I recently bought a 1997 buick lesabre custom with the 3.8 L v6 series II. this is my first GM vehicle, i used to always own old honda’s. Anyways it was 400$ with 97,000 miles. I’m gonna give back story on the car, it’s kinda unique and not sure if it has something to do with my current problem.

Car has a full car fax. regularly serviced it’s whole life (conventional, though). Whoever owned it either knew their way at the dealer or had issues with the car.

ECM replaced, Alternator and calipers replaced at 40k.

Transmission replaced at 60k. Early 2000s

Immediately after purchase, I replaced the serpentine belt, Oil change (All oil changes for this vehicle i used Valvoline full synthetic high milage 10W-30 with a purolator boss filter). I also replaced spark plugs (All replacements were AC DELCO). Also dropped the pan on the trans, replaced the fluid and filter. Inspecting the vehicle i noticed there was an issue immediately after start up where the engine seemed to be “chugging” faster and fast for about 3 seconds, then the engine would reach regular idle. this was in around 50 degree weather. did it both in warmer and cooler conditions.

Around 101k It was running really rough after about 100miles on the road, so i parked it and it wouldn’t start after. towed it home. pulled the plugs and the engine was hydrolocked, 4/6 had coolant come out of the plugs. After confirming it was the upper intake gasket, i replaced both it, the upper intake manifold, the EGR valve (the smaller diameter one they include in the kit), PCV and pressure sensor that plugs into it (I broke it), as well as thermostat and gasket, spark plugs again, oil change again, and a coolant drain and refill. Cleaned the throttle body (very dirty), MAF, and everything around the upper intake manifold since i had it off. replaced fuel injector O rings, cleaned area where they went in, etc. put it back together, drove it 500 miles, no longer was eating coolant. Then did another oil change and replaced spark plugs again, and confirmed that there was no coolant in the oil/visa versa. Was throwing a code running lean, figured i cracked a vacuum line, and didn’t mess with it since it was running fine and wasn’t doing anything new. eventually got around to it just a small leak and fixed it.

Just to note the repair took me all day to do (I took my time) and had my grandfather with me who i would consider a master in engine repair, but he doesn’t work on cars so he doesn’t do diag.

Fast forward to now, 105,500 After driving about 400 miles over the span of 8 hours with a few breaks, i notice my issue now. I can feel it in my feet when it does it, it’s like the engine is stalling/just running rough or possibly the transmission? it mostly does it at the bottom of 4th, and sometimes i think it will do it but it doesn’t, and visa versa. has done it once or twice at lower speeds but i’ve noticed it almost only occurs on a hill. replaced the TPS after reading online it was common and my symptoms matched it, as well as all the coil packs. After this, The idling issue went away, so i’m thinking it was unrelated. “Hiccup” in trans/engine got i think maybe better but i also let the car sit overnight. It’s been about a week now since i replaced the TPS and the issue has gotten worse. it’s mostly when going uphill, however is seems like overall when i give it too much gas the whole car starts to shudder and shake and i have a large loss of power. have to let off the gas and reaaply so that it goes down a gear. Also to note there is a slight sound coming from one of my pulleys, not sure which one but wasn’t sure if that could be an issue. transmission has proper level of fluid and is very clean. I’ve smelled and tasted it, no issues. engine oil is good too. not eating coolant, or leaking it. engine seeps a little oil out but it doesn’t leak or eat oil either. Oil pan is dented which happened around 103k but it was because someone lifted the car partially on it, but also lifted off the frame at the same time. not a big dent, i don’t even think enough for it to affect the dipstick, doesn’t leak either. transmission has always felt normal for a 23 year old car. Other things to note, my speedometer and gas gauge don’t work. When i put my turn signal on, most of the time the speedometer “jumps” like it gets a surge of power. all other electronics are good, car is in good shape inside and out good brakes good suspension not a lot of rust. I read online that the ECMs can be problematic and it makes sense since it was replaced before, but i’m not sure. Or maybe something electrical? I replaced the radio and have 2 10” subs running at 200W. i’ve checked the battery and alternator and they read good levels, don’t rememebr exactly what they were it was when i first got the car but I wanted to make sure that it wouldn’t be an issue to run them. Any help is appreciated, thanks!