Knocking When Warm...
Thread Starter
Safety Car
Joined: Jan 2006
Posts: 3,566
Likes: 4
From: Mt Laurel NJ
St. Jude Donor '06-'07
Knocking When Warm...
Hi everyone,
I have a stock 1990 Corvette w/ L98 and 93,000 miles. When the engine is cold, it sounds fine, but as it warms up, it starts making a knocking or clacking sound, most noticeably from the exhaust. The car runs fine, does not overheat, the oil pressure is good (40 @ idle, 60 @ 3000 rpm).
I have tried pulling a coupe of spark plug wires off, but that did not make a difference. The other strange thing is that the sound is NOT consistent, in that it will some times “skip” a beat or two.
In searching the forum, I have read that sometimes-bad precats will cause this kind of sound. Has anyone ever heard of this?
The car has been doing this for a while; I first noticed it coming home from Carlisle, but you cannot hear it when driving or even sitting in the car.
In the time I have owned the the car, it has not been abused or raced in any way.
Thank you for your help.
Randy
I have a stock 1990 Corvette w/ L98 and 93,000 miles. When the engine is cold, it sounds fine, but as it warms up, it starts making a knocking or clacking sound, most noticeably from the exhaust. The car runs fine, does not overheat, the oil pressure is good (40 @ idle, 60 @ 3000 rpm).
I have tried pulling a coupe of spark plug wires off, but that did not make a difference. The other strange thing is that the sound is NOT consistent, in that it will some times “skip” a beat or two.
In searching the forum, I have read that sometimes-bad precats will cause this kind of sound. Has anyone ever heard of this?
The car has been doing this for a while; I first noticed it coming home from Carlisle, but you cannot hear it when driving or even sitting in the car.
In the time I have owned the the car, it has not been abused or raced in any way.
Thank you for your help.
Randy
Thread Starter
Safety Car
Joined: Jan 2006
Posts: 3,566
Likes: 4
From: Mt Laurel NJ
St. Jude Donor '06-'07
Well, I guess it could be, but I when out and started the car, and held a broom handle to the engine, and all I heard was the sound of the engine no knocking sound. I then held the broom handle to the precat, and heard the knocking sound,
so I guess it is the precat coming apart. 
I was thinking of replacing the exhaust system anyway, so now it has become a priority.
Randy
Team Owner
Joined: Apr 2001
Posts: 36,453
Likes: 2
From: Overwhelmed as one would be, placed in my position.... DFW, TX
St. Jude Donor '05
Well, I guess it could be, but I when out and started the car, and held a broom handle to the engine, and all I heard was the sound of the engine no knocking sound. I then held the broom handle to the precat, and heard the knocking sound, so I guess it is the precat coming apart. I was thinking of replacing the exhaust system anyway, so now it has become a priority.
Randy
so I guess it is the precat coming apart. I was thinking of replacing the exhaust system anyway, so now it has become a priority. Randy
Team Owner
Joined: Oct 2004
Posts: 81,241
Likes: 3,062
From: altered state
St. Jude Donor '05
If the precat is coming apart its probably destroying the main as pices will get in it. Id just replace with all new and forget about it.
Just reread your post, if the sound "timing" isnt consistent its definitely the cats.
Just reread your post, if the sound "timing" isnt consistent its definitely the cats.
Thread Starter
Safety Car
Joined: Jan 2006
Posts: 3,566
Likes: 4
From: Mt Laurel NJ
St. Jude Donor '06-'07
Yeah, this all started when I installed the Flowmaster 50's on the car. I guess the lack of back pressure did something to the cats, because the car is much louder now than when I installed the new mufflers. The funny thing is that the car passed emissons w/ flying colors in September.
Last edited by RandyJ75; Dec 19, 2006 at 07:14 PM.
