M20 Stalling When Warm/ Operating Temperature

[KIRIEIW]

Hey guy's, I've been having a problem with my new convertible and I'm a bit stumped on what else to test. I'll start off by saying the records I got with the car show that it went in for service 2 days before me purchasing it for the exact same problem, for whatever reason they recommended a thermostat replacement (which I don't see how that would help a stalling issue). Of course, the sellers didn't disclose the problem but I'm still happy with the purchase which is what counts.

The car starts right up with no hesitation at any given time while cold, idles great, drives great, very solid e30 with no clunks or weird noises. Highly maintained car with receipts that date back to the car being under warranty. Once the car warms up to normal operating temperature the issues occur.

Symptoms; Video Bellow Once the car gets to operating temperature, and only when its at operating temperature, the car will hesitate if you tap the throttle from idle, and the Check Engine light will flash. If you hold the throttle from idle it will sometimes stumble and take a second or two until it starts to gain RPM's. While driving, it drives great and purs like a kitten, until you come to a complete stop where it will occasionally completely stall at the light/ stop sign. Once the car is up to speed, it drives great the problem only happens when you start off from idle, or come to a complete stop. When the car stall's, it will start up every time with absolutely no excess cranking but if you put it into drive it will stall again. 3 or 4 attempts later you can get it in drive with out the car stalling and it will take off fine. On other days (such as the day I bought the car, which was a month ago and I put on maybe 30 miles) it runs fine when warm, with no stalling or hesitation. At this point in time however, it's stalling and has hesitation as soon as you start off from idle, whenever it's warmed up.

What I've checked:
-Smoke Test (Thank you Kronus)
-CPS Sensor
-Idle control valve (it buzz's with the ignition on, also cleaned it)
-New Spark Plugs
-Thermostat (under the previous owners ownership; two days before my purchase)
-Swapped AFM with a known working one from my other e30
-Fuel Pressure regulator (Disconnected vacuum hose and plugged both ends, might replace but actually plugging both ends should have fixed the problem if it was bad)
-New Fuel Filter
-Disconnected the O2 Sensor. I read online if the O2 is disconnected the ECU will run of the the programmed data rather then live, not sure if that's true please confirm.
-Fuel pump was replaced 100k miles ago with a OEM pump at the dealer so I assume it's still fine
-Tested Ignition Coil
-New Cap and Rotor
-Swapped ECU's
-NEW Coolant Temo sensor
-Tested TPS switch
-Cleaned Throttle body from oil and replaced gasket
***Replaced ICV and problem seems to have went away (crossed fingers) Put on close to 60 miles now with out it stalling, it usually would stall within the first half mile of driving.
EDIT: The car stalling issue seems to be fixed, but is now/still idling a little un happy


Up next on the list was to check the C191 connection (I found it weird this only happens when warm, so I doubt that's the issue) and I also have a brand new Cap and Rotor which I should have installed while doing the timing belt change. I might even buy a new coolant temp sensor just because it's cheap and the car appears to have the original one still installed. I also have rebuilt M20 fuel injectors laying around which could be worth installing. I was also thinking the TPS, but the TPS only works at idle and full throttle from what I remember and the car runs fine in both those positions.


I really hate to shot gun parts into the car, any recommendations would be a great help. Car smog is due next month otherwise I need to PNO it so really need to figure this out ASAP!


Thanks for reading.

[Bullet Ride]

Still test the TPS to make sure that it's switching on at idle and off when you bump the throttle.

[dnguyen1963]

I had a similar problem a long time ago and it was due to loose aluminum elbows connecting to the throttle body. They will come loose only when the engine warms up. And it is only intermittent. Smoke test did not detect the leak until I wiggled the elbows. Lesson...shake everything up good when you do a smoke test (and plug the exhaust) or you might miss some minor leaks that can create big headache.

[KIRIEIW]

I had a similar problem a long time ago and it was due to loose aluminum elbows connecting to the throttle body. They will come loose only when the engine warms up. And it is only intermittent. Smoke test did not detect the leak until I wiggled the elbows. Lesson...shake everything up good when you do a smoke test (and plug the exhaust) or you might miss some minor leaks that can create big headache.

When I did the smoke test, the only places I found leaking/ smoke coming out where the camshaft plugs, only one plug against the firewall, and a little hole on the transmission which allows you to look at the starter teeth (in the pic you see two holes, it was one of the two). I'm assuming to get to that hole the smoke must have traveled past the rear main seal, and the only place I can find oil leaking is the rear main seal on the car, I scrubbed everything clean to make it easier to diagnose leaks. Such a small amount of smoke came out of both those places I'm surprised I even noticed it. The tester was on the car a good few minutes till I saw smoke come out of the transmission hole.

The elbows have a really tight fit, I can't even turn them but I'll check them out again when the car's warmed up. I really don't think its a vacuum leak, especially considering the car runs excellent when it's cold.




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[Bullet Ride]

Try swapping coil and plug wires from other known working E30.
Also possibly the DME, occasionally they can end up with a cracked solder joint that causes weird behavior.

I remember my car had a weird occasional gremlin that would cause it to start bucking, I couldn't do anything to reproduce the behavior, it would be fine for days, weeks at a time, then it would just happen all of a sudden, and the only way I could successfully get it to go away was to shut the car off, unplug the AFM, O2, and coolant temp sensor, and then try starting the car while pressing the accelerator ...It obviously wouldn't fire up with all that stuff unplugged (in retrospect maybe I was triggering a flood clear?). Then plug it all back in and the car would start up and act normal... for 5 minutes, 5 days, or 5 weeks, depending on what the car felt like doing lol.

It never occurred again after I converted it to over to megasquirt for my engine build, which made me believe it had to do with some flaky component(s) in the DME.

[noid]

Check the main relay, CTS and DME. You are correct that disconnecting the O2 sensor will make the car run off a default map.

[KIRIEIW]

Bought a used ECU today and swapped it out, same problem.


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[KIRIEIW]

Took a quick video, this is the second time I decided to take the car for a spin because it stated and idled great when cold. Huge mistake, once it gets warm it stalls out at every stop sign and I get strand for a few minutes at a time. Once the car is moving however, it runs pretty good (its starting to have a few stumbles when driving/moving but it doesn't flat out stall).

I have a new temp sensor coming in the mail, the specs for the Bentley show a resistance value when the cars fully warmed up, and I'm not 100% sure if it's even getting to full operating temp. so testing it wouldn't be completely accurate.

Today I swapped ECU's (Junkyard got a 1991 Automatic yesterday so I ran over there today before anyone could grab it), cleaned all the plugs/ sensors I could reach in the engine bay with contact cleaner, checked the C191 and C101, both where in perfect shape with no visible corrosion but cleaned anyways. Also unplugged the O2 sensor, the car ran with it disconnected but still had the same problem.. found it weird however the check engine light wasn't illuminated with it being disconnected.


Link to video:
https://www.youtube.com/watch?v=vZ9YKU7oiC4

[noid]

Check DME main relay (I once had one work great at higher RPM's but not at lower).
Check the pulse generator wire off of the spark plug wires
Load test the battery to confirm if you have a dead cell (also monitor voltage when driving).

How did you check the CPS?

[dnguyen1963]

Ok, I watched the video and it looks like as soon as you released the throttle, the air is cut off from the engine. Have you cleaned the throttle body and plate?

[KIRIEIW]

Check DME main relay (I once had one work great at higher RPM's but not at lower).
Check the pulse generator wire off of the spark plug wires
Load test the battery to confirm if you have a dead cell (also monitor voltage when driving).

How did you check the CPS?

I have an extra DME relay so I'll swap my extra in and report back

Is there a spec to check the pulse wire resistance in the Bentley? I'll look at my book later today. I always thought your car would still run with that wire disconnected, I know the 325e's don't have one.

I'm assuming the battery's good because it was replaced about 6 months ago, and I also have a receipt for a new alternator a few years back. I suppose it's still possible but I highly doubt it.

[KIRIEIW]

Ok, I watched the video and it looks like as soon as you released the throttle, the air is cut off from the engine. Have you cleaned the throttle body and plate?

I have not! I planed on cleaning it this weekend (my work keeps me from morning to night so there's sadly no time to do work on the week days.) I have an extra throttle body gasket already so I'll swap that in at the same time. I had the boot off and was able to see inside the throttle body a few weeks ago, little oily inside and I gave it a quick whip but didn't bother cleaning the pivoting points for the butterfly flap. I know the Bentley calls for lubricating the throttle body every so and so miles. Also, your right the car does only stall whenever the throttle is released completely, when operating temperature.

I picked up a used TPS at the junkyard after Bullet Ride's suggestion. Oily as hell but I sprayed a ton of contact cleaner inside hoping to clean most of the oil up that got inside. In case mine is out of spec, I'll now have an extra to swap in and test.

[Som]

Few things from my perspective. Not sure how helpful, but who knows...

The TPS test is pretty easy if you have a continuity tester or ohmmeter. There's 3 pins. The middle one is a common pin. When the throttle is closed, you should see continuity with one of the pins, and when the throttle is about 2/3rds to WOT, you should see continuity between the middle and the other pin.

Before doing that, though, I'd first check to see if you can feel a noticeable click when you close the throttle by hand. Open the throttle by hand and slowly release it. Just before it hits its resting point you should feel a click. If it's not there, that's a problem. Remove your throttle cable and try again -- if you feel the click now, then your throttle cable might be too tight. That said, just having a click doesn't mean you're problem free -- you'd still need to do the above continuity test because the idle switch inside the TPS could be bad.

Regarding the camshaft plug leak. I'm not really that experienced with the specific symptoms of vacuum leaks and all that. I will say, though, to keep in mind that the valve cover has a breather hose that connects to the throttle body. So, conceivably, a camshaft plug leak is still a leak that affects your intake pressure. One way to test the significance would be to have someone with a working E30 loosen their oil cap and see if it exhibits the same symptoms.

It seems like there'd only be a handful of likely sources to the problem you're describing, and I'd think the ICV would be one of them. TPS and intake leaks would be the other two. Not saying there can't be a million reasons (right down to the dreaded wiring harness possibility), but I think there's definitely more testing to do with the most likely suspects first. I believe all the other failure items are things the ECU is supposed to be able to deal with. I was under the impression you could even just disconnect the AFM/O2/Coolant Temp sensors and still expect to have the engine idle since the ECU would be able to substitute default values.

Given that, despite the ICV being cleaned and buzzing, I'd see if I can borrow someone's known good one to test with.

Good luck. Curious to hear what it turns out to be. Fortunately I haven't had to deal with any idle gremlins yet, but it's good to have data points so I know where to go when I do. :)

Som

[Bullet Ride]

have someone with a working E30 loosen their oil cap and see if it exhibits the same symptoms.

I've forgotten to put my oil cap back on before after adding oil, the car was only idling slightly rough. It definitely wasn't anywhere near stalling out.

I was under the impression you could even just disconnect the AFM/O2/Coolant Temp sensors and still expect to have the engine idle since the ECU would be able to substitute default values.

I think this might only true if your TPS is functioning because if the idle switch isn't made then the DME won't trigger the idle table (which the car can run on with the AFM unplugged)

[Som]

I've forgotten to put my oil cap back on before after adding oil, the car was only idling slightly rough. It definitely wasn't anywhere near stalling out.

That's my recollection from how my E36 behaved, but I don't know if I ever really waited for it to warm up or anything.

I think this might only true if your TPS is functioning because if the idle switch isn't made then the DME won't trigger the idle table (which the car can run on with the AFM unplugged)

Makes sense. That made me realize -- I guess yet another way to test is to unplug the TPS completely and just short the middle+idle pin in the plug with a paperclip or something. If that fixes the idle, then the TPS is busted.

Som