Hi, Coupe,

Gus plopped this on my plate a couple days ago, needed to ponder a while, re-read and such. You've posed lots of questions, so I don't know how many I'll hit.

The original post, about the RPM dip or stall coming to a stop; I'm starting to wonder if you have an issue with the torque converter clutch. This may be sort of hard to follow, but rarely I have run into one that is basically stuck on. The TCC CAN apply in all gears except 1st. But in 1st gear, it's not possible for it to be applied. Here is what can happen. If the TCC is stuck on, and you start coming to a stop, the engine is directly linked to the propshaft. As you slow down the trans begins to downshift to lower gears, as you reach a stop the trans is near the 2-1 downshift. If you come to a stop rather slowly, the trans has time to make that 2-1 downshift. But until it does, it starts to drag the engine speed down, sort of like leaving the clutch engaged on a manual trans as you stop. But at the point of making the shift to 1st, the TCC is forced to disengage by it's design. That can account for the dip in engine speed and the 'catching itself' so it continues to run. If you come to a stop more quickly, the trans may not have the time to make the downshift to 1st, so you actually reach a stop in 2nd gear. If that happens, you stall, just like that engaged clutch I mentioned. I've tried to come up with a way for you to prove this, but not having much luck. You might have to take the car to someone experienced with the ZF transmissions and let them drive it. Confirming such a thing is basically an experienced seat of the pants "I can feel it" kind of thing.

The throttle not reading a full percentage......
I wouldn't worry about that until you conquer the rest of this. More than likely, you may have to alter the stud the kick-down switch is mounted on. That's below the throttle pedal. I've had to shorten that stud a number of times to allow a full stroke on the demand sensor at the throttle body.

The MAF charts.....
Gus got those from me. As you noted, they are nowhere in any documentation. Those were made many years ago as a way to quickly gauge whether further scrutiny of the MAF sensor was warranted. I arrived at those figures by charting many many cars over time, they area averages of the various models when a new MAF was first installed. They have proven very valuable over the years. If a certain model varied more that 2-3 g/sec from that chart, I could be pretty sure the MAF was part of the problem, or I needed to really look over the induction system for leaks or blockages.

Gotta run!
Hope that helps some.

 

Thanks very much for taking the time to think it over and write down your thoughts. That's an interesting hypothesis. So it sounds like you're not in the throttle position sensor camp.

Would a stuck TCC be temperature-related...as in fine for the first 8-10 minutes from cold start, and THEN presenting this behavior?

ALSO, would a stuck TCC cause this behavior when the car is in REVERSE?

I was all ready to send my throttle body off to Automotive Scientific for a test and possible rebuild...now...well, I dunno.



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Installed the TB and set the idle at 650rpm with the a/c on and it comes up to 695rpmwith the a/c off and all seems ok so far but I will not know totally until I am driving with stop and go traffic and I am unable to do that until I get my bracket from ASI. I am going to pull the control module and check for any abnormalities.

 

Did you try disconnecting the MAF yet?

 

Continuing with XJRGuy's thoughts below on a possible torque converter clutch that is stuck ON...here's a bit from Jag in their technical introduction of the 5HP24:

Torque Convertor Lock-up
The torque convertor lock-up clutch is engaged as a function of throttle position, output speed, oil temperature, gear shift and shift program.
Lockup is possible in 2nd, 3rd, 4th and 5th gears but is usually restricted to 4th and 5th gears. During a gear shift the TCM controls the amount
of slip of the lock-up clutch to enhance shift quality.


So...if the torque converter was truly stuck ON...or partially ON...wouldn't the engine run roughly AS I WAS COMING TO A STOP and the engine was forced to slow down?

What happens now is that the RPM is completely normal as I'm coming to a stop...even if I'm literally crawling along for a while, foot on the brake but moving slowly, the RPMs are fine. The sudden RPM drop ONLY happens once the car has come to a COMPLETE stop.

Also, there is a trouble code for "Torque Converter Clutch Stuck ON"...the code is P0742. I'm not getting that code, or any other.

Still thinking. Feel free to offer your thoughts!


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I did...as expected, the engine runs quite roughly with the MAF sensor unplugged, and smooths out perfectly once it's plugged back in.


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UPDATE:

The car spent the day at a local transmission shop with a lot of experience in ZF gearboxes. After driving the car and running a few tests, they were able to replicate the RPM drops at complete stops, BUT they do NOT believe that the torque converter clutch is the source of the problem.

I'll be fiddling with things a bit more, and then perhaps a rebuild of the throttle body is back on the table.

I would appreciate hearing anyone's thoughts.



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I think you need a avos twin screw kit to fix it

 

Thank you so much, Bill. I should probably get a supercharger first, tho.

UPDATE:

I can now replicate the RPM drop and stall or near-stall simply by putting the car into reverse once the engine is at operating temp.

Car not moving at all, foot on the brake, move gear selector from DRIVE to REVERSE, and the RPM dives to about 300-400, the dash lights dim as the engine nears stall, and the engine either catches itself and RPM returns to normal, or the engine stalls. This all happens in about 1 - 2 seconds. Move gear selector from REVERSE to DRIVE, sometimes the same behavior, sometimes not.

A mystery!


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Today I pulled the control module broke the seals and looked at both sides of the circuit board (my module only had one board) and it was clean and without leakage and I looked it over hard. Well I am at a standstill until I get my cable bracket.

How does yours reach with the A/C on?

 

Same behavior, A/C on or off.


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You have problems with the throttle body electronics. I have experiences with those odd stalls with my bored TB, even it's brand new. No codes. Change your TB and you are most probably ok.