I've done a search for engine knock and found a few posts pointing toward rods and assorted belt tensioners, but none are really clear as far as helping me diagnose my issue. I pulled out my biggest screwdriver and started listening around. Nice and clear hum everywhere except directly behind the breather box. On the hex head bolt directly underneath the breather hose on the valve cover, I am picking up a definite repetitive knock as well as the background steady hum. Scared it's a rod. Here's a breakdown of the situation.

Most noticeable at idle
NO CEL, Loss of Power, or any restricted performance
If I rotate throttle, the knock increases momentarily, but almost immediately disappears.
I took it this morning and had oil changed, nothing odd looking about previous oil.

Any opinions and or additional diagnosis direction would be appreciated as Jag mechanics in my are are few and far between.

 

I have not had this issue, but I have heard that the VVT (variable valve timing) units can start to make noise as they get older and that a thicker weight oil can help. The VVTs are under the electrical plug that goes through the top of each valve (cam) cover. It could just be the one on the driver's side that's making noise. I'm not really sure by your description where the noise is coming from, but check the archives and listen some more and see if you can narrow it down to this. Apparently this is fairly common and not too serious.

 

VVT noise is a good guess. Since you have just changed the oil why not try a can of 'Restore' additive to increase the oil 'weight'.

 

I used 5W30 as recommended. I hit AutoZone on the way back home, it wasn't Restore, but about the same size and thick just like it (Oil Additive). But I'm still getting the knock. Since my first post, I've come across a video that sounds just like it. The only issue with the video is that his made the knock only after it warmed up. With mine the knock is there cold and warm. The video issue apparently turned out to be secondary tensioners on the timing chain. My wife took the car to run to work while I was making the first post, looks like it's going to get parked for a bit once she gets home.

 

If you are not sure the tensioners have been upgraded, I would sure get after that soon. I have heard of few reported lower end problems on this engine. Apparently sometimes valve seats come loose, but even that is infrequent. Lots of VVT and tensioer issues. Be aware, your engine has a different VVT setup from the 98 -99 model that has many reports on here. I do not know if the later VVTs were more or less prone to noise, but they are certainly different.

 

I tried the restore but it only helped a little.

Now that your engine is a little older and all of the clearances have opened up a thicker oil helps take up the space so the oil pump can keep up at idle.

My 99 has 144K on it and I was getting some noise from either the VVT's or the Rod Bearings. Even though I live in Michigan and have already seen temps well below freezing I have to run 10-30 in the winter, 15-40 (not a diesel blend) in spring/fall and 20-50 in the summer to keep things quiet. Works like a charm.

Like mentioned above, I'd make the first order of business removinging a valve cover and look at the tensioners.

Ken

 

I got a chance to talk to a mechanic I've used for my BMW (non-Dealership shop) seems to know his way around Jag's as well. He knew exactly what I was referring to when I told him about a knock that was symptomatic of a failing timing chain tensioner. I'm going to drop it off in a bit so he can take a quick look and run the vin to see if we can get an idea on what we're looking at.

 

Oh well, caught up with my mechanic this afternoon, he's confirmed the timing chain tensioners as the culprit. Not sure if it's the typical secondary unit or a combination of it and others. Got lucky though, he said the car is running fine and there is no sign of slippage. His first preliminary estimate is encouraging, and he knows the LS uses the same parts so he's already looking to see if he can save me some money, so I'll most likely let him do it. I can use the time I don't spend on it to make the money to pay for it.

 

bluedg, what do you mean by "the LS" uses similar parts? Is that the General Motors Racing motor having the same timing chain?

 

The Lincoln LS appears to utilize the same upper tensioners as the XJ8. Not sure about chains or other tensioners.

 

Im picking up my car tonight at the jag dealership, I'll grab the part numbers of all the timing chain parts, and go into the ford dealer down the street and check the town car part numbers. I sort of doubt the numbers will match but with that ill even ask the dealers if they match. It might be a dumb question but what are they going to do? unfix my fuel lines?