Popping sound - ?backfire?

[rold5]

'97 103K miles. I bought it a year ago at 88K. It's been maintained since then, but the prior history is pretty much an unknown. Physical condition is reasonably good.

Any diagnostic help would be appreciated.

My 97 is sometimes making a popping sound sortof like a soft backfire. The sound has an "in-a-tin-can" kind of sound to it. Almost like a cough in the muffler. the sound definitately emanates from under the car and well behind the driver position. The sound is not clearly on one side of the other that I can tell.

It is not related to engine speed.
It is not related to clutch-in or out.
It is not related to coast or under power.
It is seems to happen most often when the vehicle is cold.
It is definitely related to physical movement of the car: going over a rough road, it pops a lot.
It doesn't pop when the road is smooth.
It stops popping when the vehicle is thoroughly warmed up. (Note: I get good n' hot air from the heater long before the popping settles down).
The amount of popping is variable even more than described above, in that this morning (42 degrees), the amount of popping was moderate, but yesterday, going home around 6pm, I though parts were going to fall off the back end because it was popping so much.

I drove it over to pronto muffler today (18th & 75) from near 10th & jupiter and it was popping moderately most of the way, but by the time I got over there, it had toned down quite a bit.

I had the exhaust system looked at & the guys put it on a lift, listened for leaks, shook the exhaust train with the engine running & pronounced the exhaust system healthy. They suggested that if it was a backfire, it must be coming from the engine & I should go to a mechanic.

As I drove off, it was still popping a little. I got back to old downtown plano, and it popped once or twice as I drove in to the mechanic. When the mechanic got in for the test ride, of course: no more popping. So, no diagnosis.

FWIW, _I_ think the exhaust started smelling odd a few months ago.... almost like there was leftover fuel in it. In fact, I even went looking for fuel leaks. The fuel smell has mostly gone away, but the exhaust still smells like there's some alcohol or something in it.... just odd, to me. I have *NO* idea if that's related, but I guess it could be.

Any help appreciated ..... I'm just trying to get things back to normal.


Thanks

[sammm]

Dead or dying cat? Maybe its' innerds are rattling around inside itself. Just a SWAG.

[Sharpie]

That's my guess too. I'd order a catalytic converter and bolt it on. You can take it off and inspect the cat, once it's off if you shake it around and hear anything that's your problem. Takes longer to jack the car up than to check it. Good luck.

[tailchaser]

Dead or dying cat? Maybe its' innerds are rattling around inside itself. Just a SWAG.

Dead or dying cat, you say? Yeah... I'd probably lean in that direction. Let us know if you need any help pulling that loose. A shame you didn't ask before you ran by Pronto.

[POS Racing]

I'm surprised the guys at Pronto didn't think it was a dead cat?

[black roadster]

Funny...

I just came back from Pronto a few minutes ago. I had an exhaust rattle I wanted them to look at.

First I'd like to say I am very pleased with there customer service. We thought the rattle was from my exhaust hitting the rear diff. They welded on another hanger free of charge. I did tip the guy though.

I drove the car and realized it was still rattling. Final diagnosis, the cat is breaking up. Seems they didn't catch my bad cat either.

Regarding the cat, I had this cat put on in California about four years ago. (45k miles) I thought they lasted longer. The other sucky thing is the California muffler shop cut off one exhaust flange and welded the cat on so I can't just bolt on a replacement cat.

[miataspeedracer]

Did you run over Orville Redenbacher?

[rold5]

I took it back to the mechanic this afternoon and it was popping a little. Enough for the guy to hear it. He put it up on the rack & started vigorously pushing the exhaust plumbing around & found that the heat shield just in front of the cat would rattle a little if pushed just so. He recommended just doing a quick tack-weld & see if that fixed the problem. He did it at no charge. [Alisons and 3A muffler next door -- thanks!]

It didn't pop after that, but the car was warm. Just took it out again now that it's cold, and only heard about one tiny pop as it was warming up. Also, there's been a 'buzzing' sound related to engine rpm's I've had 2 mechs try & find without success, and it's gone now too!!

I think that heat shield was the source of 2 noises!!!

I'll keep an ear to the ground for that cat if I get any more popping....

Thanks for the pointers on it!

[rold5]

Spoke too soon. It was popping again like mad this AM .... this time I shut the engine OFF and coasted while it was popping, and it KEPT popping!

Looks like I get a new cat real soon now.....

[rold5]

Spoke too soon. It was popping again like mad this AM .... this time I shut the engine OFF and coasted while it was popping, and it KEPT popping!

Looks like I get a new cat real soon now.....

And FINALLY .... the solution! ..... After not finding it, we decided to just let it go. The sound may develop into something more recognizable & then we'd fix it. With the weather getting colder now, that "popping" eventually became a loud squeak.

And the answer was: grease the rear sway bar bushings!!!!! Doh!

Cheaper than a new cat though...

[channelmaniac]

And FINALLY .... the solution! ..... After not finding it, we decided to just let it go. The sound may develop into something more recognizable & then we'd fix it. With the weather getting colder now, that "popping" eventually became a loud squeak.

And the answer was: grease the rear sway bar bushings!!!!! Doh!

Cheaper than a new cat though...

And there was much rejoicing.... yay...

(for all the Monty Python fans...)