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94 Buick regal: drive train..2k..Engine was machine shop rebuilt..iac

Resolved Question:

I have a '94 Buick regal 3.8 drive train. All was well untill recently, when it began to high idle(as much as 2k) after an hour or so of driving. Engine was machine shop rebuilt about 20k , computer was replaced at that time and IAC was just replaced recently. It will reset for a while if engine is restarted , but soon begins to increase idle again.IAC seems to step up but not down. I tried a second one to make sure it wasn't a defective part. This last trip I "cured" the problem by unplugging the in place IAC after an idle of about 650 was reached, and plugged the loose IAC into the harness to keep the PCM "happy" No DTC's are thrown. The IAC counts up and down show normal in real time. I'm chasing my tail. Any ideas?
Jon
Submitted: 5 years ago.
Category: GM
Expert:  John replied 5 years ago.

HelloCustomer

There is always that your ECM could have a faulty quad driver commanding the IAC to retract for a fast idle.The 94 had no on board diagnostics .First thing to do is reconnect everything,cycle the ignition a couple of times for a reset & look at the IAC counts on a scan tool.The # XXXXX go over 100 (high counts represent a fast desired fast idle command) & usually sits around 30-55 in drive,according to load.Look at all the data parameters TPS should be around .36mv at closed throttle,MAF around 4-7 gps at idle,fuel trim around 128 (give or take 9 counts).The ECM will raise the idle speed if it has an input from a sensor that the engine is under load.Check the wiring harness at the engine TPS,IAC,MAF for a broken wire.if necessary disconnect the harness at engine & ECM use an OHM meter ,all readings should be .002 or less (move the harness around while checking.If all look good I would suspect ECM fault.

Customer: replied 5 years ago.
What sensors indicate to the ECM that the engine is under load? For the first hour of driving just now, the TPS,MAF &trim were right where you suggested they should be. After engine is thoroughly heated up, the 3 numbers incrementally climb each time the shift is put from N to D. They stop climbing when the idle is at about 2500. Loading and unloading the engine with the A/C still makes the idle rise and fall, but at the higher numbers. As soon as the engine is stopped and restarted, everything drops to normal.
Expert:  John replied 5 years ago.

That sounds like a bad PCM where the internal logic is correct if you can cycle the ignition & all is ok.

Input sensors are MAF,MAP,IAT,ECT,TPS,O/2,FUEL TRIM,KNOCK SENSOR,CRANKSHAFT SENSOR,CAMSHAFT SENSOR,GR POSITION SENSOR,VSS,P/N SWITCH,BRAKE SWITCH,AC PRESSURE SENSOR

John, GMC Technician
Category: GM
Satisfied Customers: 9176
Experience: ASE MASTER TECHNICIAN/40 years experience
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