This isn't a thirdgen, but it is gm 2.8 in a 87 fwd cultlass supreme. The car has been sitting for a while and has a constant backfire through the intake. It is more pronouced under load at lower rpms. There was bad valve tap as well, and after some investigation a worn lifter was found. So all the lifters were replaced. After reassembly the backfire was still there. The engine only has 25,000 miles on it, but it was sitting for a year. The ignition is distributer-less so I don't think it is a timing issue. Any thoughts?

 

i'm having the same problem, let me know if you find a way to fix it

 

I'm going to run a compression test. That should confirm if all the valves are functioning ok. I suspect that the cam might be bad since it wore down a lifter.

 

You've got valves not sealing, either from sticking or burning.

 

What causes a valve to stick? Is it the valve getting stuck on the valve guides? Or is it the lifter getting stuck in the lifter bore.

 

The problem sounds like the valves...Has the exhaust system been checked?...I had a friend that had a 2.8 that backfired and it was because the Cat was clogged. Just an idea of another thing to check. If it is backfiring at lower RPM's, it could definately be the cat.

~Sean

 

i don't know if it could be my cat, valves maybe. The cat on my car was changed about 11k miles ago.

 

odds are, you are looking into having to do some motor work. Good luck to you

 

I ran a compression test this weekend. 180 psi in all the cylinders. But I could here a poping in the intake when I was turning it over. The spark plug from the cylinder where the lifter was replaced was very black and carbon covered. I suspect that the cam lobe for the exhaust on that cylinder is shot. I don't think the exhaust valve is opening on that cylinder. My next step is to remove the upper intake and see if I can verify which cylinder is popping.

 

Well, my brother and I pulled the upper intake off yesterday. Then we cranked the motor and we could see that the #6 cylinder was popping. So we suspected that the exhaust valve wasn't opening. We pulled off the rest of the intake and found that the exhause lobe on the cam had completely worn down. So the exhaust lifter wasn't moving at all. So it looks like a cam change is in order. Since its fwd I'm going to have to pull the motor. My only other concern is that the motor was tapping before. Hopefully it was the cam and not a rod knock.