Engine/Acceleration issues with low revs

[MattyK]

Here is my problem, some things ive experimented with, etc.
Car is a '91 318is (obviously ).

When the car is cold, its fine. After driving for 3-4 minutes, problems start, and continue until you let the car get cold again, like overnight. Problem is that when you let the clutch out and go from a stop, the car lags and sounds awful. Idle is a little rough (900-600), but when the revs drop at all from 800, the car sounds like its misfiring a bit. Then say I rev it up to 1400 and let the clutch out, then move the accelerator down a bit more (say to 1/2 - 3/4 of its max) to pick up speed, it doesn't really die when you let the clutch out, but when it should start to accelerate, it does so very slowly, and you can hear the engine stuggle (sounds sort of like misfiring, or a bad A/F ratio?). At some point, keeping the accelerator down at the same point, it will suddenly react the way it should - with the revs jumping up. Sometimes it gets itself together near the end of 1st, sometimes between the shift to second, and sometimes at the start of second. After it recovers and sounds and acts fine, it will remain fine until the revs drop down below 2000 for a few seconds, then I will have the same symptoms again.

Im not much of a mechanic myself, so I haven't checked too much myself. First mechanic looked things over, didn't find much except that my ignition coils were f*cked, so got that fixed, though it didn't solve my problem (and I didn't think it would). Then decided to try a fuel system clean from Mr. Lube. Ended up smoking up the joint, so that was a good thing (or a bad thing, depending on your point of view), and the car ran pretty solid after that, probably 85% better than before, so that was good. But my gas was pretty low, and when I filled up 3 days later, the car started acting like sh*t again.

I checked to make sure I had no obstructions in airflow, which I didn't, and changed the ECU (cause that takes 5 minutes) with a spare I have. The car runs worse with the spare - Im guessing this is because the spare has a stock chip, my standard has an aftermarket (TMS I think) chip - but what would the difference in chips change in mapping to make the stock one worse?

Ive watched my fuel efficiency, it hasn't noticeably varied (Ive driven a bit more highway, so I went a bit further on my last tank).

Ideas? Things to check? Anything else I should try while driving to try to isolate the problem?

[cutncheese]

Did you check for any error codes from the mototronic? That would probally help  alot!

From the sounds of it my first reaction would be fuel, a clogged line or clogged injectors. Also, try hooking up a fuel pressure gauge to the fuel line in the engine compartment to ensure your getting enough fuel up there.

[silverblades181]

I get the same problem except it happens sometimes when I start the car. Then after a few problematic accelerations it goes away until I stop the engine. I thought about O2 sensor...

[Jtuner]

This couldn't be a coolant temp sensor could it? possibly an AFM? It sounds like what my car was doing when I first got it.. I changed the coolant temp sensor and it was fine... just a thought

[MattyK]

coolant temp sensor? Now thats a thought - a very random one at that! Doesn't make much sense, but its sure worth a look at. I had some coolant issues 2.5 years ago in the winter, maybe something decide to mess up now.

I haven't had an O2 sensor for about 4 years, so its not that (unless it just decided to matter for some reason).

I would think clogged Injectors or clogged fuel line would make me have poor acceleration the entire time, not just after the revs drop for a couple seconds though (or at completely random times) though, right? I was thinking about a problem with fuel pressure since it acted up after I filled up with gas. As well as something that needs some power, but isn't getting enough until the revs get high enough - so Im leaning towards fuel pump right now, though since I have a full tank of gas I want to wait until I have a bit less fuel pressure before I start disconnecting stuff with that to test it just to be safe. Ill take a peak at the AFM while I'm at it.

And ill try to get her to talk to me with some fault codes too.

[silverblades181]

I changed my fuel pump and it didn't do anything....why would the coolant temp sensor do anything with acceleration?

[MattyK]

Doesn't look like it should - "This sensor is used to provide the idle control unit with information to keep the car idling before the oxygen sensor heats up enough to take over mixture control" - from Pelican. So apparently might cause idle problems, but nothing with acceleration (though if its broken, you can get a bad mixture, which might cause problems in the first couple minutes of driving, but not after that).

Im almost certain its running too rich - am still puzzled as why filling up the tank would cause the problem to increase though. Unfortunetely I don't have much time during the week, and am out of town this weekend
Planning to check AFM and fuel pressure when I have time.

[D. Clay]

"This sensor is used to provide the idle control unit with information to keep the car idling before the oxygen sensor heats up enough to take over mixture control" - from Pelican. So apparently might cause idle problems, but nothing with acceleration (though if its broken, you can get a bad mixture, which might cause problems in the first couple minutes of driving, but not after that).

Im almost certain its running too rich -

From your description this is engine temperature related. A faulty temp sensor could be telling the system that the motor hasn't warmed up yet. That it still needs a rich mixture. The temp sensor may be overriding the other inputs. Otherwise you wouldn't need one. Just a timer on the O2 sensor.

[silverblades181]

my problem happens what ever the temperature. the only constant is that it happens right after I started up the car...never when I've been driving for a while.