Thread: Backfire at high RPM

Hey everyone, I hope you guys can offer some good advice.
I have had my G20 for about 5+ months. It has done pretty well so far. Last year I did a low port to high port swap. I replaced the 96 engine with a 91-93 high port. The car drive very well and strong. The 91-93 engine was an e-bay engine. I have had it about a year between two cars.


Now a new problem has developed. When I hit a high rpm, over 5K, I get backfiring back into the intake. When this happens there is an obvious loss of power.
I have been told this may be a clogged CAT. How does one know if the cat is truly clogged?
I've been told it can also be a possible valve floating and letting exhaust back out the intake valve on the compression stroke.
Has anyone seen this before? Any advice or troubleshooting tips are greatly appreciated. Thanks!

Someone using the word backfiring correctly!

Let me think for a bit and see if I can help. I like your two first ideas though. Both seem plausible. Maybe a skipped timing chain tooth, or something else to cause mechanical timing to mess up. Or maybe an ignition timing issue...

Thanks Ben, we replaced the crank pulley because it spun. After that we timed it back to 15. Sometimes, usually in the morning, it seems to run better. But as soon as it warms up, the backfiring happens anytime I take it above 5K rpm. If I drive it gently, you would hardly know there is a problem. At idle, there is a slight stutter ever so often. It will idle smoothly then drop about 200-300 rpm. It will then go right back to it's normal idle... strange one here....

How are your plugs gapped? Have you checked them recently? Maybe it's misfiring instead?

Hey Ben, The plugs are new as of last december. I haven't pulled them to check their condition so I'll do this tomorrow when I get off work. i'm pretty sure it's not a misfire. There is a distinct sound that the backfire makes into the intake. It's almost a popping sound with an echo. It's hard to explain but I'm almost certain there is an actual backfire into the intake. I'll post some more info tomorrow... Tks for the replies!

backfiring into the air filter is caused by a timing issue, wether mechanical or electrical.


i replaced an engine once on a Suzuki grand vitara, and i ddnt line up the flexplate to its original position, it was off by 90 degrees and it too blew back into the airbox.


it must have something to do with you replacing the crank pulley. i would start there 1st