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My 91 Fleetwood 4.9 runs quiet after cold start in the morning until fully warmed up. The car idles as smooth as glass- oil was changed faithfully according to records. (There is an oil leak from the front timing cover.)
If I press the gas with my foot on the brake, as the load increases, the engine has a low knock. (It has 88,000 miles on it, I have owned it for 5k of those.) Are these engines prone to bearing noise-or failure? I am running 10-30 oil in it. Would 10-40 help? Is there any other common cause for engine noise when warm? Would any ignition difficiency cause this, or carbon in cylinders...or?
 

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We have the same car, what color is yours? The owner's manual says 10w30 is best, do not use 10w40. If the knocking occurs at around 40 mph, then I would say use premium fuel. It can knock if you use regular.
 

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My '95 4.9 DeVille says to use only premium fuel right next to the gas gauge on the dash. I wouldn't dream of using anything less. The '95 4.9 had a bulletin sent out to the dealerships from Caddy wanting them to change the main bearings because of some kind of fault in them, I'm not sure exactly what, but our previous owner changed the bearings in ours.I don't know if anything was sent out for your model year....They want you to use 10-30 oil because of the tight clearances the engines run. A 40 weight is a little too thick to try to squeeze through there. By the way, does it knock when climbing a hill while accelerating hard also? Or just while holding the brake and giving gas?
 

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Go4it,
I have owned three Cadillac pushrod engine cars. An 85 Edlo with a 4.1, a 90 Deville with 4.5 and a 92 STS with 4.9. The all knocked some exactly as you describe. I talked to the service manager at the Cadillac store and he said this was the main bearings and that it would not harm anything and there was a replacement bearing kit available but the cost was pretty expensive.

Here is a quote from my service manual acknowledging the knock and about some corrective action that was taken in late 91 and 92 to enhance the block and main bearings.

"During the 1991 model year the cylinder block used for the 4.5L and 4.9L transverse V8 engines received a series of structural enhancements that have made dimensional adjustments in main bearing clearance necessary to prevent main bearing knock (mallet noise) in some vehicles. These crankcase changes are not incorporated in all 1991 vehicles, but are included for all 1992 and later engines. "

I would just change the oil frequently and keep driving!
 

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Could it be spark knock?

4.x engines don't have an absolute crank reference (like a crank position sensor) and rely on reading the distributor. The ECM assumes a certain base timing, which is adjustable and needs to be set manually with a timing light. Then, it makes "relative" timing adjustments on top of the base timing.

4.x engines do not have knock sensors so, if the timing was too advanced (for whatever reason), the ECM would just keep happily adjusting timing to its preset parameters - it isn't going to retard the timing like more modern vehicles would.

Beyond that, the engines do have a front main bearing issue. This can be addressed with a service replacement bearing that has a smaller tolerance (there's a service bulletin on this, probably available at Alldata).
 

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Discussion Starter #6
Thanks for the responses. The knock seems consistent with main bearing noise. I'm using 10-30 as recommended, the knock is noticable even when normally accelerating from an intersection. Thanks for the caution on using 10-40; I was tempted to try it to see if it would reduce the noise. I can't even imagine what GM would charge for bearing replacement up here in Canada.
My Fleetwood is Dark Garnet Red- cloth matching interior. Great ride- but the engine noise has me worried!!
It burns no oil, although a slight leak from the front seal. The 4.9 is surprising for the power it generates-and the highway economy is impressive.
91'fleetwood FWD
 

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I don't believe the bearing itself is all that expensive. However, there would be quite a bit of labor involved, and its one of those things that, while you're in there, you may as well address other things.

You can also buy the parts through Sealed Power. Significantly cheaper than GM, but may or may not be as good.

Depending on how easy it is to get the oil pan off (some Cadillacs have the front exhaust routed under the pan, others route it over the top of the transaxle), you might be able to check the bearing clearance yourself, especially if you have the factory service manual. All it requires is time, plastigauge, a torque wrench and metric sockets. The manual will tell you if you can simply pull the cap and gauge it. I suspect you could if you kept the tensioned serpentine belt attached to the crank pulley - that would be a pretty accurate measurement IMHO.
 

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Do a search... Someone posted (I think P.P) something about this, and it is normal i think.....
 

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go4it said:
My Fleetwood is Dark Garnet Red- cloth matching interior. Great ride- but the engine noise has me worried!!
It burns no oil, although a slight leak from the front seal. The 4.9 is surprising for the power it generates-and the highway economy is impressive.
91'fleetwood FWD
I have a 91 DeVille and I have the Identical worries you do. Mine is noticable at take off speeds up until I reach about 35 or 40 and then it fades away. It kinda causes me to hit the gas gently instead of driving like I would my other cars.

ipncjh1 said:
........"During the 1991 model year the cylinder block used for the 4.5L and 4.9L transverse V8 engines received a series of structural enhancements that have made dimensional adjustments in main bearing clearance necessary to prevent main bearing knock (mallet noise) in some vehicles. These crankcase changes are not incorporated in all 1991 vehicles, but are included for all 1992 and later engines. "

I would just change the oil frequently and keep driving!
So you think this thing isnt a big problem?? Is yours doing this??
 

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I have an 87' Coupe DeVille with the 4.1 and noticed the same noise at about 86,000 miles. I'm now at 105,000 miles and still running strong for now. I was a little cautious for a couple thousand miles after noticing the noise, but after that began to drive as normal. I faithfully change the oil using 10w-30 every 3,000 miles and no problems so far.....
 

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In general, you are all describing the main bearing thump that is not too uncommon on the 4.9... It is the crank moving up and down in the front main bearing due to just a .0001 too much clearance....LOL. The basic reason the engine is sensitive to front main clearance is that the serpentine belt pulls the crank up and the combustion loads push it down....thump...thump...thump.... Completely harmless but annoying.

You can confirm that this is the problem by loosening/removing the serpentine belt so that there is no side load on the crank pulley and run the engine. If the noise disappears then that is the problem. Often you can easily duplicate the noise by getting the engine good and hot and have a helper LIGHTLY power brake the engine against the torque converter while holding the brakes. Once you can duplicate the noise then just use a breaker bar in the square drive lug on the belt tensioner to loosen the belt so that it slips. The noise will suddenly go away with the relaxed belt tension if that is your problem.

An easy possible solution is to switch to a slightly heavier oil such as the 15W40 Delo/Delvac/Rotella brands of heavy duty diesel oil available in gallon jugs at discount stores most everywhere. That is a preferable oil for the 4.9 anyway since that engine still has a distributor gear on the cam that appreciates the extra antiwear package in the HD oils. The extra viscosity will help the main bearing thump.

There are service front main bearings specifically released for the service of units with this issue and they are available at any Cadillac Dealer. It requires dropping the pan and replacing the front main bearing. It can be done with the engine in the car and is not too difficult but a bit time consuming.

Try the oil first and if that quiets it down then forget about it. In any case, it will not hurt the engine in the least (the main bearing thump) but it is annoying in some cases.
 

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********* said:
Try the oil first and if that quiets it down then forget about it. In any case, it will not hurt the engine in the least (the main bearing thump) but it is annoying in some cases.

Thanks, it all sounds comfoting
 

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I did a switch in oil.
I couldnt find 15w40 so I used 20w-50. I still hear the same noise but Im not sure if its just as loud. maybe its gotten a bit quieter. Ill have to drive it at night like usual to see if theres a difference, because driving in the day I cant tell competing with others noises in the streets.
 

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Nice!
I have a 94' Deville and have had that same problem for about 30k miles now. I knew it was from the front main bearing but didn't know it was benign. Thanks. I have 2 questions though, since the noise is actually getting rather loud by now:
1 - Can I help the problem with just oil in my model car?
2 - Is there a replacement kit for my model car? Any ideas how much that'd be?

Thanks.
 

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omarietta said:
Nice!
I have a 94' Deville and have had that same problem for about 30k miles now. I knew it was from the front main bearing but didn't know it was benign. Thanks. I have 2 questions though, since the noise is actually getting rather loud by now:
1 - Can I help the problem with just oil in my model car?
2 - Is there a replacement kit for my model car? Any ideas how much that'd be?

Thanks.
Try the 15W40 oil or even a 20W50 to see if it helps. You'll just have to try it and see. Hard to predict but it is directionally correct. I wouldn't use the heavier oil during the winter if you live in a cold climate as it will cause slower cranking speeds on cold starts. If it is a moderate climate then there would be no problem.

When the engine is hot hold the brakes, put it in drive and lightly load the engine against the trans (power brake it) to see if you can duplicate the noise in the driveway. If so, then loosen the accessory belt to see if the noise goes away. If the noise dissappears when the accessory belt is loosened then it is the front main bearing thump as described. This is a simple and easy check to see if that is the problem.

Yes, there is a service main bearing for the car. Only the one main bearing is replaced....just the front one....with the special bearing. The bearing can be replaced with the engine in the car as the oil pan can be removed with the engine in the car and the front main replaced. I have no idea how much is costs to have done..... Only the Cadillac service parts has the special service front main for the main bearing thump. Aftermarket bearings will not have the special clearance and special shape required for the main bearing thump fix.
 

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********* said:
Try the 15W40 oil or even a 20W50 to see if it helps. You'll just have to try it and see. Hard to predict but it is directionally correct. I wouldn't use the heavier oil during the winter if you live in a cold climate as it will cause slower cranking speeds on cold starts. If it is a moderate climate then there would be no problem.

When the engine is hot hold the brakes, put it in drive and lightly load the engine against the trans (power brake it) to see if you can duplicate the noise in the driveway. If so, then loosen the accessory belt to see if the noise goes away. If the noise dissappears when the accessory belt is loosened then it is the front main bearing thump as described. This is a simple and easy check to see if that is the problem.

Yes, there is a service main bearing for the car. Only the one main bearing is replaced....just the front one....with the special bearing. The bearing can be replaced with the engine in the car as the oil pan can be removed with the engine in the car and the front main replaced. I have no idea how much is costs to have done..... Only the Cadillac service parts has the special service front main for the main bearing thump. Aftermarket bearings will not have the special clearance and special shape required for the main bearing thump fix.
Would this same front bearing be responsible for the cold start knock I hear when I start my 91 Deville 4.9 on cold mornings? The knock will last several seconds. Hell of a thing for GM to do to such (otherwise) wonderful cars!
 

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omarietta said:
Nice!
I have a 94' Deville and have had that same problem for about 30k miles now. I knew it was from the front main bearing but didn't know it was benign. Thanks. I have 2 questions though, since the noise is actually getting rather loud by now:
1 - Can I help the problem with just oil in my model car?
2 - Is there a replacement kit for my model car? Any ideas how much that'd be?

Thanks.
Just got off the phone with cadillac service dept. $350.00 to replace old front main bearing with the replacement bearing. I am taking mine in! Can't stand that thumping a day longer.
 

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robtpala said:
Just got off the phone with cadillac service dept. $350.00 to replace old front main bearing with the replacement bearing. I am taking mine in! Can't stand that thumping a day longer.

Just make sure that that is the problem.....

To verify, remove the serpentine belt load (remove the belt or unload the tensioner) and start the engine to see if the thump disappears.... If it is the chronic main bearing thump it will disappear with the belt load removed as the side loading of the front main by the serpentine belt is what causes it when the front main clearance is on the high side... This is a very positive test so it would be wise to do it first.

Otherwise, that sounds like a fair price for the job at a dealership. They can drop the pan and remove the front main with the engine in the car.
 

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********* said:
Yes, there is a service main bearing for the car. Only the one main bearing is replaced....just the front one....with the special bearing. The bearing can be replaced with the engine in the car as the oil pan can be removed with the engine in the car and the front main replaced. I have no idea how much is costs to have done..... Only the Cadillac service parts has the special service front main for the main bearing thump. Aftermarket bearings will not have the special clearance and special shape required for the main bearing thump fix.

wait a second , do i also need to drop the crankshaft? ...cant figure out how i would get that bearing out without dropping the crank ....unless its some kind of one peice deal that slides out horzonatllly ....

If all i have to do is remove the oil pan ,main bearing cap and maybe the harmonic balancer then ill do it at next oil change .....donesnt sound like a big deal at all ....man itd be nice to have a silent engine again ....delvac is only keepin it quiet at cold engine temps....
 

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The main shell will rotate out with the crank in place. Take the front main cap off and use a small screwdriver and mallet to tap on the edge of the upper main shell that is on the opposite from the locating tang. It will loosen and rotate around the crank. Slide/rotate it on the crank main 180 degrees and it is in your hand. Rotate the new, service bearing into place the same way. It really isn't tough to do at all. Pulling the pan is the hard part. To remove the pan in the car you have to rotate the crank so that the counterweights will clear so there is some trial and error involved but it is not that difficult.
 
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