91 Regal shuddering

Would like some advice. I have a 91 Regal Custom with the 3.8 liter, has about 128,000 miles. About a month ago, it started slightly shuddering, jerking going up hills. I replaced the spark plugs, spark plug wires & fuel filter. It was fine after that however about a week later, it started doing the same thing only worse. It was shuddering, jerking, felt like it was slightly surging sometimes & doing it all the time, on hills & flat areas. Took it to a mechanic who I've dealt with for some time. He rechecked the wires, plugs, cleaned the mass air sensor, checked fuel pressure, ohm'd the injectors, exhaust pressure, oxygen sensor, swapped out ignition coils & ignition modules & replaced a few vacuum hoses. He's positive it's not tranny related. He doesn't know what else it could be but is going to research it more. He thinks it may be a ground that is shorting out but not sure. The motor runs very good (when it's running right) so I would like to fix it if possible. Anybody have any suggestions???? Please help!!

Welcome to AF

does the car only do this at a steady cruising speed above 35-40mph? If so, it's definitely tranny related.

Take the car for a drive, but instead of leaving it in drive or overdrive, start in first gear and shift it manually. If the shuddering stops, it's probably a bad vacuum modulator in the trans. It controls part throttle shift points, as well as downshifting via engine vacuum. If you know what a trans modulator is, you can pull the vacuum line off and inspect it for the presence of trans fluid as this is a sure sign of a bad modulator.

I'll try this when I get home from work. Where is the trans modulator located? Thanks

I checked & it still feels like its missing even when in park. I rev the motor & it does the same thing as when driving-engine feels like it's missing. I replaced the plug wires for the second time & no change. I also replaced the PCV & no changes either. Does this have an EGR valve & if so, where would it be located & how much do they cost? Any of you think it could be electrical? Any help appreciated. If not, I'll probably sell it but hate to cuz the motor was running good. Thanks

You may have cracked a plug insulator during installation. (Easy to do on the back side!) Pull the plugs one at a time & visually inspect how they have been firing.

Ok, thanks for the suggestion. I'll probably end up changing all of them since they will be off anyway.

These cars were known for having harmonic balancer issues. The rubber that connects the center section that's attached to the crank to the outer ring (which the CKPS reluctors and serp belt pulley are attached to) separates and throws the timing off. Grab the serpentine belt on each side of the balancer and see if it will move back and forth. If it does, you need a new harmonic balancer.

Ok, I'll try this as well. Thanks for the info.

i whont to do a buick regal engine swap to a s10 how hard would it be

Checked the harmonic balancer & everything seemed ok. Changed out the spark plugs & it is now running fine. One of the plugs looked like it had oil on it, so we'll see how long it takes before it starts running like crap again. Thanks for all the advice, very much appreciated.

Bartma, Welcome to AF.

You may want to start your own new thread in the S-10 section of AF, since your question has nothing to do with the subject matter of this thread nor is it specific enough for anyone to be able to help you. Trying to put a FWD multi-port Regal engine in an S-10 would be way more work than swapping an engine out of an 80's vintage rear wheel drive carbureted Regal.