Backfiring - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


1992-1999 Series I L27 (1992-1994 SE,SLE, SSE) & Series II L36 (1995-1999 SE, SSE, SLE) and common problems for the Series I and II L67 (all supercharged models 92-99) Including Olds 88's, Olds LSS's and Buick Lesabres Please use General Chat for non-mechanical issues, and Performance and Brainstorming for improvements.

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Old 03-10-2007, 05:07 PM   #1
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Default Backfiring

My car has recently started backfiring. I've changed the plugs (With NGK TR55s) and the wires and that hasn't fixed the problem. It idles fine. I can tell it is having issue when I press on the gas hard, because the boost will stutter (I can hear the whine stop for a half a second and then kick back in). If I shift it into park and rev it up high, it will backfire loud enough for me to hear it (It isn't that loud). It sounds as if it is backfiring through the exhaust (It was a lot louder before the mount and exhaust were fixed).

Anyway: Things I've had done to it. We recently found that it had a broken mount. I took it to someone and had them put a new mount on it and they also repaired the rear exhaust manifold. I did have the battery unplugged for a while before all this started.
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Old 03-10-2007, 05:54 PM   #2
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The "backfiring" in park is likely the rev limiter kicking in. It is extremely hard on an engine to free-rev it like that, so GM programs a limiter into the computer. Probably right about 4k rpms.

What wires did you use? Believe it or not, some can be bad out of the box. How did the coil towers look when you replaced the wires? Any rust or corrosion? May also want to check for vacuum leaks.
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Old 03-10-2007, 06:27 PM   #3
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I used Borg Warner 8MM wires. They were the most expensive wires that auto store carried for my car. ($28 )

I usually rev it between 3-4K RPMS. Exactly how does the computer limit the rev?

The coil towers looked fine. No rust at all or corrosion at all. Vacuum leaks are possible, but I really doubt it. I had to replace the coupler in the supercharger a few months back and I replaced several of the vacuum hoses then because they were dry rotting.

I just added some injector cleaner to the fuel. I'll run the car for a while (Stay off the boost if I can) and let you know it is performing in a few days.
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Old 03-10-2007, 07:04 PM   #4
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This is a good article about rev limters.

http://everything2.com/index.pl?node=rev%20limiter

As I recall, the vacuum hoses running to the evap cannister under the battery can be an issue, and are hard to see. Check all that you can find.

Another thing that is possible is corroded battery connections. It often shows itself after battery replacement, or disconnecting it. The worst is the postive side. Remove the boot over the connections and check for corrosion there. That has solved many strange problems caused by low voltage. May as well clean up all connections while you're at it.
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Old 03-10-2007, 07:20 PM   #5
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The evaporation canister is on the other side of the vehicle and under the windshield washer reservoir. And I replaced all the hoses that went to it when I removed the supercharger to replace the coupler.

The rev limiter does make sense. It doesn't make the noises described in that article, but it does backfire after the peak in the rev (Usually it backfires about the time it hits 1K RPMs again).

The negative terminal of the battery looked okay when I had it off a couple of days ago. I'll pull the positive side of sometime and check it as well.

Thanks for the replies.
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Old 03-10-2007, 07:26 PM   #6
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Yeah...that'* what happens when you try to work from memory on where they hide the thing on the different series of engines. Let us know what you find.
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Old 03-17-2007, 03:06 PM   #7
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Bahhhh! The problem is still here. These videos make me wonder if something is wrong with the supercharger.



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Old 03-17-2007, 05:16 PM   #8
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Superchargers don't cause backfiring. You have a fueling/spark issue. Pull your vacuum line off the fuel pressure regulator and smell for fuel. If it fails, it can leak un-metered fuel into a vacuum line and manifold, pooling in the manifold and igniting occasionally.
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Old 03-18-2007, 02:02 AM   #9
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I just checked it. It definitely doesn't have any liquid in the vac line (And the engine is hot right now). There is defintely a gas smell in the vac lines though. I didn't directly smell the line that goes to the fuel pressure regulator, but I smelled one that hooked to the line that feeds the fuel pressure regulator.


Do you think the fuel pressure regulator is bad?
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Old 03-18-2007, 01:30 PM   #10
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Let'* back up a minute. You changed the plugs right?

Go back and double check firing order and make sure you have it correct.
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