Hi all, could use some help in diagnosing a disturbing engine noise on my 2000 XJR. Last month I have noticed a mild "rattle" from the motor when under heavy throttle. It would occur briefly at a very specific RPM range, maybe around 3000, then disappear, and was present whether the car was hot or cold.

Unfortunately I didn't pay the sound much attention as I was preoccupied by the massive leak from one of the two heater hoses beneath the supercharger

I removed the supercharger to repair the hoses and inspected it while off the car. I did some substantial work to the top of the motor a few years back and, in the process, installed a smaller pulley and serviced the supercharger. While it was off this time, I checked the oil, which seemed fine, inspected the gears (there was no debris or indication that the back bearings had broken), verified that the pulley has no play, and the unit spins freely by hand. Everything seemed A-OK so I put the car back together.

Unfortunately now the sound seems even louder. It does not occur at idle, again, only at a very specific RPM range, and seems quite clearly to come from the supercharger.

I hate to guess but I am thinking the snout bearings have given up.

Any other ideas how I might isolate the problem? Or, perhaps, info on replacing the front bearings? I am loathe to remove the supercharger again ... the throttle body elbow is a royal pain.

 

Have you checked the secondary tensioners? It could be the plastic body rattling?

I am not overly familiar with the supercharged version of this motor.

 

Take a long screwdriver (or stick), put one end against your ear, and the other on the motor. Run the engine to the offending speed and move the motor end of the stick until you find the noise.

 

Remove the supercharger drive belt and try driving it without it. If the noise disappears, that would point to the supercharger itself (or the pulleys on the drivebelt).

I have heard of weird noises coming from the supercharger bypass valve if it has failed.

 

@Bcrary3 I replaced the tensioners, guides, and chains when I did the heads a couple years ago, so I'm very hopeful that's not the problem ... !

@jim Thanks for the tip. I remember my dad showing me that when I was a kid and thinking he was insane.

@Mark I'm at work but that's what I plan to do when I get home, thank you.

I went out at lunch and I'm able to reproduce the noise just before the rev limiter kicks in, so it seems to be around the 2800- 3200 RPM range. Definitely sounds like it emanates from the supercharger. Wouldn't burnt front bearings be a persistent rattle, or is that just wishful thinking because I am NOT wanting to remove the SC again this soon.

 

I've had SC related bearing warbles & whines in previous vehicles that would only surface @ certain RPMs, just throwing that out there for you.

 

I have replaced the front snout bearings and rear needle bearings and mine is still loud. The noise however is constant at all rpm's and is not a rattle. I am hesitant to change the gear bearings and needing to align the rotors. For now I am just going to live with it.


As Mark suggested remove the belt to the SC and drive it (enjoy the Prius type acceleration). If the noise is gone then at least no worries about the engine.

 

@King Thanks for verifying. I was just just grasping at straws

@Rocklandjag Do you have any references you could point me to for replacing the bearings? I have JTIS somewhere on a disk but lately the PDF extracts I have found for certain repairs were more convenient.