backfire on an 89 IROC ;)

hey all, ive got kind of an annoying backfire on the IROC that i cant seem to get rid of. it only does it when you rev the engine and quickly let off the throttle. when the revs start to come back down from the high point there are some very loud backfires. at low rpms it idles like a dream, and driving it around under regular throttle dosnt seem to have a problem (at least none that i can feel or hear). it has a new timing chain, timing was correctly set, full tune up, new fuel pump, new injectors, new air filters. and edelbrock TES headers with A.I.R. pipes.

the only things i can think of that might be causing the problem is the MAF sensor, which has been on the car since i bought it, and probly since the car was new. same with the IAC valve which i have never paid any attention to. i seem to remember replacing the ignition control module but cant be sure. i pulled the cat off thinking maybe it was plugged and causing the backfire, but it still did it when i took the cat off. im kinda annoyed and drawin a blank at this point. the possible causes are some expensive things to just throw parts at it.

that could be because youre running too rich in the upper RPM's and when you snap the throttle shut some of the unburnt gas will get into the exhaust and ignite. not really a big problem IMO.. My car does this from the factory with a factory tune and if you take the cat off you get 3ft flame balls coming out the tail pipes

or possibly a sticking valve

le' sigh........ well it may be that the engine has 160,000 miles on it. but to be fair, its not burning any oil or anything. as far as the rich burn, i did put in slightly larger injectors so the computer may still be learning the new pulse width necessary to burn right.

figured it out. i decided to use the tried and true method of start unplugging stuff and see what happens. when i unplugged the MAF sensor, and reved it like before.... no more backfire.

so, i guess ill try to clean it with some carb cleaner, there may just be some corrosion on the wire, or the connector which come to think of it is broken. other than that.... guess ill just have to bite the bullet and get a new MAF sensor.

turns out leaving her sitting for so long realllly did a number on her. cant wait to find out what needs replacing next lol.

don't use carb cleaner, use electrical component cleaner. carb cleaner may be way too harsh and end up destroying the element.

well. i actually found a can of MAF cleaner and it does seem to be making the car run better as far as overall driveability, but the backfire is still there. although ive noticed it ONLY does it when i rev it up to between 3000 and 5000 rpm, and then abruptly close the throttle blades ALL the way. if i leave them open just a little bit, theres no backfire. So im pretty sure at this point its a problem with the IAC valve. ill have to pull it and try to clean and adjust, or replace it.

oh and for some reason i did have crap in the MAF sensor, inside between the screens. so i used a small screwdriver to GENTLY spread apart some of the grids to make a larger hole and did the tedious job of getting all of it out and then putting the grid back in its place. also the screens had a black sooty coating on them, so i saw a big difference for the cleaning which i think is reflected by the smoother running of the engine afterwords.

you're absolutely sure you did the timing correctly?

oh yee of little faith ;)


lol, yeah, i questioned myself, so i looked it up from three different sources and checked and rechecked my timing reference on the engine.