Hi all, I am hoping for some help. I am at my wits end here. I bought my son his first car. A 92 firebird, v6 automatic. Ive been fixing it up and am stuck. In od, as soon as it shifts to 4th or overdrive, it shakes and shudders like crazy. Acts like a manual transmission lugging down at low speeds. If I hit my brakes it quits. Now before anyone says "its your torque converter" let me explain all ive done. I have had the tc professionally rebuilt, it was spotless, the clutch disc inside it was still well within factory specs. The guy told me it looked like it just came off the showroom floor. So, ive had that done. I replaced the tcc solenoid with a new unit. I have replaced the filter. When I had the pan off, I used a screwdriver and pushed up on the od shaft in the pump, and the springs pushed it right back down. So its not sticking. Now, for the engine.... it has a new air filter. Replaced the distributor (a used but should be good one) replaced the fuel injectors, all the vacuum lines, fuel filter and fuel pump. It runs at 42 psi at an idle and raises some when you hit the throttle. Replaced the tps. Also the coolant temp sensor, and thermostat. It runs great. Sounds great. But bucks in 4th gear. Now, when I replaced the tcc, the fluid was red but crystal clear. Very clean. No friction material dust, no metal in pan. the car has 142,000 miles on it. Can someone please give me something else to check? The ecm has been replaced previous to me buying the car. Im at a loss as to what is causing my problems. Any help is greatly appreciated. Thank you.

 

Im sorry, I forgot to mention, I also replaced the spark plugs, plug wires, cap and rotor. there is no cel. So there isnt much that isnt new lol. After replacing all that I have, and its still shuddering, im sure you can understand why im at a loss.

 

Update...... found a dead cylinder. #6 has 0 compression, all the others are running 110. Strange, it sounds great and runs great. So, nevermind all. Thanks for reading.