Tapping noise

[adallak]

I have had this noise for four years and it was more pronounced at lower speeds. It is quite loud at higher speeds now. I do WOT from time to time (in second gear from a stop to 95 MPH), so I do not think it is carbon deposits. The throttle body and EGR tubes are clean. It idles quite nicely - no tapping noise after the engine warms up (in 3-5 minutes). Any ideas what could cause this?

[Scotty]

I have had this noise for four years and it was more pronounced at lower speeds. It is quite loud at higher speeds now. I do WOT from time to time (in second gear from a stop to 95 MPH), so I do not think it is carbon deposits. The throttle body and EGR tubes are clean. It idles quite nicely - no tapping noise after the engine warms up (in 3-5 minutes). Any ideas what could cause this?

Is it the number one main bearing knock?

[adallak]

Is it the number one main bearing knock?

I realy do not know. If it is the case should it will much louder when the gas pedal is applied?

[Scotty]

Yes it does get louder on acceleration. There are a few tests to see if it IS your number one main bearing. Try taking the serp belt off while cold where you can drive the car a SHORT distance (like a block with no lights, ), if you have the number one main bearing knock it will be gone or seriously quieted (Al suggests this below). We used to have this guy named Al, he was quite knowledgeable, was a really nice guy and wrote novels!

He wrote this about the 4.9 number one main bearing knock for me. The text is below here is the link: http://caddysearch.netgetgoing.com/mbarchi...id=IMPORT-14865

By the way, I found that synthetic oil made it worse, I started using 10W40 in the summer and a mix of 10W40 and 10W30 in the winters and it minimized the noise.

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It is difficult to diagnose noises from several thousand miles

away but I will hazard a guess from your description that you are

hearing the front main bearing thump. First off, other than

being an annoyance, there is absolutely nothing wrong and nothing

bad will ever happen to the engine because of this.

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......

Bruce, it would be good to get some of this info and other things

such as all the info on cooling and coolant,etc. into the

FAQ.....not much of it is showing up there and a lot is being

repeated already....Thanks.

[adallak]

We used to have this guy named Al, he was quite knowledgeable, was a really nice guy and wrote novels

I do remember Al the novelist, I did not hear from him for a long time... There is another quite active GM sertified novelist - Bbobinsky, and he is as good as Al was. Sometimes I even think it's the same writer - Al Bbobinsky (well...sounds like "Ali Baba and the Forty Thieves" )

I am going to power brake and ask my crazy GF to release the serpentine belt with a breaker bar to see if there is a difference. But I am she will put the bar right into the running motor. Soooo... let me see if I can attach the bar to the passanger seat somehow...

hey thanks for the info Mike!

[Scotty]

I think you should keep your GF out from under the hood, unless you are looking for a new one! Take the belt off altogether and drive it a couple of blocks

[adallak]

I think you should keep your GF out from under the hood, unless you are looking for a new one!

I am sure you mean a new 4.9 motor

[Scotty]

I think you should keep your GF out from under the hood, unless you are looking for a new one!

I am sure you mean a new 4.9 motor

really! Hey did I hear that you needed that ISC motor, did you install it?

[Ranger]

I do believe Al and "Guru" are one in the same.

Why not let "crazy GF" do the brake torqueing while you release the tension or is that a stupid question?

[adallak]

Hey did I hear that you needed that ISC motor, did you install it?

my ISC motor started acting up about a week ago. From previous experience I learned it would last for a couple more weeks... so I asked epricedwell (or right) to send your motor to me if it does not work for him. Looks like they last for some 60-70 kmiles...

[adallak]

Well... some update. The knocking noise starts after the engine has been entirely warmed up (a 10 minutes drive up to 190-200 degrees). It sounds like Maxim machine gun shooting with some 5-15 Hz depending on RPM. I hear the same noise while power braking at the same temperature range. There was no noise in neutral. Taking off the belt and starting the car without coolant and oil flow over 200 degrees is scary. Perhaps, I will have to call my GF for manual relief of the serpentine belt <_>

[JimD]

?? oil flow ??

[Scotty]

You will have oil pressure but no alternator, and getting the car to 200 power braking, listening and then turning it off should not be a problem. I lost my serp belt on my 91 as a result of a blown water pump on the expressway and it hit 260 before I found a safe place to stop.

[adallak]

?? oil flow ??

ooops... the oil pump is driven by a shuft and has nothing to do with serpentine belt.

[adallak]

brought the temp to 200 degrees where the knocking noise is coming up, shut the engine, removed the belt , started again ---the noise was still there! The temp jumped to 250 just in a minute. I stopped the engine looked under the hood and the coolant was boiling and fountaining from the surge tank! Guess the mix was a little bit out of range... Any idea what this tapping noise might be else? It starts at 190-200 degrees.

[Scotty]

brought the temp to 200 degrees where the knocking noise is coming up, shut the engine, removed the belt , started again ---the noise was still there! The temp jumped to 250 just in a minute. I stopped the engine looked under the hood and the coolant was boiling and fountaining from the surge tank! Guess the mix was a little bit out of range... Any idea what this tapping noise might be else? It starts at 190-200 degrees.

Now we need to get bbobynski on the case. Its not likely the number one bearing knock with that test.

Its interesting that it only occurs at operating temp, but the oil thins out a bit. My knock got really really loud when driven in heavy traffic and the engine was very hot for a couple of hours. Its possible that your knock is only at the beginning and the bearing clearance is tight enough so that when the oil is cool you dont have a knock, but when it thins out when hot the knock is louder. Have your tried 10W40, if not do a 50/50 mix of 10W30 and 10W40 and see what happens, my knock was greatly improved when I did that. Its also possible that its a rod bearing or a bearing other than the number one. Try the oil mix. Check out your coolant concentration and cap I don't think you should have boiled over..

[adallak]

Have your tried 10W40, if not do a 50/50 mix of 10W30 and 10W40 and see what happens, my knock was greatly improved when I did that. Its also possible that its a rod bearing or a bearing other than the number one. Try the oil mix

I have been using only 10w-30 for the last three years. 10W-40 hmmm that's what the manual does not recommend...Should it be sintetic?

[Scotty]

Yes I know the manual does not recommend it, but its primarily due to fuel efficiency. That is the reason why I said 50/50 mix, you have a higher mileage engine at 133,000 miles (am I right?). I think you may be surprised

[adallak]

well... I would be really happy if oil change helps. Frankly I can stand the noise just want to make sure nothing wrong is happening to the engine. BTW I saw Havoline DexCool (5 years//150.000 miles0 in local WalMart. Someone complained he could not find it.

[adallak]

brought the temp to 200 degrees where the knocking noise is coming up, shut the engine, removed the belt , started again ---the noise was still there! The temp jumped to 250 just in a minute. I stopped the engine looked under the hood and the coolant was boiling and fountaining from the surge tank! Guess the mix was a little bit out of range

actually the mix could be fine. The temp sensor is by the thermostat, and despite I monitored the temperature through DIC and shut the engine down at 250 some parts of the coolant could reach higher temp because of absence of the flow and boil. I do not recommend repeating my experiment - the tepmperature climbs realy fast when car does not move and can get out of control.

[Scotty]

brought the temp to 200 degrees where the knocking noise is coming up, shut the engine, removed the belt , started again ---the noise was still there! The temp jumped to 250 just in a minute. I stopped the engine looked under the hood and the coolant was boiling and fountaining from the surge tank! Guess the mix was a little bit out of range

actually the mix could be fine. The temp sensor is by the thermostat, and despite I monitored the temperature through DIC and shut the engine down at 250 some parts of the coolant could reach higher temp because of absence of the flow and boil. I do not recommend repeating my experiment - the tepmperature climbs realy fast when car does not move and can get out of control.

Yes it could have continued higher when you shut the car off.

[adallak]

if it is not #1 bearing but say, valves, will they tapp in Neutral at 190-200 ?

As I said before there was no tapping/knocking noise in Neutral.

[Scotty]

if it is not #1 bearing but say, valves, will they tapp in Neutral at 190-200 ?

As I said before there was no tapping/knocking noise in Neutral.

OHHHH, I did not seen that! There is no tapping in NEUTRAL? HMMM, and still be affected by engine temp.... I can't imagine your engine mounts, hotdogs, flexplate bolts, exhaust outlets, being affected by engine temp. Look for exhaust system or component touching or grounding out against the frame, I am not sure why engine temp could affect that however. It could be an exhaust leak affected by engine temp. You have me stumped because it is affected by engine temp and it stops in neutral.... Could this be detonation, related to the EGR? I don't know if the detonation could be affected by DRIVE or NEUTRAL...maybe someone will brainstorm here.

[adallak]

OHHHH, I did not seen that! There is no tapping in NEUTRAL? HMMM, and still be affected by engine temp.... I can't imagine your engine mounts, hotdogs, flexplate bolts, exhaust outlets, being affected by engine temp. Look for exhaust system or component touching or grounding out against the frame, I am not sure why engine temp could affect that however. It could be an exhaust leak affected by engine temp. You have me stumped because it is affected by engine temp and it stops in neutral.... Could this be detonation, related to the EGR? I don't know if the detonation could be affected by DRIVE or NEUTRAL...maybe someone will brainstorm here.

yep..too many questions. I still cannot exclude the #1 bearing completely since I could not conduct careful experiment with the belt off. The temperature climbed real fast and coolant started fountaining from the surge tank. Well... looks like I need an assistant to release the tensioner while I am power brakeing. It is the best way to see the difference since you can repeat it many times without beeing afraid of overheating. It would be too bad if I had to get married just for the sake of #1 bearing .

[Scotty]

The number one bearing noise would not be affected by it being in neutral.

To see if she is the ONE, you will have to have her help you bleed the brakes! Taking directions is important, DOWN-UP, DOWN-UP, etc.

Here is a good date night for you! Drive to the darkest part of town, with no lights and park the car, leave the car running. Look at her lovingly, and tell her there is something you want her to help you with! Go around the car and open the door for her and take her hand and help her out. Slowly lift the hood telling her how important this is for you. Tell her to bend over carefully, and to stare into the engine compartment and see if she sees any sparks! If she is with you after that, she is the ONE! Anything else you do to create sparks well, that is totally up to you, but then this test is invalid for checking the ignition wires!