Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
OK, maybe I am missing something simple but here goes.
1990 Chevy truck 5.7 TBI auto with a stalling problem.
New parts installed: O2 sensor, thermostat
Known good parts tried: IAC, TPS, ECT, ICM
Other parts tried: Used PCM
Throttle body and IAC passage cleaned
All vacuum hoses, PCV inspected
Throttle body gasket not leaking
All measurements taken with a scope.
All 5V reference circuits OK (scoped with key ON sensors plugged in)
MAP key off 30 inhg, idling 8 inhg (live data on scanner)
TPS a little high for that engine and year at .69 but not adjustable, sweep test good (live data and scope)
ECT reads exact temp measured at the thermostat housing with temp gun, scoped signal as well
No vacuum leaks detected
Fuel pressure good
Timing at factory spec 0* with EST disconnected
P/N switch operating properly (live data and scoped @ PCM)
Enters closed loop (live data)
Minimum idle set properly
EGR sealed
No knock counts (live data)
Tune-up questionable but the truck runs awesome when not stalling, did not scope the ignition.
KAM cleared many times after each test session or change, then road tested to relearn
Block learn 128
Integrator swings from 124 to 135
Good cross counts, scoped O2 and reacts well to lean/rich tests
Here is the part that has me wondering. Cold or hot with the O2 sensor disconnected I only get 2.25V bias voltage from the PCM. Checked right at the PCM as well with the circuit isolated (cut wire). All PCM powers and grounds scoped while running and test good. A used computer was tried with the same result. Either I was lucky enough (sarcasm) to find a used PCM with exactly the same problem or I am missing something.
I had it running great at one point and cannot pinpoint anything I did right prior to that. When it was working, move the shifter just enough for the P/N to open and the idle would bump slightly anticipating going into gear. Now it idles in Park with IAC @39, but when the P/N switch is opened, the IAC slowly closes to 0.
I believe the low bias voltage may be the key here, as it is the only anomaly, but is it bad luck to get the same problem on another ECM or is there something else that can cause it?
"Knowledge is a weapon. Arm yourself, well, before going to do battle."
"Understanding a question is half an answer." I have learned more by being wrong, than I have by being right.
Topic Author
