I recently purchased a 1998 XJ8 L with 116K miles. I've had my mechanic replace the valve cover gaskets a few days after the purchase plus a few other mechanical maintenance. The engine seems to run fine, however when idle slow it is rough compared to my 2012 XF. I've come across a few posts about timing tensioners. Since my engine has 117K miles now if it would be a reasonable assumption that the tensioners have already been replaced?...or have I just been lucky so far. When would one expect the tensioners to snap/break? What has been your experience with tensioners, did you have them replaced at a certain mileage? Was there a clear evident 'sign' that you knew you had to get them replaced before the engine was damages?

 

Your mechanic should have examined the tensioners when the covers were off. That is really the only sure fire way to know what you have. I doubt tensioners are causing a problem with rough idle. The more likely suspects would be dirty MAF, vacuum leak, dirty throttle body. I would have the fuel trims checked at idle and see what the engine is doing when it is idling.
If your tensioners haven't been updated then you have a ticking bomb. The tensioners when they fail will cause the chain to jump teeth. One tooth possible easy repair. 2 or more teeth is bent valves and damaged pistons, at speed the engine becomes an expensive boat anchor.
For peace of mind have the mechanic pull the cam covers again and make sure they are metal.

 

SOMETIMES the chain rattles on startup as the tensioners fail, but not always. When one fails the exhaust cam will skip one or two teeth and the engine will run rough or a valve will hit a piston and the head will have to come off for repairs. There have been cases where the tensioner came apart and the barrel broke the timing chain.

Never assume they have been replaced. They could fail any time as age and temperature are both factors in addition to miles. Get the cam covers off and inspect.

 

I just had my tensioners replaced and will be picking up my 2000 XJ8 Vanden Plas tomorrow. I took it into the shop when the engine started to idle funny. The car was shaking. I never heard any rattling when I started up my car, although I've read that the car will make a clinking noise when the tensioner goes bad/cracks. My tensioners were cracked and one of them were moving around in the case. It was throwing off my timing and 4 of my cylinders were misfiring. Your mechanic should've checked them when he had the cover off. Its an expensive repair... $1700 at the shop that I took it to, if you walked in and already knew that was the issue. For me its more like $2300.... But my engine was saved. And I have LOW miles, only 63,000.

 

Curt...I won't repeat what others have eluded to....suffice to add, considering the mileage and your lumpy idle....yes, it could be something
simple like a vacuum leak or cracked piping on the induction side, shrunken seals under inlet tract or even a dodgy ign. coil about to go **** up ;o]
When...if ever have the O2 sensors been repalced, especially the 'upstream' heated ones?
On a 1998 AJ26 engine there were three types of cam chain tensioners for the secondary chains.
Types 1&2 both were reddish orange which turned to a dirty dark brown with age, cracked up the sides of the tensioners and nearly always lost their plastic slipper feet.
The third type was alloy bodies with pale yellow slippers and are good to go.
The Primary tensioner were also changed to alloy bodied units in 1999.
I'm afraid you are going to have to remove those cam covers again to get a proper look at tensioners.....bad marks against your mech. for not doing this in the first place!
Here's apic of my knackered Mk1 tensioners at 76,000 miles ;o]
You will see the damage up the piston shaft housing and missing slipper feet which is typical damage.

 

Great pic!