When the engine is cold it does not knock. When it warms up (most of my trips are short) you can slightly hear it. Once a week I do a 70 mile trip to pay a bill and it is all highway. When I return back into town at lower city speeds it is a quite noticable knock. I have heard that this is related to the front main bearing? Can I replace this when I remove the oil pan? Will anything else have to be done other then remove the pan, off with the old, silicone, and install new bearing?

It is rather EGR-related knock. Read up the archives.

Believe me the EGR was the first thing that came to my mind too. When I intially bought the car that is what I suspected. I changed the EGR. It was with a aftermarket "universal" type. The last time I had the throttle body off I coat hangered both passages out underneath. The last time I drove it I pulled into the driveway and opened the hood. In park it can still be very slightly heard when you are near the belt side. I maybe wrong but I thought the EGR was disabled in park. From the inside of the car you can not hear it in park. It is not really to loud at any point but noticable.

Are you running the correct grade of fuel? Has the timing been advanced at all?

I run Shell 93' octane everytime I fill up. No timing is stock. I did a recent check when I had to change the ISC motor. It was part of the procedure and I figured it was worth checking too.

Yes, EGR is disabled in Park.

My 4.1 knocks the exact same way. You can hear it slightly in park also. I'm gonna change my #1 main, might as well while i'm in there.

If the knock you are describing is at idle in park or drive then the noise is likely the main bearing thump discussed in the past.

Spark knock would be a totally different noise and, although described as "knock" does not sound at all like the main bearing thump at idle. Spark knock could be related to EGR problems but EGR has nothing to do with the main bearing thump. EGR is indeed disabled at idle in park or drive so EGR would have nothing to do with a knock at idle.

One thing you might want to do is to switch to the 15W40 Delo/Delvac/Rotella oils. The slightly thicker oil would help the noise from the front main.

There is a service front main bearing shell that is available from the dealer that can be replaced with the pan off that will alleviate the noise.

If you want to positively confirm the main bearing thump diagnosis then you could get the engine good and hot and have a helper hold the car at idle in drive and confirm that the knock is present while listening under the hood. Take a 1/2 breaker bar and unload the serpentine belt tensioner slightly so that the belt slips and the accessories stop turning. If the noise disappears with the loosened belt then the noise is definitely main bearing thump. The side loading of the crank snout by the serpentine belt causes the noise. The belt load vector pulls the crank snout up and the combustion loads from the pistons pushes the crank down and the "thunp" is the result. It will not hurt a thing but it is annoying and can easily be remedied with the "purple" service bearing shell that is designed with less clearance and a different eccentricity in the bearing profile.

Thanks Chevelle I will try the belt test tomorrow or thursday. Whenever I can get a helper. Good thinking. I thought that would be the cause but would have never thought to check it like that.

Just for clarity. Illumina asked whether the timing was advanced. I wanted to try this with my car once I get a couple other things fixed. Would this cause a knock situation other then using lower octane fuel with the advanced timing. The main reason I went with a recheck on the timing was in conjunction with me having the distributor off to make sure I did not screw something up. I changed the pick-up coil because it was not measuring out right. I wanted to advance the timing then but figured I would wait tell I got the rest of the car running a little better so I would not complicate dignosing problems for myself. I think that made sense.

The sound went away when I released the tension on the belt. I am going to shoot to change them in two weeks when I change the oil. Got to wait to get paid anyway.

The sound went away when I released the tension on the belt. I am going to shoot to change them in two weeks when I change the oil. Got to wait to get paid anyway.


Sounds like the main bearing thump. Just replace the front main bearing shells only, not the rest of the main bearings. They are not "worn" per se....just the service bearing for the front main only is designed for a tighter clearance and with a different eccentricity for reduced noise. The others are fine and would have no effect on the bearing thump anyway.

I haven't tried that belt tension trick, however, mine sounds more like its pinging... It sounds similar to knock if you use 87 or 89 gas. I always use 91, mostly Chevron. I thought that it might be the EGR system, however, it will do it in park with a little pressure on the throttle.

91 Deville, 40K Miles

If the timing is advanced and you try to use lower grade octane it will ping. If you use the stock timing people say you can use lower grades. I personally would not. It is a little cheaper I suppose but you get better gas mileage with the higher octane.

Were the 4.5L's sensitive to this? A friend of mine just picked up a 90 Deville and it sounds quite solid (125K miles). Just wondering if commonly shows up in 4.5's too.

Sounds like the main bearing thump. Just replace the front main bearing shells only, not the rest of the main bearings. They are not "worn" per se....just the service bearing for the front main only is designed for a tighter clearance and with a different eccentricity for reduced noise. The others are fine and would have no effect on the bearing thump anyway.

Yeah the main bearing thump is common on 4.1,4.5, and 4.9s. The 4.1 has another problem internally that can make a similar sound.