I finished the job of replacing the timing chain and gears. I switched out the new CKP sensor just in case it was bad. Checked the harmonic balancer vanes and they were as they should be.

Started the car and it started without much hesitation but was idling rough. Finally after a bit of time the check engine light lit. Checked and I still have the code 18 now joined by code 17.

I have gone through all the tests in the GM shop manual. I really need to figure out this problem.

To recap, the coolant sensor, the cam sensor, the crank sensor, the ignition module, and the MAF sensors have been replaced on this car.

The wiring seems to check out fine in the cam and crank circuits.

you have a wiring diagram and test them all the way back to the icm plug? make sure no corrosion or loose pins.

Yes, did all the checks and everything checked out fine. Guess I will do it again. I even checked the connection between the PCM outside connector and the inside solder joints.

The GM shop manual states that code 17 can be caused by a faulty PCM. I have yet to check the circuits mention for code 17.

I have checked the wiring once again, voltages on the PCM connectors read as they should. Today I checked for the "spark reference" (Code 17). Tested at the back of the green connector for CKT 647. Got a very low reading so bumped the starter as the GM manual said until I got 5 volts as it should be. Manual said to start the engine and let it idle for one minute to stabilise, voltage should read 3 volts. I started the engine and noticed the voltage jumped between 1.X and 3 for a while until it settled down at 3 volts steady.

Rechecked for Code 18 and found no faults, Voltage steady at 5 volts and shaking wires did not do anything.

For Code 17, if voltage at back of connector GD6 is steady at 3 volts it is either a bad connection on GD6 or a faulty PCM.

Maybe I will just go ahead and buy a new PCM, this one is 19 years old. What do you guys think?

I am assuming that since I am getting the 3 volts on CKT 647 at the PCM connector that the ICM is working and that it is getting the pulses from the crank position sensor and sending it out to the PCM. I am also seeing the 5 volts from the Cam sensor so assume that circuit is functioning as it should. I don't see what is causing Code 18 to be set as the wiring seem to be OK and the sensors are working as they should.

Update: Code 17 is now gone, still have a Code 18. Perhaps the Code 17 was just a fluke. Still wondering if I should replace the PCM (also called ECM)? Will continue to check circuits.

I can not find any fault. Checked again today and even tried to tighten up on the connectors but was not sure how or what to use. Reattached the battery cable and started the engine and it idle rough, surged a few times and then the check engine light came on, Code 18 again. It did not even idle for a minute before the check engine light came on. I am lost as to how to fix this.

I posted at another site that claimed to have GM techs. His final answer to me was:

"Your first mistake is using aftermarket parts on the car. There has to be a GM dealer somewhere in your area. If that is not an option, call a GM dealer and get the part numbers and go to a factory authorized AC Delco parts center and get the factory stuff. All your tests seem ok, but something is glitching out. Unless you have known good GM parts on the car, you're at a standstill."

I certainly can not afford to pull all the sensors off and the ICM and replace them with ACDelco (or whatever the OEM was for the parts).

With the Code 17 that appeared to only disappear and the Code 18 that does not seem to have any faults associated with it, could the PCM be bad? It is the original one, and is 19 years old.

i wouldnt give the gm "parts replacers" opinion too much weight. i would guess your on the right track with the pcm being the next step, seems thats the only thing left.

The car ran great, after I finally fixed the stalling and not starting again problem, for several months. Then BAM one morning while warming up I get this Code 18 problem. I feel like doing this to the dang car I am so frustrated. I like my old Buick, she has been a really good car until now. I was planning on giving her a new coat of paint this summer since she looks so worn.

Put a new computer in, same old problem. Can the PROM go bad in these things?