Just put a new belt on our 2005 XK8 and it is thumping like crazy. Looks like it wants to jump off running back and forth on idlers making the tensior pulsate. It's a Gates belt but understand Dayco made the original.
Have any of you had this problem? If so, which belt did you end up using?

Jack
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I would take the belt off and check the pulley bearings for looseness or difficulty in turning by hand.

also check the belt over closely for stiff sections or other anomalies.

The Gates belts are less prone to squealing than the Dayco ones so you made the right choice. It sound to me as if something is not right with the installation, either the routing or the belt is hanging one rib over the side on one of the ribbed pulleys. Are you sure that the belt is the right length? You should have to fully tension the tensioner pulley with a long spanner or socket into order to slip the belt over the final pulley (alternator or whichever depending upon how you installed it). If you were able to install the belt by simply pulling the tensioner by hand with no tools then your belt is too long.

It could also be a driven component failing just as you installed the belt but that would be an unlikely coincidence.

Richard

I checked the idlers, and belt length was checked at parts place. I marked the tensioner arm before removing the old belt then again with new, it hardly changed at all.
I only changed old belt while I had access because the old belt is 21 yrs old. The car only has 34,000 mile on it.
I did read somewhere else that another person had exactly the same problem with The gates belt.
I will check it all out again as I remove it next week.
Thanks for the help.

Thanks
Jack

2006 XK8 built in May 2005....

When I had the alternator refurbished in July 2018, I used that opportunity to install a new Gates K060910 serpentine belt from rockauto.com. It was a perfect fit and has never given me any problems....

Belt part number ?

Gates Micro-V K060910
13/16" x 91 5/8" OC
20MM X 2329MM OC

my car is a 2002

your belt has six ribs. The gates belt on my 2002 has seven ribs and the part number is K070922

So according to RockAuto (infallible??) the 4.2 cars use a different belt

^^ For some reason, Jaguar appears to have reduced the belt width on the 4.2 to 6-rib from 7.

cheaper ?? Nah, they wouldn’t make a change purely for the money

No idea, but it means that all the ribbed pulleys must be also different from the 4.0L to accommodate that change. Seems like a step backwards, but I presume there's some other driver apart from cost??

Our 1997 4.0 XK8 is 7 rib whereas our 2006 4.2 XK is 6 rib as I've changed both at one time or another. I don't know whether this helps the OP but it would be impossible to remove one type of belt and replace it with the other as the difference between them is so obvious that you would spot immediately that something was wrong.

Richard

I just went out to the garage to check, old and new belts have 6 ribs. Grooves in old belt seem to be deeper, could be wrong as bifocals don't allow me to see the one on the car that clearly. I will put in on the lift tomorrow and check it all out.

Jack

No progress on belt. When first started cold the belt looks like it's trying to jump off the alternator pulley. looks like the belt has a knot on it going across the pulley but it doesn't. The pulley is running true . After about 30 seconds the belt runs great and doen't act up again until after cooling off all day when it will do it all over again.The routing is correct also.
I do however think it is a defective belt. Never a problem until I installed the new belt.

Jack

It does appear to be defective if it works when warm.

Do you still have the old belt ? If so reinstall it and you’ll have the comparison needed. Otherwise, get a new belt.

I've never heard of anything like that before but a faulty belt sounds to be the most likely explanation.

Is it only the alternator pulley which seems to affect the belt? If so, that could be because the alternator is a small pulley with a larger percentage of wrap-around and the alternator usually imposes a higher load on the belt when the engine is cold so any problem with the belt internal structure will probably show up on that pulley.

The other alternative is that there is a problem with the alternator but I would expect to hear noise from the cold alternator and you could test it by removing the belt and spinning the alternator pulley by hand to check for resistance. Replacing the original belt, if you have kept it, would be worthwhile.

Richard

I do still have the old belt and plan to re install it for comparison probably next week. I've never seen anything like it.
I have a 1989 ford F150 that had a very old and frayed belt so decided to change it. The first new belt lasted maybe 20 miles and the second new one lasted but a little longer. after a good look I found the tensioner swing arm tilted about 2 deg leaning the pully back toward the engine.
I went to the local scrap yard and found 2 others made the same way and one that was straight. I got the straight one and it's been fine ever sense. The previous owner had apparently installed the wrong tensioner. The old limber belt ran fine while the stiffer new belt revealed there was a problem. I can only see movement at alternator pulley and only for 30 to 40 seconds then it is fine. My son cranked the car while on lift so I could observe it. There could be a stiff place in the belt as someone suggested that limbers after several revolutions. I'll look for problem at alternator first.
The new belt in this situation may be telling something isn't right. The idlers felt like new, roll smooth and no loose bearings.
I just dread draining the coolant and removing the hoses. I have very large hands and arms and there is no other way that I know to get it off.
Hope I only have to do it one more time.

Jack

That is interesting. I've changed the belt on the 4.0 XK8 and on the 4.2 XK and didn't need to remove any coolant hoses.

Richard

Please tell me how. On this 2005 you can hardly even see the belt much less touch it.
Thanks very much.

Jack