MAF away when I got the new one. Stupid, stupid, stupid... At least I would have something to test with. The MAF in the Town Car is not the same by any means. I will see if I can track down the leak, but will have to get a diagram of the vacuum lines and check each one. A lot of times, from what I have seen others deal with when faced with a vacuum leak, it is something you won't even see until you bend the vacuum line to see if it has cracks. Most of the upper vacuum lines was replaced not too many years ago, but there are still some I never touched. I will update when I find out more, could be a while though. Always a pleasure to hear your comments, thanks Terminator!