Superchexx

Hi,

I have a 91 C4. I did a valve adjust. Put in new plugs. Had a problem getting out #3 exhaust, but managed. Cleaned up the plug hole with a spark plug chaser and installed new plug. New valve cover gaskets and nylock nuts and new washers. New cat bolts and seal. New 02 sensor. Put in Mobile 1 0W40 and new oil filter. Started her up to opperating temp and added remaining oil and went back and tightened up cat and 2ndary muffler seals after they expanded from the heat. I also did a dist belt rebuild and new caps and rotors.

With all that being said (not sure if its relevant or not) I have what sounds like a rattle on the left side of the engine by the cat. It would happen on acceleration and I just figured it was a broken bolt on the heat shield (I don't have the engine encapuslation on) or a muffler leak. I put the car on the lift and started it up to opperationg temp then reved the engine in neutral and around 4K rpm's I get the same rattle. Under load while driving this rattle happens at lower rpm's. Any Ideas? Sorry for all the maybe non relevant info but Im trying to be as detailed as possible.

Thanks,
Brad

MassGuy

First off I take my hat off to you for getting all that work done. Sounds like my "to do list".

I wish I could offer some precise help....but I can only suggest you continue to attempt to locate this minor but annoying problem. It will soon reveal itself.

Good luck and keep the faith......

Superchexx

Brian,

Thank you for those kind words. Its not really work if your having fun, right?

Brad

Richard Curtis

If you can put the car up on the lift again, use a long screwdriver to help locate the sound. Start engine. Put your ear to the handle of the screwdriver and the tip of the screwdriver at various places. It's sort of like a cheap stethoscope. Even better is to buy a mechanic's stethoscope made specifically for this. I'd start with listenign to the valve covers to ensure that the rattle isn't coming from within. Otherwise, it's probably just something loose like the tinware or ... Good luck, and let us know what you find.

JasonAndreas

Check the bolts on the front of the catalytic converter where the bottom heat shield is mounted to the front/side heat shield. The nut-locks that are used there have been known to crack.

911URGE

Hmmm.....could this be fractured internal ceramics of the cat rattling? How old is this cat?

Superchexx

Jason,

I haven't put the bottom heat shield on yet, but I will check the bolts on the side heat shield. If they look suspect I'll remove it completely to see if there is a change.

Thanks for reading and replying to my problem.

Brad

Superchexx

The cat is original. When I removed it from the primary muffler to do the valves, the ceramic insert or some sort of ceramic pieces came out of the cat. I thought for sure that was my problem, but I checked in the cat looking for a broken honeycomb and it looks all intact. Anyhow, I cleaned out the ceramic bits and no change.


Thanks for your suggestion

Brad

Millemiglia

I had the same symptoms as Brad, only on the right side of the engine.
Turned out the secondary muffler had rot, so I replaced it with a g-pipe which save weight, money (compared to a new sec. muffler) and gained in sound experience...

J.PELLEGRINO

I would wonder if the noise was present before you performed the service work...?

Good luck, Jerry

Marc Shaw

I had a rattle in the same area before and turned out to be the brake dust shield was loose.

Marc

Superchexx

Hi Jerry,

Yes it was there before I performed the service work.

Brad

Superchexx

Mark,

I didn't think of that, but I'll check it. Would yours rattle in neutral if you reved the engine?

Brad

Superchexx

Ok I had the girlfriend rev up the engine while it was on the lift. It definitely sounds like it’s related to the cat. I also noticed that it’s not at all related to rpm but rather how fast one 'gets on the gas'. A slow rev up to 4k rmp yields no noise. But if you slam the accelerator down I can hear the noise. So my questions are now this:

1) Can an original 16 year old cat that appears to not have any broken 'honey combs' still be suspect?
2) Since the problem happens under quick acceleration regardless of rpm's could it be exhaust leak related?

Thanks for any and all replies.

Brad

dutchcrunch

take the lower heat shield off the cat an the one between the cat and plastic shield outside the valve cover. that noise should stop. if it does check the parts and make sure no cracks and reinstall and check for clearance everywhere. i bet 1 dollar thats your problem!!!!!! i think Jason mention this before that the noise was coming from these.