Hey everyone, I'm Dave and I recently picked up a '95 Bonneville SE with a nasty problem. I've been tinkering and fussing with it for a couple weeks now and I'm at a loss as to what could be wrong. To explain the problem is a task because it comes and goes (seems to be with the weather). When I got the car it would hardly run at all. Backfiring, smoking like holy hell, rough idle...had to keep your foot on the throttle just to keep it running. Changing coils didn't help (I have a whole parts car as well) put in a new idle speed motor, didn't help, voltage regulator in the alt. (fuel pump changes tone and I get a drop in voltage on the gauge), ignition module, cleaned the throttle body....none of that seems to help. The only thing that got it to run right was two bottles of fuel injector cleaner. Drove the car down the highway and about as soon as I got off it just shut off while moving. Couple weeks later the car comes back into my possession, but now I'm holding the title to it as well, my new daily driver if i could get it to run right. So I tag the car and dump Iso-Heet in like it'* going out of style. This seems to help the car but it still won't run right all the time. Doesn't matter if I'm running the **** out of it or at a dead stop, if it gets to fussing you may as well pack it up. The revs will drop to as low as 300 and it may pick right back up, might just shut off. When the engine is bucking back on me, I can floor it and all it will do is backfire very violently, but then 30 seconds later it might smooth right back out and run great for 3 more days. It'* the craziest problem I've ever seen and the only things I haven't touched are the fuel pump, knock sensors and the crank sensor. It'* got good fuel pressure on the rail, even when it'* having a spell, knock sensors aren't supposed to register at an idle and I REALLY don't want to mess with the crank pulley. Anyone ever had a situation like this?

Are you throwing any Service Engine Soon (SES) lights?
Does the little engine icon light up on the dash?

Couple of things to try......

Try a Seafoam Treatment https://www.gmforum.com/t284616/ and then change the oil and filter after a couple of WOT.

Next take a can of carb cleaner and while the engine is running, spray around vacuum lines. If the engine RPMS increase where you are spraying, you have vacuum lines that need to be repaired or replaced.

After that and the problem still exists, come back in here and we will move on to other repair options.

Have you changed the fuel filter? Test the fuel pressure as well. Does the pressure drop when it starts to run bad? If so, the problem is fuel. If the pressure stays up, then start looking over the ignition, pull the plugs, how do they look?

Vacuum lines have been replaced, fuel filter has been changed in the last year or so, had the date written on it...

Next step is to test the fuel pressure and see what happens.

Fuel pressure is good, injectors open and close properly, holds good pressure after shut off, had a new crank sensor put in, ignition module good, coils are good. Oddly enough, last week the car was running flawlessly, then it developed a misfire. It would only miss accelerating from a dead stop, maybe once or twice, then normal running it was just fine. I had a known working ignition module off to the side, swapped it out and the car went right back to where it was, sputtering, won't hold a steady idle, shutting off.. Put the original back on and it was STILL back to its' old problem. Ran fine for three days and bam, right back to where we started...and essentially nothing had been changed other than me swapping then re-swapping ignition modules!

I think it may be time to invest in a new computer...

Any input is greatly appreciated