: 4.6L engine knock at idle



david_peak
11-05-03, 09:26 PM
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?

elwesso
11-05-03, 09:55 PM
No!!! You wanna keep this car, since its got new head gaskets.....

Anyway, I think that post mainly referred to the 4.9 if I recall correctly.... maybe not, its been a while since Ive seen that thread....

It could be carbon buildup......? Also, do you have any DTC codes???

Ultra Slow
11-05-03, 10:16 PM
Rev it up a bit and see if it has a rod knock sound in the 2500-3000 range... You will ususally here it there. Also since you ahd the motor apart, do you think you may have gotten trash in it causing a low oil pressure condition?




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?

zonie77
11-05-03, 10:59 PM
The only thing I'll suggest is get a mechanics stethescope. Harbor freight has them cheap. I couldn't believe how much better I could locate noises when I finally got a set.

Aurora By Olds
11-05-03, 11:09 PM
The only thing I'll suggest is get a mechanics stethescope. Harbor freight has them cheap. I couldn't believe how much better I could locate noises when I finally got a set.

Ditto.
Also, dont forget about anything that's belt driven either.

david_peak
11-06-03, 12:39 AM
Only time noise is audible is between idle-1100 rpm. I checked the belts and no luck. It only does this after car is completely warmed up. I have the el cheapo oil 10w 30 in it since i've been draining it and changing the oil filter frequently trying to remove any residual particles left after head gasket replacement. Is it possible this could be the problem. As for carbon, I cleaned the combustion chambers and pistons until they looked like new. I'm hoping that nothing dropped in the intake. I ran a pretty clean shop during the process. I am changing to mobil1 oil tomorrow and checking it out w/ stethoscope. I will let you all know what happens. Thanks for all the help. I really appreciate it.

david_peak
11-07-03, 08:16 PM
changed the oil and filter today to 20w-50 and car still had low hollow knock at idle through 1100 rpm. Stethoscope didnt reveal much except it is coming from the back side of the engine. Knock sounds same on #1 as #3, 5and 7. Even listening through the steth, the noise goes away after 1100-1200 rpm. I'm thinking it could be oil pressure related or ignition timing related. Does the ignition timing change once the motor is warmed up? Thanks for the help.

elwesso
11-07-03, 08:20 PM
If the engine is knocking the knock sensor should pick it up......

Are you sure its detonation, or could it be something rattling...... Maybe lifter noise???

david_peak
11-08-03, 01:25 AM
i suppose it could be, the only thing I can pinpoint is that it is coming from the top side of the motor on the right side of the engine. I disconnected the different spark plug wires and it still made noise. I'm still curious as to why it would knock only after completely warming up.

elwesso
11-08-03, 08:48 AM
I dont think I asked, but do you have any codes coming up??? Knock sensors will throw a code.....

If in fact it is spark knock, it makes sense that it WOULD only do it when its warmed up.... The piston expands, and prior to reaching operating temp its probably on the verge of detonation.....!

david_peak
11-08-03, 11:14 AM
setting code pcm 1441(current). This is for the evap purge solenoid. Could this thing be the culprit.

elwesso
11-08-03, 11:17 AM
Its possible...... What that thing is the solenoid that lets gas flow into the evap container.... it contains a charcoal like substance, and it absorbs gas vapors from the gas tank, and then delivers it to the intake collector.....

Its a long shot, but its possible.....

david_peak
11-08-03, 11:34 PM
tried diconnecting plug wires again and VIOLA! #7 is the source. Knock stops completely when disconnected. I didnt hear it the other day because of the snap of the electrical spark. Could anyone tell me what might be the cause of #7 knocking. I will summarize things to maybe help:
1. low, hollow knock from #7
2. occurs ONLY after completely warmed up (fans kick on at least twice)
3. occurs ONLY between idle and 1100 rpm.
4. changing to heavier oil did not quiet knock. (warmed up motor until knock began, drained out hot oil and added 20w50 and new filter. Cranked it up and 10 seconds later the knock was back)

elwesso
11-09-03, 09:58 AM
Well, it does sound as if its detonation...... Funny why the knock sensors wouldnt have picked it up, unless they are already retarding the timing fully.....

Im gonna say to use a can of TOP ENGINE CLEANER..... That may help...... And its easy, go to your local GM dealer, pick up a can (liquid, not aerosol) for like $7 (I used it on my Q, good stuff), and follow the instructions...... It will smoke like a mother, so be ready......

Try that and report what happens..... It may/may not help, but its the cheapest fix I can think of..... Plus, then youll have clean intake valves and a carbon free combustion chamber......!