For people knowledgeable in TPI, please help me out. This is a long post, but I've included as much info as possible.


I'm working on an '88 IROC with 5.7L TPI/700R4. Stock engine with 113K on it. It was wrecked sometime around 2015 and has mostly been sitting since. My goal is to get the engine running well and then pull it to use for another project.


The car starts up and runs pretty well (you can see and feel a very slight engine shake) at 1000 rpm for about 2 minutes. While running well, the engine pulls a steady 20 inches of vacuum. Once it hits closed loop and tries to idle down, the idle speed becomes very erratic, floating between 800 and 1200 rpm, and it develops an obvious misfire. The engine shakes pretty bad and it gets worse if you rev the engine up. As the car gets warmer, the problem gets worse.


In an effort to fix the problem:
I have drained the tank of the old fuel and replaced it with fresh 91 octane. The pentle was frozen on the old IAC, so replaced it with a "standard ignition" part. The ICM tested bad at NAPA so I replaced it with the OEM AC Delco part. The ignition coil and pickup coil both check out per the factory service manual test procedures. I installed new AC Delco cap, rotor, plugs, and wires. I set the TPS to .54 volts at idle. I replaced the stock rochester fuel injectors with Bosch D3 injectors from South Bay fuel injectors. I have 43 psi of pressure at the fuel rail with key on and engine off and 36-38 psi at the rail with the engine running. I also tried another OEM MAF from an '86 camaro with no change in the problem. Initial timing it set at 6 degrees BTDC with the ESC wire disconnected.



I have a Moates adapter to read live data off the ALDL plug. I can see that the O2 sensor fluctuates well between 100 mV and 800 mV while in closed loop. On a whim today, I disconnected the O2 sensor to keep the engine in open loop to see if it would maintain a smooth idle. The engine ran smoothly at 1000 rpm without throwing a code. When I plugged the O2 sensor back in , the car went into closed loop and the erratic idle and misfire returned.



At this point, I'm suspicious of the ECM since it seems like the O2 sensor is sending the data it should be, and I didn't trigger an engine code by running the engine with the O2 sensor disconnected. I don't have another ECM lying around to test this so I'm kind of stuck and very frustrated.



Does anyone have ideas of what to try next? Thanks for reading.

 

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I would suggest first that you buy a shop manual for the car. Read up on what is happening during open loop and closed loop. I don't remember all the details, but in open loop, fueling is determined by the ecm reading the MAF, and coolant sensors. In closed loop, fueling is determined mainly by reading the O2 sensor. It is weird that the ecm did not set a code when you disconnected the O2. The GM shop manual is a wealth of information as well as the members of this forum.

I suggest: 2) Clean the IAC passages. The passages may be clogged, 3) Replace the ECM, 4) Do a pin-out, make sure the ecm wired connections are sat. A car that has been sitting since 2015 could have allot of electrical issues. When a car has been sitting for that long without being started and run, the electrical connections tend to get oxidized.It would be a good idea to disconnect all the connectors and spray an electrical contact cleaner in the connector (male and female).

Good Luck