I replaced the original secondary tensioners a few weeks ago with the new, all metal type.

It seemed to be OK for a few days and then the engine developed this thump . . . thump . . . thump . . . thump on start up. There is no sound of chain rattle.

The longer the car sits the more the number thumps occur. If the car sits overnight it will thump about 3 times. If the car sits for 3 days it will thump about 7 times.

It is coming from the front of the engine but I have not been able to localize if it is Bank A or Bank B. It stops so quick it is hard to figure it out.

I am assuming that it is the oil pressure going up in one of the newly replaced tensioners or maybe a VVT unit.

I did a test last week where I rev'ed the engine up real high a few times and this seemed to stop the thumping for a day or two.

I am seeking a little input before I took the whole thing apart. It sounds like the oil is leaking down from some charged unit. Does this line or reasoning make sense?

Thanks for any comments.

 

This is not the chain tensioner.

Shine a light on the serpentine BELT tensioner in front of the engine. Is it jumping in time with the thumps?

If so, your belt idler pulley is the most likely culprit, and the belt tensioner itself is also a possible culprit.

 

Doesn't sound like either the VVTs or the metal tensioners pumping up: use a long tube or mechanics stethoscope up front.

Someone noted a while back that the plastic tensioners were a more sophisticated design than the third gens, but not your sound.

Does it run and idle well after it starts?

 

Runs 100% fine. Even during start up.

And with the pulley suggestion, would that not be constant? And the number of thumps are related to how long the car sits.

I don't know why I am stuck thinking it is a charging up issue. You can detect a swish-thump if you watch the video again.

I will try to use my stethoscope but the problem is that the number of thumps for an overnight shut down is 3 and that happens in 3 seconds so I don't have much time.

Thanks.

 

I have seen this precise problem before and it turned out to be the idler pulley. On the one I had seen it seemed to happen after the car sat overnight. I theorized that the bearings were failing and they hang up when the grease is cold.

Just have someone crank the engine and observe the belt tensioner under a light after the car has sat a while. It is a quick thing to check.

 

You were right on here. I wanted to see if I could localize which side the noise was coming from so I climbed up on the engine and had my head situated so I could determine this. I also had a bright drop light.

On start-up I could see the mono-belt slip slightly forward on the alternator pulley and ride up on the pulley's forward lip. It would then slip/snap back down. The "swish" was the ride up and the "thump" was the belt snapping back in place.

I did change the belt to a Gatorback when I did the tensioners a few weeks ago. I also replaced the idler pulley (it was available at aftermarket part store for $ 20.00) and cleaned and repacked the bearings on the belt tensioner pulley.

I thought I had read that the Gatorback was the original equipment on the car. That Gatorback came as a spare when I got the car a couple of months ago. Note that the belt jumping DID NOT occur until a couple of weeks after I put the new belt on.

Is it possible that the serrated Gatorback is allowing it to ride forward and up on the lip? Would a constant grooved mono-belt be better? And along these lines, why would the belt want to move forward anyway and why would it stop after just a few seconds after start-up?

I will probably try re-installing the original belt (in pretty good condition) and see if the symptoms abate.

Thanks to all!!!

 

Sounds like a frozen idler pulley. Change both back, if the old pulley spins cleanly (?)

 

I did change the idler pulley when the tensioners were done a few weeks ago. i repacked the bearings of the tensioner pulley.

The idler pulley that came off AND the replacement was smooth and had no lips. A Gates replacement photo shows lips and may be just what is needed to keep the alignment correct for that start-up period.

You can view that Gates pulley here:

Gates® DriveAlign® 36270 - Idler Pulley | O'Reilly Auto Parts

Both pulleys turned very freely when re-installed and the tensioner seemed pretty tight as I moved it to get the belt back on.

Is the tensioner/pulley assembly Goodyear 49420?

 

The Gates 36270 may be too narrow. The correct idler replacement is Gates 38028.

 

And a suggestion for the best place to get the tensioner/pulley assembly? Is it a Goodyear 49420?

Do I need to go to a Jag supplier or will an auto parts house have one?

 

And here is a video of the belt moving on start up. So is it safe to assume if the belt tensioner was tighter this would not happen?

 

I am missing something: I thought it was already determined that your idler pulley (not tensioner pulley) was too small -- ergo, the ^^^^ing belt is not going to be tight enough!

 

No, idler pulley is not too small. It was replaced when I did the secondary tensioners a month ago. I also repacked the bearings on the belt tensioner pulley.

I did replace the standard belt with a Gatorback.

The confusion may come from a post above (Post # 8) when I mentioned that I was going to replace the idler pulley with one that has lips on the edges to help with start up alignment. There was a subsequent comment that the referenced pulley may be too small in width. The pulley with the lips has a 31 mm width and the belt is 25 mm so I don't think it will be a problem. That O'Reilly Site also list that lipped pulley fits 65 different Jaguars. It just seems to be an upgrade from the standard no-lip pulleys.

I am also considering replacing the belt tensioner mechanism. I was trying to get some input if the Goodyear 49420 referenced in an above post was the proper replacement. There are lots of those on eBay at a good price: about $ 80.00.

 

why don't you just replace the pulley bearings, I listed the part numbers and a 'how to' post a while ago, fit an oem belt and be done. Hope that helps!

 

I seem to vaguely recall (from my year in Jaguar parts) something about an upgraded or superceded belt from Jaguar that also involved needing to change the pulley as well. Could you be running a new style belt with an old style pulley, or vice versa?

I could be wrong on this, but I know there was some confusion over this. I just can't recall which year XJ8s were involved.

Does this sound right to anybody else?

 

Seems simple enough to reinstall both the old idler and belt to check if your changes caused the problem.

Easier yet, check to see where the arrow on the tensioner arm aligns with three marks on the housing. A tight fit with a new belt will align between the two marks that are close together, if it is near the third mark (or outside of it) the belt is the wrong size or the idler diameter is smaller.

The 15 mm tensioning bolt broke when I tried to reinstall the belt after changing the water pump, hoses, thermostat and tower: I bought both a new tensioner NCA7739AE and an OEM belt NCA2211DA from the local dealer ($111.50 tensioner; 99.75 belt) since Advance Auto Parts had neither.

 

It's starting to come back to me now, I think. I believe the grooves in the newer replacement belt are a different width than the ones on the original belt, and installing the newer belt requires a different tensioner or idler pulley (I don't remember which). Try contacting Gary at Jagbits. He should be able to give the full details.

 

I noticed the wear pattern (of the paint coming off of the new idler pulley) was uneven. Like maybe it was out of round.

I returned that replacement pulley (Dayco) and put on a new one. I also changed the
"direction" of the Gatorback belt. Sounds crazy but a site on Fords said they work better when the lettering reads properly as if one was standing in front of the car. (I know, crazy.)

This seems to have stopped all the wobble on start up.

BTW, I believe the original equipment on these Jaguars was a Goodyear Gatorback belt AND Gatorback tensioner. The Goodyear 49420 tensioner and the Jaguar NCA7739AE appear to be the same. Right down to being made in the same plant in Canada.

I am playing it safe tho and I ordered a Jaguar NCA7739AE today. The tensioner I have seems tight but I am going to replace it anyway. I figure after 11 years . . . .

I will say that the belt rides close to the front of the car on the idler pulley. Especially when compared to the belt position on the tensioner pulley.

 

So we can safely add resolved to the thread title?

 

Dare I say yes?

OK, "Yes."