Manifold / heat shield expansion (99 3.1)

[MagnumFiredNeon]

I have a '99 Lumina base sedan with the 3100, and have a question about a noise it makes. Hopefully someone has heard it before or knows what it may be, but here goes:

There is this sharp pinging noise that comes from the upper manifold in the front ('left side") of the engine. Also, when I use my old-school mechanic steth to listen to it, you can feel the pings go through it. You can also "feel" the noise through the left engine mount (seems like it supports the manifold's heat shield somehow).

Anyway, I've only heard two other cars do this and they were both Chevys as well (Tahoe and TrailBlazer, both newer models). Car seems to do it while it's warming up, while it's idleing (unless it idles for a long time, then it stops) and when it's shut off.

I brought it to an ASE mechanic who also does exhaust work but he just listened to it and thought it was normal. He said the car isn't running to hot or anything, and it isn't a mechanical problem, so it's nothing much to worry about.

I'm not worried about it, but do any of you have a remote clue what does this?

p.s. if you would like an MP3 sound clip (I know it won't help that much since you can't see the car in person, but it's something) let me know.

Thanks

[jeffcoslacker]

Shields make all kinds of weird pings and ticking sounds when heating or cooling off.

Don't trip on it.

Go listen to a Honda Accord when it's cooling off. Sounds like a Grandfather clock.

[jeffcoslacker]

As far as what it is, it is just the sound of a steel shield that's tightly bolted to a cast metal manifold with a different expansion rate, friction stress between the two builds as they heat, until it is released in a very small snap of movement between the two, that resonates through the shield and gets amplified, using the shield's surface area as a sounding board.

If you know anything about earthquakes, the process is almost identical

There's lots of mismatched material matings in modern engines, you've got iron, steel, cast magnesium, phenolic plastic, etc, all joined together in different places, and they all produce expansion sounds when heated. Some you can hear, some you cant. Something like a shield, though, that is free to resonate will be loudest.

[MagnumFiredNeon]

Thanks for the reply, interesting to know about that.

The reason I was wondering is because I had a 95 with a 3100 (that was lost in an accident ) and it didn't do it as loud as this one does. Not a big deal at all, I was trying to remember how the 3100 was configurated in that one. But it's all good.

You know your stuff man. Nice Lumina, too. Yours is an LTZ, right? At first it tricked me out but I new it was too long to be a Monte.

[jeffcoslacker]

Seems like the older they are, the less they make noise, probably because the parts have been expanding and contracting against each other for so long, they've worn somewhat where they meet. A brand new motor will really tick and ping the first few times it runs and cools off.

The different expansion rates are why newer gaskets have so many tricky features to them, like they integral steel carrier that's inside the rubber of lots of pan and valve cover gaskets, and the teflon coating on other rigid-type gaskets. Without this stuff, the differing expansion between them would push the gasket out of place, or slowly grind it up.

Thanks for the compliment! It's an LS, I just blacked out the tail and installed the LTZ spoiler for a better look.

[jeffcoslacker]

You made me think of something funny from the past along these lines. Back in '99 I bought a new Kawasaki 1500 Vulcan Classic, about September.

One real cold morning in October, I started it up after it had sat outside all night, and man it made so much of the kinds of sounds we're talking about, I halfway wondered if it was gonna blow up.

And I had single-wall drag pipes, the exhaust port end of those probably hit 500 degrees or more within seconds of starting. With full length shielding, there had to be some serious differences between the shield and the pipe itself.

I mean these sounds were cracks, rather than the normal tics and pings!

[MagnumFiredNeon]

Man, that would freak me out. I thought the pinging every few seconds was bad enough, but if it started crackling like that, I would have shut it off and had it towed for sure!

Cars can do crazy things sometimes. I used to have a 99 Plymouth Neon which would make a constant ticking noise which the mechanic figured was a worn wrist pin. Oddly enough, after the EGR was replaced, it only did it once in awhile. Then again, it could be my hearing.