: 4.6L engine knock at idle



david_peak
11-05-03, 01:37 AM
Just replaced headgaskets on 97 deville concours. After car warms up, there is a very light knocking noise coming from the rear of the engine compartment at idle through 1000 rpm, then it goes away. Car is running premium gas and has no valve clatter, even during heavy acceleration. Heads only had .003 shaved off of them at the machine shop. Car runs SUPERB otherwise. Car did not make noise before gasket replacement. I read on other post that this could be the #1 bearing thumping caused by serpentine belt lifting against damper. I hope this is it at least. Maybe its time for marvels mystery oil and a for sale sign?

Aurora By Olds
11-06-03, 10:57 PM
Well, just a thought. You say you had .030 shaved off the heads. Now that means that your heads are that much closer to your crank, which IS going to change your valve timing ever so slightly as well as move your valves closer to your pistons.
I dont know what the specs are on shaving N* DOHC heads, but you may want to check into that.
It wouldn't make sense as to why it would only knock at low RPM's, but it could be possible.
Good luck.

crazy4asista
11-09-03, 10:58 PM
Yo im new here but i just thought id trow my 2 cents in ... ..my 98 sts sounds like its about to trow a rod at low rpms... guess what the problem is damn a/c compressor has bad bearing ... do this check get down under right front of car while running while its making the noise see if its the a/c compressor on mine u can feel the bearing knocking in it ... Saftey first though make sure parking brake is on and a/c is truned off cause if u touch the compressor while its on and the clutch ingages might break ya finger ..hope this might help if not if u every hear a diffent knocking sound tryig looking at that darn a/c ....my car has 193000 miles

zonie77
11-10-03, 12:08 PM
I've been giving this some thought and about the only thing I can think of is a bad piston skirt, either cracked or worn. The only reason it might not have made noise is the same as why it stops when you pull the plug wire, less compression(if that was the bad cyl). Normally I think that's worse when cold and quiets when it warms up so that may be the wrong diagnosis. Another possibility is the piston pin worn.I didn't do any searches for info so that may be worth looking around the web.
I hope I'm wrong but that's what I thought of. I'm not sure how to test for either without pulling the engine.

david_peak
11-10-03, 10:41 PM
tried the top engine cleaner. No luck. Even let some set in cylinders overnight. My god, this thing smoked like crazy after startup! Knock is not getting any worse and i've been driving it a lot lately. Anybody else want to take a guess?

zonie77
11-11-03, 05:52 PM
I talked to a friend of mine who rebuilds a lot of engines. He feels it's piston, wrist pin or rod bearing.
Especially after head gasket failure. Antifreeze in oil can wipe out the bearings fast.

david_peak
11-12-03, 12:14 AM
It is still hard to explain why it did not knock before the head gaskets were replaced. It seems like if it were a bearing, wrist pin, rod or piston it would have done it before gaskets were replaced. Also, wouldn't it knock throughout rpm range if it were one of the above problems? My brothers co worker is a factory trained GM mech, he listened to it and he said he was stumped, he is bringing his scanner by sometime this weekend to run some further test.

zonie77
11-12-03, 10:18 AM
Was number 7 the cyl with head gasket leak? If it is that might explain no noise before.
Usually when these problems first start they show up more at low rpm's.
As I said before, I hope I'm wrong on this but couldn't think of anything else that would make that kind of noise.

david_peak
11-12-03, 08:00 PM
leak was between #2 & #4. probably putting a for sale sign on it if it is too expensive to fix. I'll find out this weekend. Thanks for all the help everyone.

jdp6000
11-25-03, 06:20 PM
leak was between #2 & #4. probably putting a for sale sign on it if it is too expensive to fix. I'll find out this weekend. Thanks for all the help everyone.


Have checked the timing???