Intermittent MAP stuck at 2.9 inHG ( P0107 ) solved, finally
Bought 96 Impala for a good price because it had all the usual issues. You all educated me and got me through it for cheap, so while I hope this doesn't happen to you, I'd like to add this fiasco to the database.
Relatively unmolested 96 DCM, 168K, P0107 showing. I used to make my rent money with electrical diagnostics, especially emissions stuff, and this is one of the strangest one I've ever seen - which makes sense because it's my car.
Summary: Bad MAP connector, possible intermittent bad ground from ECM, MAP sensor intermittently bad. Story is as follows.
Wiring connector to MAP was crunchy, back-probing 5v ECU reference wire found no 5v, $9 to Rock Auto for pigtail. 5v now seen at connector.
MAP input (inHG) on scan tool , readings bouncing around as usual, good 5v reference and ground:
Engine off: 29.4
Scratching my head, I took it on a 2 hr drive to see family.
During drive: Engine tone changes, scan tool shows MAP signal stuck at 2.9 inHG.
Pull over, good 5v power, good ground, sending MAP signal.
Blip throttle from engine compartment with DVOM attached, signal wire fluctuating in voltage, approx 1.7v.
Scan tool PID still shows it stuck at 2.9 inHG.
Motor idling a little rough but not bad, exhaust smelled a little rich. Got back on road, 20 mins later MAP starts switching normally again, I felt it when it came back online. Later on the drive it got stuck for rest of the drive.
Over the next few days, it would do it and I'd pull over and check everything: Identical symptoms. However, one time it started working again the second I touched the voltmeter to the ground wire from ECU to MAP. Aha...
Checked and cleaned all grounds. Stripped the factory wires, the very thin ground wire had a hint of greenish corrosion on the bare copper wire. Kinda like green acne. I ground MAP to solid battery ground, leaving ground wire from computer disconnected.
P0107 returns with this exact same, randomly stuck at 2.9 inHG foolishness. Exact same behavior (will go days without getting stuck, then get stuck for random length of time), except now I only see millivolts on the signal wire. I have a good ground, good 5v, but only millivolts on the signal wire. When it is switching normally, I see normal voltage on signal wire.
So, only true failures exhibited was no 5v before pigtail replacement, now no activity on signal wire. I have never seen a bad GM MAP sensor before, but it would be exactly my luck.
$20 to Amazon for the bottom-shelf Standard ( AS5T ) vs the $46 ACDelco because I am that skeptical it's the MAP sensor.
Code has been gone for three weeks and 300 miles, haven't caught it getting stuck. Gas mileage slightly better.
I think it was the pigtail and the sensor. I am still on the fence about the role that the sensor ground from PCM to MAP played. Intermittent ground can do weird stuff.
Hope this helps someone navigate this horseshyte.