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

[David Allen]

Is this a MAP sensor system?  If so, make sure the MAP sensor hose is not 
compromised in any way!

That will cause a rich condition.

David
  ----- Original Message ----- 
  From: Cowen
  To: gmecm at diy-efi.org
  Sent: Thursday, November 18, 2010 6:14 PM
  Subject: [Gmecm] 1990 Cavalier 3.1l running very rich, ECM?


        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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