1998 XJ8 Blue

When I used to COLD START (about 6-8hrs being off) my car there was always this faint knock (about 5-6 months actually) ONLY until engine got warm. The more days it sat usually the louder it sounded when turned on.

About 3 weeks ago I went out to start it (been below 40F here since) and there was a really loud KNOCK KNOCK KNOCK, my old BELT tensioner actually started moving completely, moving from side to side smacking the metal when the belt would run over it. Immediately turned it off. Figured it failed since it was original. (it was not lose or anything, completely tightened) no clue why it gave up ive stlill got it in a box.

I got a brand new one, mechanic installed. Worked great, completely silent. Not even the faint knock that I used to hear for months (which confirmed that it was the slightly moving belt tensioner on overnight cold start the whole time) It even took away the annoying belt/pulley squeal It had on coldstart if below 50F while warming up. (i've already replaced idler pulley before as well squealed like banshee lol.)

Today it was 4F. It's probably like -10 below with the windchill, but I had to get out. Went out to start it, same thing unfortunately. A loud knock knock knock, left side of the engine, I remembered the noise. Lifted hood and yes brand new belt tensioner was moving again. Very sad stuff :P After about 3-4 knocks it stopped (guess it 'set' or warmed up or something?) took it right to mechanic, Everything appears ok, this new belt tensioner has only been in the car for 2 weeks and about 300 miles so whats up?

btw only thing I found out to get rid of the belt squeal last year was what someone else stated, a very small dab of di-electric boot grease on the belt (works for about a month here between applications)

1998 XJ8 blue - WholeTiming system replaced 2008 to 3rd gen Metal, New alum thermostat housing, New valve covers

* Edit - I just replaced the belt February 2008 with this Dayco poly cog belt below:

http://www.autopartswarehouse.com/de...85c7ab9e&gan=1

I did have a very small coolant leak for about 6 months through the summer last year that dripped coolant onto my drivebelt as it went around the pulleys, I wonder if it soaked into the belt overtime causing issue? I see the belt tensioner is only pulley that can actually pivot aside from rotating. Im curious if it's posisble for the di-electric grease I put on the belt over time (a dab every month or so during the winter) to actually bond the belt to the tensioner pulley from the bitter cold start somehow?

Could a bad harmonic balancer actually cause my belt tensioners to start moving on cold start? Thats whats causing the knock, the belt literally moves the tensioner (when its at full tension/completely tightened) as the belt goes around i can watch it happen. it has only happened twice now, and it was on days below 20F. The tensioner must have just been moving very slightly before when the noise was faint. I want to add that aside from the newly moving knocking belt tensioner on cold start, there is no vibration in the engine at idle or anything

 

Could it be the belt and not the tensioner? Stiff when cold?

 

This is just a guess, but check the harmonic balancer for cracking. If the rubber is beginning to split, you may have some movement due to that. You can mark the inner metal and outer metal rings with a paint pin in a solid line from the outer lip to the inner metal ring. Then check it latter to see if the line is solid or if it has moved.

 

Sure sounds to me (as test point mentioned) as if the belt has cold flowed and stiffened to take on and hold the curves of any pulleys it has been in contact with, and until warmed, when those bends pass over the tensioner, it bounces as described - the curve in the belt overcomes the spring tension causing it to bounce. This would be similar to flat spots on tires when very cold.

Did you replace the belt also, or just the tensioner?

 

try replacing the belt

 

No sir, thats the one thing I've yet to replace since the belt is less than a year old. It seemed like a good belt, $20 more than all the others, had great reviews. Is this a common problem on all vehicles that are not garaged during the winter?

That would also indicate that my old tensioner really isn't bad either, since this one did exactly the same thing. It must be the belt because it only actually moved when it sat for a couple days in the bitter cold (assuming the belt is really taking and holding the shape the longer it sits) I used to drive it daily so I guess it never really got to sit, now I only drive it about 3 times a week. I do start it up and let it come to op temp every other day though, so it never sat more than a week since I got it.

* Now If its just the rubber is cold and holding shape, can't I just drizzle some blazing hot water over the belt tensioner pulley area then turn it on to test? (its above freezing today) That would definitely loosen up the rubber there. If that relieved the cold belt, and it doesn't move tensioner, that would confirm that its in fact the drive belt just failing out.

I guess ill give it a shot! Thanks will update when its put on.

 

Yes...went out check it this morning, wasnt really stiff or what not.

I ran some hot water over the whole belt in the front, started it up, absolutely no problems. Belt tensioner in place no movement, no noises of anykind. Its actually 8F now so yeah, I guess this belt didnt like the cold? hah... no clue, anyways new belt purchased will install soon enough. I guess we'll see how long this one lasts!

 

It seems that we both have the same problem only mine is on a 99xk8. Shame on Tom not to tell you or me that!!!! Take a look at my post https://www.jaguarforums.com/forum/s...ad.php?t=29520 I replaced the belt for the second time the other day and still have the problem. I am going to replace the tensioner and idler tonight and see what takes place. I will let you know!

 

I have the same issue on my 98 xj8. On a cold start the tensioner pulley moves and makes a clicking sound as it moves back and forth. It does it 2-3 times. From the look of it both pulleys have been previously replaced as they are not original as they're the updated kind. the belt is new.

From what I can work out the belt is not perfect over the idler pulley - it's not dead centre so basically the tensioner is kind of doing its job bringing it into line as it runs and settles.

I've bought the bits to to do a replacement and we'll see if it goes away when i get it done.

 

You can certainly have a failed component - like a tensioner .... but I will tell you from personal experience that aftermarket belts can cause problems.

We stopped using them years ago. When we had a fleet of Mercury Sable V6's we learned that you can actually have slight differences in a specification for a aftermarket serpentine belt -- it explained a lot of issue we had over the years on many makes. Some of the belts are just a bit longer or shorter -- or a tad thicker or thinner ... you can not tell unless you have them sitting next to one another.

Surprisingly we had the most trouble with belts from Gates -- Especially the conventional ones on Toyotas and Nissans.

No more aftermarket belts for us.

Also -- Clean up the pulleys and NO belt dressing!

 

On mine it is an after market belt - so maybe it's just that. It makes sense when you think about it. The jag belts are not anymore expensive in the uk so I may have to take advantage of my parenets UK address to get a jag one.

Thanks for the tip.