95 Bonneville Running Bad

Car was running great(Getting close to 25MPG) and suddenly started to miss and backfire. The exhaust now hums and every 10 seconds or so you can hear backfires in the exhaust and they seem to come in pairs. There is so much backpressure some of the engine gaskets are starting to seep oil(They leaked slightly before but it's increased). I replaced ECM as I was not getting codes. So far with new ECM, Check Engine light hasn't come on. I did some intial testing on sensors and they all seem to check out ok.

I am leaning towards an ignition problem, clogged catty or stuck values. I'm going to start with checking the coils for wear and damage. If they check out I will change plugs and wires. If that fails I'll replace Ignition Module. If that fails I'll have the catty checked. Is there anything else I could be overlooking? Like I said the car ran great and suddenly went bad.

It's a 95 Bonnville SE 193,000 miles on it. It has a rebuilt engine in it with about 60-70K on it.

P.S.

No exhaust color but I can smell it's running rich.

If you've got gaskets blowing out, you've got backpressure,(as you stated). Disconnect the connection at the exhaust pipe at the firewall to bypass the catalytic converter and wire up the pipe to keep it from dragging, and see how it runs around the neighborhood. Sounds like its cogged.

Thanks for the response.

Sensors, ignition module and coils all checked out ok. I still feared it was the cat but I didn't think it was that as the car starts up real fast.

Today I changed the wires and plugs. I find that the plugs were not platinum but the standard copper. Three spark plugs were ok and 3 had issues. One had white ash and a "nick" in the electrode and two were fuel fouled. The two fuel fouled were in the front by the radiator and I suspect the wires were bad. The bonny now runs great! Now I have to change the oil (diluted with fuel from not running right), change a few radiator hoses, flush the radiator and I'm(actually my daughter) is good for a while.:grinyes:

P.S.

I also changed the PCV value. The old one rattled but the new one rattles a lot more!

Make sure the o-ring is installed with the PCV,(on the base and the cap too). It will make a huge racket at operating temp, at low speeds and idle. Sounds like the lower end of the engine is coming apart. Its a loud vacuum leak squeal making the noises.

Make sure the o-ring is installed with the PCV,(on the base and the cap too). It will make a huge racket at operating temp, at low speeds and idle. Sounds like the lower end of the engine is coming apart. Its a loud vacuum leak squeal making the noises.


Thanks for the heads up. When I mentioned the rattle it was when I held it in my hands. Both gaskets are in place. Thanks again!