I am having a bit of a problem with my 89 iroc. I have a new 350 engine in it, it was an L98.
When I drive it, it starts ok and then it starts backfiring and hesitating. It doesnt completely die, but the rpms drop really low. I hold the throttle down and revs way up, they starts backfiring and dropping the rpms again. It started doing it all of the sudden.
It has been running fine for the last 460 miles. I adjusted the iac the othernight, and its the only thing I have done to it since the first oil change at 100 miles.
I drove 82 miles after adjusting the iac without a problem. The next morning this started. Could a messed up IAC cause this problem. Im suspicious of the IAC valve. I got a great post from vader last night on setting the iac and stuff, but I got hung up at one of the first parts. It says to put the car in diagnostic mode by jumping a and b, which it did fine. It says the iac should run till its fully closed. Mine never stopped running. It just ran and clicked forever, well, at least for over half an hour before I gave up. Is this normal? I found a new iac that I can go buy, but its 50 bucks and I dont want to go buy it if you guys dont think it could be the problem.
Thanks and im sorry for the long post.
Mike

 

mine was doing the same thing and the timing was off so i set it 8* btdc. double check that was my problem.

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86 berlinetta
350cui w/.30 over
.526 hilift cam
hooker supercomps
3" duals
edelbrock streetmaster
holley 750 dp
bad asshauler

 

I just checked my timing, its still right at 6* BTDC, right where its been since I installed the engine. Any more ideas? Im still suspecting the IAC because it never stops running and clicking in diagnostic mode. Am I barking up the wrong tree here or could it cause the problem im having.
Thanks for the response.

 

Check the VALVE timing. that would be my guess.

 

I thought about the valve timing, but this is not constant, it only happens after the car warms up a little, like after just a few minutes of running, but not before.

 

Check the AIR system and the ecm. If its not functioning right it won't divert the air to the aircleaner during deceleration ie when there is high vacuum which will cause backfiring. Of course if the ecm isn't getting accurate manifold pressure readings problem like this can ensue. Check the sensor as well.

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'87 Red Formula Firebird
-mostly stock LG4 w/cowl induction setup borrowed from '83 TA and T5 tranny

'85 Corvette
-stock L98/4+3 manual tranny
-z51 package

 

I don't think the IAC is the issue...could cause some popping out the exhaust on decell...but not on acceleration..IMO.

I'd be looking into the ignition system...possibly PU coil or Module.

Does the SE light flash when it's backfiring?

 

I dont get any codes or anything. I took it out for a drive last night though, and it seems to be an intermittent problem. I drove it around for about 20 minutes with no problem, then all of the sudden it started running like crap again, it never actually died though. I let it run for a minute , giving it gas to try to rev it , and eventually it revved up and went back to normal. It was fine for the next few miles on my way home. Ive been told it could be bad gas, so I put some fuel system cleaner in, and im gonna test it tonight. Thanks for the replies.

 

check the fuel pressure, my car was intake backfireing and hesitating, and it was the fuel pressure

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90 RS 305 tbi, 4 3/4" cowl Hood, Edelbrock TES headers, Flowmaster exhaust, Crane roller cam, comp roller rockers, vortec heads/intake, ported tbi, w/new injectors, MSD probillet Distributer/ coil, 1 inch tbi spacer, air foil, 14x4 k&n flat base air cleaner, Rebuilt 700r4 w/ B&M shift kit, B&M Megashifter, 3.23 posi, 2 12 RF's with mtx amp, top of line alpine cd player,autometer phantom gauges, soon to have 91-92 gfx, z-28 wing, new paintjob, rims, all new suspension, and new inteior