Code 51 ECM PROM

[Guest]

I've been reviewing posts all day, trying to piece this one together.  I haven't been able to, so here's my sad story:

('90 3.1L V6 Auto, Stock)

I suddenly began having an unusual idle situation: sometimes it will idle at 2000 RPM, and sometimes stall with an idle in the low 100's.  If you keep the throttle open it doesn't stall.

I recently replaced my cracked EGR with a junkyard unit, replaced my coils, wires and plugs, MAP sensor, CPS, and some questionable vacuum lines and, finally, hollowed out the cat converter.  It sounds like a can of angry bees, but after all of the new stuff I get amazing fuel economy and more power.

My speedometer has gotten progressively worse over the past few months, reading higher than it should.  When the idle problems began, the speedo needle layed down at zero and plays dead when it isn't lying to me about my speed.

My Low Coolant light comes on once in a while as well, but temperature stays okay, and there is coolant in the reservoir.

Finally, I couldn't get a trouble code when I scanned for it.  In 'field service mode' I didn't even get the blinking 12 of the SES light.  Once I'd removed the chip (The PROM, right?) and plugged it back in I got the reassuring 12 of the SES, followed by code 51 ECM PROM.  A fancy Snap-On scanner verified this diagnosis, but couldn't stay connected to the computer for long.  Sometimes I can get codes, sometimes I can't.

So:  Idle is either too high or too low, speedo got progressively worse and now goes out intermittently, Low Coolant comes on once in a while, and when I can get a code, it is 51.


I am going to double check for broken vacuum lines, start locating and cleaning grounds, and remove the instrument cluster to inspect for loose/dirty connections.  Waste of time? Is there a way to test the computer?  Where is it grounded?  Do they go bad?  Which part do I replace if it is bad?  Is the speedo a coincidence?

Any advice would be greatly appreciated.

[1988GTU]

You replaced everything but the TPS.  Maybe you could try replacing that if you have the cash to do so.

[Mr Goodwrench]

Thinking abought the old obd1 computers.  sounds like it gowing bad.  causing things to read incorectly an run irraticaly.  try a junk yard or see if someone in your area has one u could try..  
i think their were some kind of tests you can run on it.
ill check it out an luk.

Mr Goodwrench

[Guest]

Took me a while, but I solved the problem.  Found a junkyard computer from the same year, and it runs good now.  Idling properly, speedometer is back on line as well.  I just wonder if the settings are different because it doesn't have the same punch that it used to.  It'll get me by until I can get a performance chip...

First I checked as many grounds as I could find, then I pulled the guage pod to see if there was anything loose.  I didn't find anything amiss, but since the trip meter wasn't working I swapped it for a junk yard pod.  While I was at it I replaced the broken off power lock button.

Any one have personal experience with upgrading the PROM for performance?  Are the 60degreeV6 chips still considered the best choice?  Any one have experience with 'soft' programmable computers?

Thanks every one, once again.
Bill