First off, I will conceed that this might be potentially off topic for this forum. But I'm posting here because the solution for my problem is something that might be of value to ECM tuning in general, and frankly I'm out of ideas...

The patient: my daily driver, 1991 Sunbird, 3.1L v6, $A1 mask, 3spd auto, stock motor, stock cal.

The symptom: BLM's go to 100-105 at all load ranges. Idle speed sometimes oscillates when warm (in and out of gear). Cold start up will sometimes go to ~1600 and keep climbing >2500 (but will decay back down to normal idle speed once car is in gear and the ECM sees mph >0).

What I've done to try and fix it: checked plugs, replaced all 6 injectors, replaced pressure regulator, new O2, new wires, new plugs, air filter

Notes: The original motor had eaten coolant (intake gasket) when I bought it and I replaced the motor with a junk yard donor. The donor has run okay for ~6 months, but never really idled smooth. Passed emissions within 1 week after installing the donor. The oscillating/racing idle is something that developed over time. All the sensor readings on ALDL look normal, it doesn't look like the ECM is being lied to. Since I started watching the BLM to try and debug this, I've never seen anything higher then 105 (after it has stabilized from being rest to 128). Noticable idle quality improvement when I installed new wires and plugs, but now I get O2 rich code...

I'm actually starting to wonder if the motor is a 2.8L badged as 3.1L...

Any ideas or suggestions to debug this further would be greatly appreciated.

 

Check if the CCP can is full of fuel. Could also be a bad ECM.

RBob.

 

RBob, I've tried your two suggestions (unplugging and capping the CCP line, and swapping ECMs). No change in behaviour observed.

I unplugged the O2 and reset the BLMs to 128, it runs MUCH better overall in open loop. In open loop the cruising MAP is ~2.1V, with the BLM pegged at 100 it is around 2.4-2.5V. I think this is a good indication that the ECM is being lied to somehow.

I guess another logical thing to do is a compression check. The first 1-2 minutes of running with a load after a cold start is always met with a clacking noise coming from what seems like the valve train, but it's always been there and I've just not worried about it because it always goes away once it has warmed up a bit (i.e. gone after I've driven down the road about a 1/4 mile).