I purchased a 1991 Cadillac Deville with 77,000 miles. A week after I bought it, it started knocking. I took it to my mechanics, they said it needed main bearings. They installed new main bearings and new oil pump. They didn't want to do this because of warranty issues. They told me to take it easy on the car for 500 miles. Drove car and it's still knocking. Tried heavier oil to no avail. Still knocks. Knocks only when engine is warm and only on take off or brake torque. I am at wits end with this problem. Should I replace the motor or drive it the way it is. Car is from Florida and is in absolute mint condition. Any help would be greatly appreciated.

Sounds like you are a victim of "main bearing thump", a common problem with the 4.9 and while annoying, it is harmless. Ask the mechanic if he put in an OEM bearing or a service bearing. The service bearing is slightly oversized to cure the problem. They may have used the wrong one. Also try Rotella 15W40 oil. Do a search for "main bearing thump". There should be lots of info to put your mind at ease. No need to replace the engine.

Knoch at slight throttle when engine is hot is a clear indication of EGR problem. Try to clean up the EGR tubes inside the throttle bodey, replace the EGR valve. Check out all vacuum tubes. This knock is harmless though.

I purchased a 1991 Cadillac Deville with 77,000 miles. A week after I bought it, it started knocking. I took it to my mechanics, they said it needed main bearings. They installed new main bearings and new oil pump. They didn't want to do this because of warranty issues. They told me to take it easy on the car for 500 miles. Drove car and it's still knocking. Tried heavier oil to no avail. Still knocks. Knocks only when engine is warm and only on take off or brake torque. I am at wits end with this problem. Should I replace the motor or drive it the way it is. Car is from Florida and is in absolute mint condition. Any help would be greatly appreciated.

what kind of knock are you hearing.. is it a thumping knock that increases as the rpms increase? or is it pre-ignition knock(pinging)? i dont care what the other guys say, the 4.9L engines do have main bearing noise issues, but it shouldnt be ovewhelmingly loud.. if it is, chances are it is a bad rod bearing, since you had the mains replaced already. if this is the case, it will only be noticeable when under a load, not in park.. have a friend help you out, let the car warm up, and have him hold down the brake hard while stepping down on the gas pedal, to try and recreate the sound..remove 1 spark plug wire at a time from the distributor cap, always keeping 7 of them hooked up... if the noise lessons or goes away, chances are you found yoru bad rod bearing.. make not of which cylinder bank that wire was running to, and then have the mechnaics replace the rod bearing on the correpsonding connecting rod,,

try all the EGr stuff first, but if it is a loud thumping type of knock that increases in frequency with the engine rpms, i doubt its an egr problem...
ohh. and use ONLY the oem EGR valve replacement.. do NOt use an aftermarket EGR valve, they are complete garbage. ive NEVER had a good experience with one..

Sounds like you are a victim of "main bearing thump", a common problem with the 4.9 and while annoying, it is harmless. Ask the mechanic if he put in an OEM bearing or a service bearing. The service bearing is slightly oversized to cure the problem. They may have used the wrong one. Also try Rotella 15W40 oil. Do a search for "main bearing thump". There should be lots of info to put your mind at ease. No need to replace the engine.

am I assume that this is the number 1 main bearing? do you have the part numbers for the service bearing AND the oem bearing numbers?

cob,

After checking out the EGR tubes you can bypass the EGR solenoid and connect the vacuum tube from the Throttle Body directly to the EGR valve. You'll get nearly stall condition at idle but will be able to restart the car and drive. If the engine is hot (190 plus) and you do not have knock at slight throttle or poerbraking (say, taking off from a stop) it is the lack of EGR because of aged EGR valve not beeing able to open enough.

stickpony,

just like the main bearing knock the knock caused by lack of EGR is very common among 4.9 motors, and I really do not care whether you care or not what Ranger or I say. LOL

what kind of knock are you hearing.. is it a thumping knock that increases as the rpms increase? or is it pre-ignition knock(pinging)?
I am having a similar problem with my 92 deville. My knocking is defninitely a pre-ignition type of pinging that only occurs when the engine is hot and about mid-throttle. The pinging is random and does not increase with RPM. The fuel injection system has been serviced with no improvement. Does the 4.9 have a knock sensor that adjusts the timing when the engine gets warmer? I always use premium grade fuel and the engine only has 56K miles. Believe it or not, but the miles are mostly highway miles.

Any fixes to pre-ignition pinging would be greatly appreciated.:banghead:

#1 cause, EGR system plugged under throttle body (very common), bad EGR valve (not very common)
#2 cause, wrong timing set (10 degrees for 91 octane, 6 degrees for 87 octane, as per the 1991 Cadillac FSM)
#3 cause, bad thermostat (verify temp with P.0.4)
#4 cause, clogged radiator (verify temp with P.0.4)
#5 cause, fans not operating (verify temp with P.0.4)

None of the 4.1L, 4.5L, and 4.9L's have a knock sensor.

Preignition would ruin your engine in seconds. Do not confuse preignition with spark knock. The latter is annoying but harmless. Pay attention to NODIH's post. #1,2 are most likely the problem.

Try GM engine oil suppliment and mobil one devlac 15w40 before attempting main bearing replacemnt ....it made my thump go away ....and is allowing me to put off the rebuild a little longer ...

i have the same exact car as u... it has nothing to due with any of the things that people are saying. its the comprestion of the 4.9s, just use the high tess gas 93oc. is the reading by me. the pinging is cause due to the compression being to high and the intake lobes not being machined finally enough in the 4.9 to handle the low octane gas.....try the high tess gas and tell me what happens..

Well...If the rod bearing have been replaced corectly and with the proper units, it must be a wristpin knock. But if it for sure knocking, you may want to see what bearing units were used to replace yours, if they were standard, oversize, or the service ones, they did have a few problems with this, and if your average joe-schmo or redneck mechanic just threw in some basic parts house bearing, it wont solve a bearing problem, so once you're sure you've got the correct bearings, it could only be where the rod attaches to the piston via the wrist pin. Are you sure your fuel system is pure and operating correctly, if you have a cylinder that fires on a parial fuel charge or no fuel charge, it may tend to get a spark knock. The octane, egr, and compression all have to do with a higher pitched ping sound.