[Gmecm] 1990 Cavalier 3.1l running very rich, ECM?

[Cowen]

Hello all, I'm having trouble diagnosing a student's car.? He's resurrecting a car that came in not running and, after doing a bunch of top end work, it's running now, but very, very rich.? It will barely idle, and the SnapOn MODIS reportsO2 voltages around 950mv most of the time, especially at lower RPMs, and the integer is usually around 120 or lower.? It is entering closed loop.? So, the ECM "knows" it's running rich, and it's pulling fuel from the mix, but not enough.? You can smell the fuel when it's running, the cat can't keep up!

We've pulled the fuel rail, and nothing is leaking when the rail's pressurised.? All the injectors show a very similar resistance, and we've been unable to find any vacuum leaks.? Strangely, it doesn't run much different when the 02 is disconnected.? I haven't checked for codes (don't know if the student has), but it's not showing a MIL.? We sprayed tons and tons of water all over the top end but couldn't get any definitive change in the way it runs, so I really doubt there's a vacuum leak.? Not that that would necessarily cause such a huge over fueling, but I thought I'd pass along what we'd done.

Here's the thing, it's got the "wrong" ECM.? It's a 1990 3.1l 5spd Cavalier, and it has a 1989 2.8l 5spd Cavalier ECM.? Could this be the problem?? Shouldn't it run pretty well, maybe a bit lean at WOT (which we haven't even attempted)?? I have had a lot of success running my 3.4l MGA conversion with several different stock 3.1l ECMs, including passing emissions with flying colours.? I expected this car to run pretty well.? Would it likely run better with a 3.1l auto ECM?? 

This is not actually my student, but I am supposed to know a bit more about these GM systems (ha ha!), so I've agreed to help this kid -he's a go getter, and that's rare amongst the teens we see in shop today...

I admit we don't know if it ran before being donated to the shop with stripped spark plug holes and sand in the intake and heads (!), but it runs well now, except for the fueling issues.? So, any suggestions? Thanks a bunch for any ideas!!


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