I just had my cat replaced and muffler because my cat went bad. I replaced the muffler becaust it was plugged with the material that came out of the cat. I left the muffler shop today and when i rev it up to 5000rpm's, it backfires in the muffler. The strange thing is that it does not do it all of the time? 20 inches of steady vacume at idle, fuel psi is at 45, timming is 6deg advanced. Nothing has changed since i had the repair and now it runs like crap? Help.....please!

 

how loud is it? cause mine kinda makes a pop'ing sound when i rev it high.

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88' GTA 350 MODS---> air foil, K&N, Shift Kit, 180* therm, TB bypass

 

Well, its not like gunfire however, it is noticable and the car surges when it backfires. It only happens under hard acceleration and around 5000 rpm's?

 

maybe ur just running really rich. which could be cool when u start having flames shoot out.

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88' GTA 350 MODS---> air foil, K&N, Shift Kit, 180* therm, TB bypass

 

If I remember right, backfire's at hi rpm's indicates a lean condition? I cant remember

 

well, its not really backfire, its better known as "afterfire" l think its a combination of high pressures and heat inside the muffler building up, and after you let off the throttle and cold outside air is finally able to enter the muffler, the combination causes a "pop" to happen inside the muffler.

if anyone has any other theories please tell

 

Same thing happens to me, so how do you fix this "afterfire" ?

 

earplugs.. ok i don't know

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88' GTA 350 MODS---> air foil, K&N, Shift Kit, 180* therm, TB bypass

 

Thanks for the reply but the backfires are durring hard acceleration and i hear them as i am approaching 5000rpm's...my foot is still on the gas..hard acceleration. I would not be too concerned about the backfires if it was when my foot was off the gas, but they happen at WOT around 5000rpms? HELP

 

K....Check This out...
I disconnected my fmu and my car does not backfire! However, at higher rpm's it hesitates a little when i wants to shift. When i first put the fmu and untill i had the exhaust changed, it would slam into gear so one of two things is going on. First, I know that the fmu was creating a too rich condition, hence, my cat went bad. It either was way too rich from the start or it is not functioning properly? I know i need more fuel on the top end so maybe I'll try to get a kit to drop the fmu from a 8:1 to a 6:1 or even a 4:1 if they make one that small. Any other thoughts? Thanks for all of the replys!!!!