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sts70004
04-28-2009, 12:06 AM
So I just finished a head gasket replacement on the car, and I was in the process of idling it to circulate some new oil to clean out the oil pan (gonna do another oil change in a day or two) and I gave it a little bit of gas to help out the idle and there was a loud Snap or Pop from under the car, about where the manifolds meet the exhaust pipes. At first I thought it was just a backfire, cause by old gas or something, and I put some new gas in it and it did it again. It only pops when you sharply tap the gas. It seems independent of RPMs. It doesn't sound like a backfire, more like hitting a piece of metal against another, except without as much resonation.

My only thought is the exhaust flange isn't tightened all the way and I'm hearing a sudden shift of the joint between the exhaust manifolds and exhaust pipe. Sound like a possible cause to you?

pablon2
04-28-2009, 08:02 AM
I would make sure your exhaust mounting points are secure. This includes making sure you have a good seal between the exhaust manifolds and downtubes. Do you hear any ticking noises? Also, have you checked your motor mounts for sagging or tearing?

sts70004
04-28-2009, 09:09 PM
I haven't noticed any ticking. I haven't checked the mounts but I guess that if they're bad the engine will be rocking a little more. I tightened the exhaust flange a little bit too, and I made sure they were even but it still made the noise, but I'm not sure its as tight as it needs to be. How hard are they to change?

sts70004
04-28-2009, 11:06 PM
Anyone? I am really confused. I checked exhaust manifolds and they seem very secure. I don't know if its a backfire, I haven't heard one in a car newer than 1985. If its a backfire I need to know whats causing it before I can take it in for Emissions. I'm not getting any trouble codes or check gauge light. :confused:

Previously, it only did it when i pressed the gas sharply. I noticed I had lost a screw on my hose clamp on the throttle body so I just replaced that, and now it idles a little bit better, but when I first started the car, it had trouble going and it made the sound when it was coming back from the rough idle, when I revved it slower, it also did it once the boost started to build, and the boost gauge sorta responded to the sound. I don't know if thats related to it or if its just responding to the sudden vibration.

When I did the head gasket, my EGR tube broke, so I put a block off plate and a gasket on the intake manifold, and also a flange fitting on the exhaust manifold until i can get it fixed. It seems like the exhaust manifold is leaking after I let the car warm up. Could this cause the backfire? I also didn't check the valve lash when I put it back together.

rickbtbird
04-28-2009, 11:18 PM
Can you hear it in the engine bay or only when you're driving it? If it's only happening when you drive it, it could be you bushings.

sts70004
04-28-2009, 11:24 PM
Ok well I fell like a major [email protected], but its fixed now. I didn't think it was the case since the car was idling pretty good, and I figured it wouldn't work if the firing order was wrong, but it turns out that was it :-) Someone suggested it to me earlier, and I did a search "backfiring" and it brought up a post where someone had done the same thing I had. So its fixed now. Just firing order. Turns out the car is fine, the only flaw was me hehe.

TinManSC92
04-28-2009, 11:36 PM
i don't think that will cause a backfire by itself. you might want to take another look at that connection you kinda threw together. other then that ..don't know

sts70004
04-28-2009, 11:40 PM
well it might not have been a backfire i was hearing, but thats what a few posts i saw said. anyway, i started it up and its doing wayyyyyy better now, no backfire, no rough idle, no missing.