How did the plenum gasket get fixed? Just the gasket or did you replace the pan as well?

 

Backfire isn't caused by lack of a cat..... nor is mufflers blowing up. Sounds like maybe a leaky injector, or a valve that isn't sealing as well as it should.

 

back fire is created by lack of o2 sensor when there is no cat by no cat i mean its off at the manifolds so yes no back pressure no o2 sensor on a 99 = back fire its not an 89 which would also have a back fire if reved and the lack of a cat will put excessive heat in the muffler of any kind due to the fact they are not designed to burn of gasses which would happen at that point


and to answer your question about the gasket i replaced the pan also and i rebuilt the tranny then

 

What? No cat would allow more unburned hydrocarbons out the tail pipe, not more heat.

 

My cat is gone also and it dont backfire at all.. You got something else going on..

 

You didn't fool around with the distributor adjustment when you did the plenum did you?

 

no i didnt it ran fine for 8 months

 

Have you tested and verified the fuel pressure is correct also?

 

yea the fuel is fine it was the first thing to get checked it was at 49psi