Hello, I recently acquired a 1994 Bonneville--base model--with about 200k miles on it. It is in great condition for it's age and miles.

Issue that I've continue to have is related to a hiccup while running followed by the Service Engine Soon light popping on.

Here's what happens: Car starts fine, runs fine, then randomly--sometimes withing 2 minutes, sometimes 20 minutes, and sometimes after several days---I'll be idle or driving down the road and the engine will sputter (sometimes shut off in the least opportune times, like an intersection), rpms will fluctuate for about 30 seconds then go calm and the Service Engine light comes on and everything is smooth again. It's like the computer decides to do something, tries really hard, gives up, throws on the light and lets everything go on as normal. I turn the car off and restart and the light is off again, but then shortly does its thing again, leaves the light on, and then runs fine.

After trying to unsuccessfully get the error codes from a couple diagnostic scanners, I went to the GM dealer and they pulled the following codes:

P0123- Throttle Position Sensor Circuit (Signal Voltage High)
P0341- Cam/Crank Error
P0705- TRans Range Switch Circuit
P1350/P1361- Ignition Control Circuit
P1558- Cruise System Problem (SPS Indicated Low) - I use cruise control a lot and it works intermittently.

Where should I start? What can I do to narrow it down and knock out the issues one at a time?

I'm not too mechanically inclined, but I'm willing to give it a shot. I recently replaced the thermostat--while simple to do, a major accomplishment for a computer geek like me.

Thanks in advance!

Jeff

My 94 se was doing similar things when it was given to me. A good used ECM fixed mine. Be sure to address any neglected tune-up items as well, like fuel filter, spark plugs, plug wires, etc.
Oh, and since you replaced the thermostat, be sure you don't have any air in the cooling system. There is a writeup here somewhere on properly bleeding the cooling system. Just do a search.

Thanks for the reply!

Where did you get the good used ECM/PCM? Are they hard to replace yourself? What is a rough price range for a used ECM?

Is there anyway to tell it is the ECM or sure? Which so many issues triggering, it makes sense it could be the ECM causing it. Is there a way to check the other issues pulled off the codes to rule them out before replacing the ECM?

Thanks, Jeff

Got mine off eBay for around $40.00 IIRC. You will need numbers off of your original to match the replacement. Not hard to replace. Up under the dash on the passenger front. With so many codes, I decided not to pinpoint diagnose, and went for the shotgun method of the ECM.

Great! I'll check ebay.

I called a local pick and pull and they have a few I can pull for $40 as well.

Thanks!

-Jeff