1989 Pontiac Firebird 305 TBI suspected cylinder 5 misfire 174,000+ miles automatic
When my bird is idling when stopped on the brake she vibrates and pulses foward when put into drive, same with all gears. I initially thought it was my torque converter as it is high mileage, but I took it to a shop and one of the technicians thought it was a dead misfire. I knew it would be cylinder 5 as that was the only plug I couldn't get to when I tuned it up. I replaced it today and she ran smoother than ever but she was cold, once I drove around a little bit the issue came back and I think its missing again. Im not too sure what is going on and wanted some feedback on what might be the cause of this.
When dealing with a nearly 35-year-old car everything is suspect. Unlike a modern car there is no comprehensive data available, though you could try pulling OBD1 blink codes to see if anything comes up. Other than that, old-school diagnostic methods are needed.
https://mechanicbase.com/trouble-code/obd1-codes/
Do you have a shop manual? That could be helpful. Some libraries have service information for older cars in their reference section. Shop manuals are also available online from sources like rockauto.com.
Given the car's age and that the problem appears to be with one cylinder I would do a compression test. If compression is no good nothing else is going to help until that is addressed.
Vacuum leaks due to deteriorated hoses and gaskets would be a prime suspect on an old car.
When it was tuned up were the distributor cap, rotor, and ignition wires replaced?
How do the spark plugs look, particularly in that #5 cylinder? Carbon fouled? Oil fouled? White (overly lean)?
Have you checked fuel pressure? Checked fuel injectors for clogging or leaking?
