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4.9 knock


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Do 4.9's normally have a little knock? My previous '92 Deville would knock while driving, mostly under load. My current '93 knocks a little while cruising...step on it and it doesn't knock. It's just a little rattle...car runs great, starts great, will flat haul a** when you get down on it. Just wondering about it and what can I do about it. I currently use Valvoline Synthetic oil and it was just changed again. Only use Shell Premium or Philipps 66 Premium gasoline...she doesn't like anything else. (picky drinker) :P

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Here is a discription of the bearing knock on the 4.9 by bbobynski from the old board:

If you look at the 4.9 accessory drive you will see that the

serpentine belt wraps around the bottom half of the crank

pulley. This is necessitated by the location of the accessories

and the belt routing required to reach all the pulleys. The

rather unfortunate result of this arrangement, however, is that

the resultant load vector from the belt tension pulls the crank

upward. The gas pressure loads on the pistons acting on the

rods and crank journals tends to push the crank downward. The

belt pulls the crank up and each time #1 or #2 fires it pushes it

down. Up...down...up...down...thump...thump...thump...

The nose of the crank moving up and down (in reaction to the

accessory belt load and the cylinder firing loads) within the

small clearance of the front main bearing is what is making the

noise. As the clearance changes minutely with temperature and

the oil viscosity changes with temperature the noise will change

slightly and come and go. Nothing will ever come of the noise.

The engine will run forever. It is annoying, however.

I have never heard or the thrust bearing on the crank causing any

kind of noise. There are no alternating forces in the engine to

push the crank back and forth to cause a noise in that fashion.

The thrust bearing serves the purpose of absorbing thrust created

by the torque converter ( and the throw out bearing reaction

force in a manual transmission car) but I have never heard of a

single problem related to the thrust bearing in a 4.1/4.5/4.9.

Those engines are bulletproof in that respect. The thrust

bearing idea sounds like something someone came up with to

explain the noise and it turned into an "urban legend" type of

thing. That is not the problem.

If the noise becomes too annoying, go to a Cadillac dealer and

have th front main bearing replaced. I believe there is a

service bearing that is several ten-thousands over size that was

released to retify the main bearing thump in complaint cars.

This is not a difficult repair.

The particular engine in your car, Bruce, was probably borderline

for this noise and as the miles accumulated and the bearing wore

a few "tenths" it allowed it to move enough to start thumping.

If you want to verify the diagnosis, release the tension on the

accessory belt when the engine is thumping by holding the

tensioner off with a breaker bar. Or remove the belt and start

the engine if it is at the temperature range that you would

expect to hear the thump. One way to do it is to get your tools

ready and do a trial run then warm the engine up until the thump

is evident. You might have to put it in drive and hold the brake

and torque the engine gently against the torque converter to put

some load into it. If you have one person do this while the

other goes under the hood and releases the accessory belt tension

with the breaker bar on the tensioner you can turn the noise on

and off at will.

With a transverse engine going up or down a steep incline doesn't

do anything to move the crank one way or another. If the thrust

bearing idea was true you should be able to initiate the noise by

weaving left-right-left-right violently with the steering wheel

thus sliding the crank back and forth in the engine. Good luck...

When the main bearing thump phenomenon was recognized and

investigated many years ago we were curious as to any inpact on

the crank or engine durability. To investigate this we severely

opened up (only) the front main bearing clearance to about .003

inch. The normal clearance is about .0008 to .0015. Sure

enough, this created a rather severe main bearing thump so we

knew we could creat the situation. This engine then ran a full

300 hour endurance test without incident. There is absolutely no

durability nor reliability issue with the main bearing thump. It

is purely an annoyance.

You'll notice the Northstar accessory drive is designed such that

the belt tension load vector pulls the crank to one side rather

than up. This is to avoid the same phenomenon.

Hope this helps......

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