Intermittent Engine Light

[wsanders123]

Went through the boards, looking for a previous similar situation and found a few that were close to what I am experiencing, but my situation seems to be just a bit different.

I have a 1995 M3 with about 65K on it. A few weeks ago, I noticed that my engine 'hunts' for an idle rpm between 500 and 900 rmp for a while after first starting up. After getting down the road a bit, when approaching a stop and pushing the clutch in and rolling to the stop, the engine rpm fluctuates a bit, hunting between 500 and 900 rmp again. As soon as I come to a full stop, the rpm settles back down. This will continue for a few miles and then everything seems normal. I have seen similar posts on this topic. But......

A few days ago, I noticed that after a moderate accelearation and rpms around 3000-3500, when I push the clutch in and as the engine rpms drop, my check engine light will flicker on. By the time I get into the next gear and engage the clutch, the light goes back out (it only flickered on for less than 1 second). This will happen going between 1st and 2nd and between 2nd and 3rd. It seems that when the car gets warmed up, the engine light will no longer flicker. The light also does not seem to come on if I take it easy while accelerating, at least not that I have noticed.

I have not checked the engine codes, as I noticed I can do without an actual code reader. I have been wondering if my O2 sensor might be the culprit.

Any ideas? Thanks in advance.

 

[Ben]

i think you might have a small hole in your intake, either the rubber hose or one of the fittings after the AFM. just an idea. but i dunno.

 

[wsanders123]

Thanks, I'll look into it.

 

[jrt67ss350]

I'll think out loud to see if I give you some ideas based on my knowledge of domestic car engine management.

I'd bet that it's not your O2 sensor. It sounds to me like it's a problem related to the engine running in open loop mode. If the BMW engine electronics are anything like those in domestic cars (which I bet they are since they have to conform to the same emissions standards as domestic cars), the engine will run in what's called "open loop" mode when you do a cold start. In open loop mode, the oxygen sensor is essentially ignored. I will say that I don't know what kind of variables a heated oxygen sensor throws into the ring here. But, if the engine runs in a true open loop mode when it is cold, then it is probably not the O2 sensor. When the engine attains a particular pre-set coolant temp, the computer switches to closed loop mode and fine tunes the air/fuel mixture to attain the optimal lambda via the O2 sensor. At the least, the change from open loop to closed loop may help pinpoint the problem since it seems to stop after the engine reaches a certain temperature, ie, switching to closed loop.

The most definitive thing to do is pull those codes!