Idle drops after sitting/stopping for 5-10 seconds

[YoungSensei]

Hey guys, my e30 is running pretty strong and seems to drive relatively nice and smoothly. However, my idle rpm seems to drop after sitting in neutral at a stoplight, stop sign, stop, etc. It starts idling at a stop at approx 600 rpm, but after a few seconds, it seems almost like the computer is causing the idle to change, dropping it to below 500 rpm. After this occurs, the idle sounds much more choppy, and as if the engine is saving itself from stalling. On a couple of occasions, there have been times when the vehicle has stalled coming out of a higher rpm range and into neutral at a stop, although it is rare. I'm almost certain the e30 doesnt have an "eco mode" while idling at a stop, which is almost what this reminds me of. Any ideas of what may cause such symptoms? I'd appreciate the input.

[Clio320i]

First thought is probably a vacuum leak. Any other info, like what's been done to it lately? Do you get a check engine light ever? If so, what does stomp test say?

[95BMWIC]

If no vacuum leaks - try pulling the ICV and spraying it out with brake cleaner.. Make sure it moves freely..

[YoungSensei]

Havent really done much to the car recently. I did replace the injectors with some used ones I got from ebay, but I don't believe them to be the cause of my issue. I will admit however that these injectors did cause misfires when I first got them installed, as if they would seem to stop delivering fuel while trying to accelerate above 3000 rpm, although a bottle of injector cleaner has since resolved these issues. Another major thing I did was replace the ICV. I used to have a monster surging idle, so bad, that I would have to leave the clutch biting first gear so people wouldn't think I was revving at them at stop lights. Other times the ICV would remain stuck open and the car would rev up to 3000+rpm and stay there. Replacing the ICV did fixed those original idle issue, however, it seems to have introduced this new, less noticeable issue that I mention in the original post. Here is a video for reference, you can clearly hear how the idle changes and becomes distinctly more choppy. Thanks for the replies! https://youtube.com/shorts/we7W4shY5tg?feature=share

[YoungSensei]

First thought is probably a vacuum leak. Any other info, like what's been done to it lately? Do you get a check engine light ever? If so, what does stomp test say?

No check engine light, although it seems like it may be the only light on the dash that doesn't turn on with the rest of the dash lights when the key is first turned. Isn't it the most center light on the dash? Is it supposed to atleast turn on to indicate the bulb isnt burnt out? Because that's generally what I would expect.

[Clio320i]

No check engine light, although it seems like it may be the only light on the dash that doesn't turn on with the rest of the dash lights when the key is first turned. Isn't it the most center light on the dash? Is it supposed to atleast turn on to indicate the bulb isnt burnt out? Because that's generally what I would expect.

The most center one is the OBC check light. That one should flash at you when you first start the car until you tap the brake pedal or push the button on the roof panel. The actual check *engine* light is over on the left. Let me see if I can point you to a picture...

ok check this pic out. The OBC Check light is that one right in the center under where it says VDO in between the speedo and the tach. It's off in this pic. The actual CEL is on in this person's pic, and it's way over at lower left. https://preview.redd.it/t57r2bzpk8b9...abled&395b5820

[YoungSensei]

Hmm neither my CEL light nor my OBC light ever turns on, even though there is fault for the Brake lights on the overhead cluster for some reason. This brake light fault is also sporadic, and will sometimes disappear after pressing the brakes after the car has started. Other times, the whole overhead computer seems to lose power, as the backlighting and lights sometimes will fail to power. However, pressing the check button itself seems to "reset" it.

[Clio320i]

Well, dang. Both should come on when you turn the ignition key to the 'on' position. The CEL should be on steady and the OBC light should be blinking. Starting the engine *should* cause the CEL to turn off and the OBC light to continue to blink until the brake pedal is depressed OR you hit the reset button. Sounds like some connections are not great in your cluster and with the check panel. If your CEL was functional, I'd say do a stomp test and see if you can get a code; these are *sorta* useful. But chances are good that even if you did that, you'd get code 1222 (assuming your O2 sensor is at least somewhat functional). I do still strongly lean toward a vacuum leak still happening that was previously unnoticeable when you had the bad ICV in there.

If this were my car, I would:

1. proceed as though you have code 1222. Do all the googles and particularly take note of whatever jlevie says. Basically, make sure O2 sensor is good and then track down vacuum leaks.
2. remove the cluster and ensure that the bulb for the CEL is present and functional, and if so, do whatever electrical troubleshooting is necessary to get it to work
3. sort out whatever's causing the check panel to be intermittent

And, an anecdote, FWIW. Recently - well, like, since we took it to the track in July - our E30 had a 1222 code with the CEL on only at idle. Low idle speed and smelled kinda rich. Boggy at low rpm, would want to hump the pavement when going in 2nd at anything under about 2000rpm. Kind of a drag in traffic lol.
First thing we did was replace the O2 sensor since it was of unknown age and rust-welded to the downpipe. No change.
Second thing we did was replace the in-tank fuel pump and fuel filter, in efforts to rule out a fuel starvation issue AND because on hard corners to the left with less than 1/3 tank of gas, it would cut out. Classic in-tank pump failure symptom. Not much change after that, if anything it was worse.
Third thing was to replace the spark plugs, which were quite fouled after only 10K miles. That seemed to improve driveability slightly, but recognizing the fouling is only a symptom of the problem and certainly not the cause, we proceeded to the
Fourth thing, which was to rule out vacuum leaks caused by a) bitch tube o-rings (leaking there a lot) and intake manifold gasket. We also replaced a coolant hose running under the IM and the throttle body > IM gasket, which was fine. This solved the problem entirely; we found that the PO had installed IM gaskets for an eta, which have smaller diameter intake ports, so as you can imagine there was extra gasket material sort of flapping in the breeze between the IM and the head, AND, the #6 gasket was torn and partially blown out - definitely a significant vacuum leak there. Had we inspected closely before taking it all apart, we could have actually seen that torn gasket material; as it was we noticed it during the IM removal process.

Of course this is not to say you'll have exactly the same scenario, but the symptoms were close to identical with the exception of your lack of CEL info. One more possibly useless data point is that my M10 in the E21 will do a similar drop in idle speed particularly after going along at higher RPM. I have the idle speed adjusted a *little* high to compensate for this, which is definitely caused by a vacuum leak I haven't yet found (or even bothered to look for). The E30's computer will do these sorts of compensations for you, which is, as you mentioned, I think what is probably happening to some degree.

Have you tried a smoke test? That might be helpful..

[beeemdubbayoo]

Is your car a 325e? The injectors for the eta cars have a different impedance value than the injectors for a 325i car. The fact that you replaced the injectors with ebay injectors threw a flag for me.

[YoungSensei]

Is your car a 325e? The injectors for the eta cars have a different impedance value than the injectors for a 325i car. The fact that you replaced the injectors with ebay injectors threw a flag for me.

Sorry I should've mentioned that they were replaced with exact same part # oem injectors. They are the "e" injectors.

[YoungSensei]

Well, dang. Both should come on when you turn the ignition key to the 'on' position. The CEL should be on steady and the OBC light should be blinking. Starting the engine *should* cause the CEL to turn off and the OBC light to continue to blink until the brake pedal is depressed OR you hit the reset button. Sounds like some connections are not great in your cluster and with the check panel. If your CEL was functional, I'd say do a stomp test and see if you can get a code; these are *sorta* useful. But chances are good that even if you did that, you'd get code 1222 (assuming your O2 sensor is at least somewhat functional). I do still strongly lean toward a vacuum leak still happening that was previously unnoticeable when you had the bad ICV in there.

If this were my car, I would:

1. proceed as though you have code 1222. Do all the googles and particularly take note of whatever jlevie says. Basically, make sure O2 sensor is good and then track down vacuum leaks.
2. remove the cluster and ensure that the bulb for the CEL is present and functional, and if so, do whatever electrical troubleshooting is necessary to get it to work
3. sort out whatever's causing the check panel to be intermittent

And, an anecdote, FWIW. Recently - well, like, since we took it to the track in July - our E30 had a 1222 code with the CEL on only at idle. Low idle speed and smelled kinda rich. Boggy at low rpm, would want to hump the pavement when going in 2nd at anything under about 2000rpm. Kind of a drag in traffic lol.
First thing we did was replace the O2 sensor since it was of unknown age and rust-welded to the downpipe. No change.
Second thing we did was replace the in-tank fuel pump and fuel filter, in efforts to rule out a fuel starvation issue AND because on hard corners to the left with less than 1/3 tank of gas, it would cut out. Classic in-tank pump failure symptom. Not much change after that, if anything it was worse.
Third thing was to replace the spark plugs, which were quite fouled after only 10K miles. That seemed to improve driveability slightly, but recognizing the fouling is only a symptom of the problem and certainly not the cause, we proceeded to the
Fourth thing, which was to rule out vacuum leaks caused by a) bitch tube o-rings (leaking there a lot) and intake manifold gasket. We also replaced a coolant hose running under the IM and the throttle body > IM gasket, which was fine. This solved the problem entirely; we found that the PO had installed IM gaskets for an eta, which have smaller diameter intake ports, so as you can imagine there was extra gasket material sort of flapping in the breeze between the IM and the head, AND, the #6 gasket was torn and partially blown out - definitely a significant vacuum leak there. Had we inspected closely before taking it all apart, we could have actually seen that torn gasket material; as it was we noticed it during the IM removal process.

Of course this is not to say you'll have exactly the same scenario, but the symptoms were close to identical with the exception of your lack of CEL info. One more possibly useless data point is that my M10 in the E21 will do a similar drop in idle speed particularly after going along at higher RPM. I have the idle speed adjusted a *little* high to compensate for this, which is definitely caused by a vacuum leak I haven't yet found (or even bothered to look for). The E30's computer will do these sorts of compensations for you, which is, as you mentioned, I think what is probably happening to some degree.

Have you tried a smoke test? That might be helpful..

You've given me alot of helpful advice here. I will follow up on your suggestions. I'll probably get a smoke machine soon.