This week, I started my X308 and heard a thumping sound. The serpentine belt was causing the tensioner pulley to be rocked back and forth (first video) and the belt to move all over the idler pulley. While I was trying to diagnose, 2 minutes later everything smoothed out (second video).

Video#1

Video#2


I then took the car out for a test drive and while not thumping, the serpentine belt squealed but only when I was accelerating. Approximately 2 miles later everything smoothed out and was quiet.

Other facts - I checked by hand each of the pulleys, they rotated well and none of them were loose. I did remove the idler pulley to check and it was well seated.

While driving, the car never went into restricted mode, the temperature remained normal, there was no fluids dripping (i.e. water/coolant),and steering was fine. A/C on or off did not make a difference. I tried the screwdriver stethoscope method, but none of the components were registering any unusual sounds (clicking, etc.). The belt has no noticeable wear or cracks.

I had replaced the belt, the tensioner pulley, and the idler pulleys with new parts ten months ago. There have not been any issues until this week.

New parts used from amazon:
Gates K070763 Multi V-Groove Belt
ACDelco 39049 Belt Tensioner and Pulley Assembly
Dayco Idler Pulley Part#89016


Any suggestions to help determine the culprit are appreciated.
Sincerely,
Tom F.

 

Tom, have you left the car stood for a while? I noticed a similar issue when starting a car after it being stood for 4 months....a new belt sorted it.

 

That was also my first view, a hard angle in the belt.

 

Sean & Eric,

Brilliant! My sincerest note of gratitude!

I had kept the old serpentine belt in the garage since it still looked to be in good condition. After reinstalling the old belt, my X308 is purring beautifully.

Based upon this I cannot recommend the Gates K070763 Multi V-Groove Belt.

A few points for anybody that has a similar problem. Except being slightly thicker than the previous belt, the newer belt had no noticeable differences, including signs of kinks or sharp angles.

It looked good and a visual inspection might not be enough.

The car is driven approximately 2 times a week, but perhaps because of the cold weather it made the bend/flaw more pronounced.

Without your diagnosis, I would been going after imaginary problems.

Thank you for your help.
Tom

 

Glad it seems you got the mystery solved.

I got a Gates belt (2 years old, driven once every 2-3 weeks) and have no issues whatsoever.
Maybe winters are not really existing here, that might be the difference.

 

If the problem recurs, replace the idler pulley (or its bearings). Most likely the new belt will stop thumping if the idler was replaced.

 

I think that is what he did already.

 

I seem to remember someone having a problem like this that was caused by a loose power steering pump or bracket.

 

That belt shouldn't fit. It's 76.3 inches in circumference, while the correct Gates belt (K070922) is 16 inches longer.

EDIT: Gates K070916 is also listed as fitting. Regardless, the smaller belt should not fit, unless a pulley was left out of the loop (A/C pulley?).

 

Just bumping this topic as I've run into similar issues.
My VP needed a new tensioner. I ordered the superseded parts (tensioner NCA7739AE, pulley C2C22758) from Jaguar Columbia and the belt that they had listed to go with these was NCA2211AC (aka Gates K070916).
After getting those in, the rough idle did not get any better, and it would make the slapping noise on start up and take a minute to tension properly.
Looked through these topics and went out and got Gates K070922 (Jaguar NCA2211DA) which was very easy to find, at O'Rielly's. Got that in there and no more slapping noise or rough idle, whole car is running much better now.

 

Does anyone know why there are two belts listed (in the UK catalogs) one for steel idler pulleys, and one for plastic pulleys. What's the actual difference between the two?
Paul

 

The Parts Catalogue lists:

Steel Pulleys: NCA2211AE (superseded by NCA2211DA), 7PK2342 - 2,342 mm Long - supposed to be from VIN F00035
Plastic Pulleys: NCA2211AC, 7PK2322 - 2,322 mm Long - supposed to be up to VIN 878717

However, my SC engine (VIN up to 878717) has steel pulleys but the belt is 7PK2325 (2,325 mm long, close to the Plastic Pulleys belt length). In any case, there are two belts, longer and shorter, which is due to the different diameter pulleys used. It seems that it is better to go by the VIN in the belt length selection (if the pulleys have not been changed) or just see the length (like 7PK23xx) of your old belt.

 

Not sure if I caused a problem then? I damaged the plastic one fitted to mine some time ago, got a 2nd hand replacement which, was a metal one, but I used the same belt?

Should I replace the belt or pulley?
Paul

 

Maybe the tensioner arm design changed over time, but the pulley's are the same diameter for the plastic and steel ones.
I exchanged my plastic ones for steel from a later XJ8, but not the tensioner itself, and kept the belt.
No issues.

 

My steel pulleys (smooth), idler and tensioner, are both 76 mm diameter and the belt is 7PK2325, matches very well. I do not know the diameters of plastic pulleys but, if they are a bit bigger, then they would require the other, slightly longer belt (7PK2342). In any case, the correctness of the belt length can be verified by checking the marks on the tensioner arm.