Mechanics - rattle noise on deceleration?

[Fraoch]

I have a 1999 Honda Civic DX coupe. 1.6L engine, automatic transmission, assembled in Kentucky (I let people know this when they say I bought "foreign"! )

I only have about 50 000 km on it = 31 000 mi.

About 2 years ago I noticed this - when the engine is cold, I get a loud rattle on DECELERATION. Yeah, you read me right, deceleration. And only when the engine is cold. And I *think* only in 2nd gear.

Whenever I take it to the dealer for regular servicing I ask them to take a look at it. They always say they "removed a heat shield". The next time they say that I'll ask them how many heat shields the thing has! They've told me this 4 times now! I think it's their excuse for "we didn't road test it, and you must be making the whole thing up, so we'll tell you we did something which you probably won't check anyway."

At least they didn't charge me for it.

A suspicion of mine is the exhaust manifold. About the same time this started I got a notice of a "warranty extension" which indicated there could be a casting defect in the exhaust manifold which causes an improper seal. The letter indicated I could tell if I had this from a "sudden very loud noise in the engine compartment". I don't exactly get that, just a rattle when I let off the gas when the engine is cold. But since it goes away, it could very well be that the exhaust manifold isn't sealing right when cold and it needs to heat up and expand to seal.

But if that were the case, wouldn't it be loud on acceleration?

Aside from that, the car runs fine. I get as low as 6 L/100 km gas mileage, which means things must be running right. I did have a comment once that my oil gets "muddy" which was unusal for a Honda. I change the oil every six months - maybe I could do better here, but I still think that's good.

Any ideas?

[BlendNo27]

Actually...i think it is the heat shield. it could be hittting your exhaust pipe. I've worked on this before. Its a pain in the *** to figure out where it is hitting though. My brothers accord 95 does the same thing when it is on low gear where the car vibrates a lot. I actually fixed it by just bending the sheild with a long nose.

[Fraoch]

Thanks BlendNo27.

How many heat shields are there on a car typically? Bottom of the exhaust manifold, catalytic converter, where else? I thought there were only 2 or 3.

And why only on deceleration with a cold engine? Maybe the engine just produces a certain "sympathetic vibration" when revving down and bingo...

(I actually encounter this in home audio. Certain bass frequencies cause objects to rattle.)

[BlendNo27]

http://www.automotiveforums.com/t151801.html

Just googled this...it might interest you froach.

[Fraoch]

Originally posted by BlendNo27
http://www.automotiveforums.com/t151801.html

Just googled this...it might interest you froach.

Aaaaaah!

So only certain RPMs huh? That would explain it. I would guess this is around the 1500-1700 RPM mark, although I don't have a tach. It's certainly higher than idle. I don't get any noise when idling or in park.

I'll take a look under there to see if I see any more heat shields and if they're loose. If they've all been removed I sure don't hope it's the catalytic converter - I hear these are $1-$2K. If this is the case I have a few weeks of warranty left...

Thanks very much for the link!!

[BlendNo27]

No prob, have fun working on your car. That's my more expensive hobby btw, If I'm not upgrading my pc, I'm upgrading my ride! Im in the process of doing an engine swap right now, just waiting on parts