Reseting DME makes it run better?

[Beej '86 325es]

I've got an odd problem where my engine starts running worse and worse as time goes on. After about an hour of driving, it starts running slightly leaner. I'll drive it a couple of days later and it's running leaner, and leaner, and leaner across the board as time goes on - at cold idle and at warm WOT.

But then I unplug the DME for 30 seconds, plug it back in, and it's back to normal. It's not throwing any codes or anything.

I tried swapping DMEs, but what are the odds that both of them are bad?

Gremlins I tell you...

[ScHpAnKy]

sounds more like a sensor to me than a DME

[LJ851]

There is a reason the Spec E30 guys reset their ecus every time they shut the car off.

[hwy84]

Leaky injector?

Adaptation might be working against you.

If you have a leaky injector or two, your 02 sensor will see a rich mixture, and adapt (shift the stoich map) which will result in smaller duty cycles for ALL the injectors. Since the other injectors were fine to start with, then will actually lean out the mixture for those cylinders.

If the leaky injector is really bad - this will continue until you adapt all the way to the lean end of the map.

Resetting the ecu will get you back to a non-adapted map, and the cycle starts over.

Just one possible cause...

[Beej '86 325es]

I actually just had the injectors cleaned and flow checked a month ago, so I think I can rule that out. It sounds like maybe I'm not giving it enough time to adapt. Either that or it's fried...it does get awful hot after driving the car.

Why do the Spec E30 guys reset their DMEs?

[nando]

Spec E30 guys do it because Motronic 1.3 will lean the motor out over time - it's basically a trick to make more power. that really has nothing to do with a 24v and Motronic 3.x...

[Beej '86 325es]

I drove the car in to work today and I was thinking...I can understand the DME adapting for WOT air-fuel ratios, but why would cold idle get leaner and leaner and leaner each time I start it?

[nando]

same thing - fuel trims.

OBD1 right? it just has 1 O2 sensor reading 6 cylinders. if one cylinder is running abnormally rich, it could cause the other 5 to run lean.

or, the O2 sensor could be reading off due to a ground offset or something.