Hey man, sorry for the late reply but no, we haven't got time to change the pump/thermostat yet
Anyway I got a chance to ride in the car, and this is how it went:
-Cold start: Louder squeal than the one in the video I posted above back before changing the belt/pulley, seems to be more consistent.
-RPM dropped to 800: The squeak is less louder, but it still exist.
-Driving: When the rpm goes up to 2000, the squeal can be heard again until the engine has fully warmed up.
The funny thing is that it didn't squeal at all when we replaced both pulley/belt, it was buttery smooth/quiet until we cleaned the Electric Throttle Body, that's where the squeal returns again and sometimes a clang/clunking sound appears as well when cold starting it.
Honestly I have no clue why that clunking sound appears along with the belt squeak returning...

 

Thanks Fraser......having read the thread and your precise explanation and reasons for the squeak, I decided to just leave mine as is. The squeak goes as soon as the automatic choke knocks off and the revs drop.

 

I would not leave the belt squealing. It only needs to slip like that a couple of times and your belt is toast. They rapidly wear the tiny Vs which drive it and then sit on the top of the pulley and bottom of the grooves. If you are sure it is the belt squealing, and pulleys/belt are good, then first look would be your tensioner, The internals may be failing/jamming or it may not be mounted correctly. You will have to look at your belt to see if it is still functional. Look at the bottom of the grooves, if they are shiny you will need a new belt again. Don't put your new one on until you have checked your tensioner.

 

+1
Have always replaced the spring loaded tensioner with the belt, & idler bearing on my cars.

Both mine were approaching a hundred thousand with no issues.

But the thermostat, & water pump were being changed out.

So while i was there, i changed them as well

 

Well, for my case, all pulleys turned freely, the tensioner works normally too, so I have no idea why.
Honestly it only came back when me and my dad cleaned the Throttle Body (Which is on another thread called "Jaguar Idling Inconsistency" where this and even more troubles came up).

 

Well, we would have replaced the waterworks-related stuff too, but we don't have enough tools to work on it sadly .

 

If you are sure the belt slipping/squeeling, is making the noise and the belt is OK - no shiny parts at the bottom of the grooves - then the tensioner is still suspect. How did you check it? When you push down on one of the long runs of belt between pulleys it should have quite a high force necessary to get motion.
Is that the same belt that was on while you were cleaning the throttle body?

 

Yes, we did that, and when we were putting the new belt on, we had to use a lot of force to move the tensioner so that we could place the belt back to where it was, so yea, seems like it's working as it should be.
And the belt that was on is the new one we replaced when we were cleaning the Throttle Body.

 

Also just a bit of a note: We did cover things up with a few rags we had laying around, so any solvent should get adsorbed by it.

 

Since you covered the belt area, while cleaning, and there are no shiny spots at the bottom of the Vs - Are you sure the tensioner was not putting up a fight, while taking it off, because it is slightly seized? It then may be putting too little tension on the belt? Are you 100% sure the belt is the correct length? 1/2 inch can be enough to make it slip since the tensioner is near the end of travel into the weaker spring force.

 

The tensioner when we took it off, well it was slightly seized (Presumably 16 years of not moving) which was resolved after we put some lubricant (WD40) and getting it moved again (Custom built breaker bar made of a 3/8 inch breaker bar with a long tube inserted because it was a bit short).
The belt length is about the same as the original belt I believe.

 

That means your spring inside is probably rusted and will not have the required force to keep your belt tight enough.

 

Our cars usually are approaching a hundred thousand miles.

With a lot of expensive to have replaced wear parts, suspension is a big one!!!!

Expect to spend two to three thousand, or more, to put one right yourself.

But what you have when your finished, is an addicting pleasure to drive.

 

Well I'm really looking forward to final part, you know, not having to worry that the belt is squeaking loudly, not worrying that the car may stall (It stalled once when engaging the AC),...
For now, we're trying to figure how to fix all the issues it have currently.

 

Understood, just keep at it, and you will succeed
Just remember anytime you buy a used something or other.
Your buying someone else's problem.
That they either don't know how to fix it.
Or just don't want to deal with it, & trade it in.

 

Hi for which vehicle is this applicable? If not an XK8 where could I find same for XK8 2001?
Thank you
Mark

 

Well, it's been quite a while, so I thought I should give an update to the situation:
Yep, the culprit is in fact the belt tensioner (never replaced in 17 years), and after replacing it, the car runs extremely quiet when cold starting the engine up, and I notice that the tension on the belt is 2x stronger than the old one, which is really good .
Really want to say thank you to everyone who has helped/support me and my dad in our quest to fix this issue, and now, we can pretty much start the car up without grabbing anyone's attention (haha, the squeal is such a headturner).

 

Just read through the whole of the thread and congratulations on sorting out the problem at last, I hope you all enjoy the car very much

 

Same here and well done- thanks for the details and videos. Mine sounds just like that on startup and I now have a plan and a part number. I'm going to do both belts for good measure with the tensioner at a minimum, and will consider the water pump and t-stat while I'm at it.

 

Great intel there and looks like you were right. Thanks!