90 tracker backfiring

[mystical_nightmare]

okay, my dad had a 90 geo tracker he let a friend of his borrow for a long while. we borrowed it for about a week and it ran fine, then about a month later we bought it and noticed the backfire.

when i accelerate it has a "pop" from under the hood and the vhicle jerks. sometimes it will make 5 or 6 pops in a row before it stops. it "mainly" does it at lower RPM's (below 3500), and while cruising at a steady pace it runs fine. when i give it gas it backfires again.

i have tried to adjust the timing both ways with no change, i checked the plugs,wires,dist cap,vacume lines and ECM codes with no luck.

what all could be causing this?

thanks.

[versatech]

Hi my 91 backfired in low rpm. It was holes in the exhaust. I found most of them and used muffler mend cream. The backfire and rumbling went away.
Hope this helps.

[mystical_nightmare]

okay, it's fixed. on christmas day i finally had the time to tear it down. after taking the fan off to get to the balancer bolt, so i could turn the engine over i took the cam cover off to check the valves. as it turns out all 4 exhaust valves were stuck open a tiny bit. after adjusting it the backfire is gone, and it has twice the power. now all i have to do is replace the cam seal and fix the exhaust leak on the down-pipe. neither of which will be fun....

thanks.