: 1993 Northstar engine knock

02-29-04, 10:17 AM
My engine (70K miles) just started knocking slightly at idle, but vigorously at around 2500 rpm. My wife drives the car most of the time, so I'm wondering whether it knock might be caused by carbon buildup or something other than a rod? I'm slightly afraid to drive the car much if it really is a rod knock, but since I've seen so little regarding rod knocks in Northstar engines, I'm somewhat confused. Any help would be greatly appreciated. Thanks!

02-29-04, 02:58 PM
Has your PCM set any error codes?

Do you have oil in it?

Do you use premium fuel?

Might be the knock sensor does not detect the knock, from to low of an octane rating, and is not retarding the timing.

Could be carbon buildup.


02-29-04, 05:35 PM
Basically it's not any of those things, it's extensive knock ( Very loud)
I believe it to be either piston slap, or rod knock, but, I need some more opinions, or preferably experiences with this problem from other owners.

P.S.- Regular maintenance has been done, generally changing fluids before they need replacement, also only use highest octane fuel available.

02-29-04, 05:53 PM
Noise's are tough to diagnose without hearing them. You might do a compression test and see if there isn't a cyl or two that is unusally high. You might aslo get one of those little flash lights that you put down into the spark plug hole, at least if the carbon builds up on the piston. Both, trying to determine if there is a carbon build-up in there.

Could also be a valve hanging up, which would be shown by a very low cyl pressure.

I think if your not able to do these tests I'd get in in right away. Don't drive it until you find the problem.

bbobyski is familiar with the carbon build up issue. Hopefully he will have some additional insight for you concerning diagnosing it. I know I have seen him post that is is a rather prominent noise, so maybe all isn't lost.

Good Luck

02-29-04, 11:27 PM
I have same isue with my 94 sls and its driving me nuts.I have 120K on my car, and the noise is not always there. Sooo,, it may be carbon? I read that GM makes a product that cleans out the carbon deposits by introducing this product through the PCV valve hose(not PCV valve) and letting it sit overnight. Maybe will try product some time in near future

03-01-04, 02:16 PM
Read this thread http://www.cadillacforums.com/forums/showthread.php?t=226Then take it out and flog it. "Rod knocks" or any bottom end problem are SO SO rare on a Northstar that it wouldn't even enter my mind as a potential problem. Generally knocking noises from the engine like you describe are carbon buildup from lack of "use". Take it out and run it hard. Forget carbon cleaning or deep carbon cleaning or solvents or anything....unless you do not know how to hold the pedal down. A few good WOT accels cures noises in the Northtstar....