Those Tensioners Again

[WULLIE4567]

Hi jag 92 HOT
thats a cracking reg Im not sure how to load pic's on post, only on gallery but I think pic's are better there because people can look through them better, if you put them in a posting nobody will know there there unless they go through the posting. The web man will keep you right if you want to put them on yer post along with a wee artical.

cheers ww

[outoftowner]

Quote yesterday Wedesday 4th - £1100.00 plus VAT, for modified tensioners top and bottom, new chain, new type water pump.
This from a specialist and approved privately owned 'small' Jaguar dealer who states that my '98 car has plastic tensioners top and bottom so all should be upgraded and replaced. Tony.

[mike020150]

Hi Tony,

Your specialist is likely to be correct about the type of tensioners fitted top and bottom. However, it's the top tensioners which split most readily. Hence, it's better job to have both top and bottom tensioners fitted but you can achieve a much lower risk of engine failure (than with 4x plastic tensioners) if you only have the top tensioners changed at a cost of about £500. Mike.

[outoftowner]

Thanks for that mike but I think I'll get all the four tensioners done for my own peace of mind, I just hope the chain doesn't break when I start the car up to get the job done, fairly soon, Tony.

[Billroot]

It is not actually the splitting of the tensioner that does the damage. The tensioner looses the steel slipper that bears angainst the chain (this gradually debonds from the plastic tensioner body). Only on the top tensioner can this feed itself into the in-running nip between sprocket and chain. The secondary chain breaks instantly and then its just luck where the camshaft stops. If you are lucky half the engine just stops; if unlucky, open valves impact pistons and its time for a new engine.

The lower tensioners do not have these possibilities, the part that bears on the primary chain is a long steel leaf. I would not loose any sleep over the lower tensioners. Also, you are unlikely to need to replace chains and sprokets even if doing the whole job.

[outoftowner]

Now I agree I'm no mechanic but you would think wouldn't you that Jaguar with all it's experience in building car engines for many years would realise that these plastic tensioners might well break, because, wait for it -- there made of plastic, why didn't they make them of alloy in the first place, or am I being quite naive, if so I beg all your pardons

[PrideofLyons]

why didn't they make them of alloy in the first place

Lightness and cheapness.

[Billroot]

I am an Engineer by profession (though not automotive) and it beats me why Jaguar used plastic for this component. Other than oil seals it must be just about the only engine internal part made of plastic. The only other one I can think of is the water pump impeller (is there a theme here!). Lightness and cheapness could be the answer although I doubt whether there is more than a few grams between the weights and more than a few pennies in the manufacturing cost. There is a great deal to admire in this V8 design and it is a pity that just a few (but expensive) issues have marred its reputation. I think it was a case of poor design QA. "Critical" components (ie those whose failure is likely to be catastrophic for the engine) should get special consideration at the production design stage. Had this been done I wonder if the materials selected for the tensioners and water pump would have got through. I guess we will never know.

One other thought - It remains to be seen whether the aluminium bodied tensioners actually do cure the issue. It's relatively early days for them!

[outoftowner]

One other thought - It remains to be seen whether the aluminium bodied tensioners actually do cure the issue. It's relatively early days for them!
Gawd save us

[bobwnm]

Perhaps its a "Jag thing". Historically the XK engines always had a tensioner problem!!
Don't think about it! just get your toe down before it manifests itself! I am ashamed to say I sent lots of MK2s to the graveyard - body rot, rear springs, tensioners and oil pressure! Happy days all the same. All for £50 a pop, inc T&T

Bob

[Billroot]

That's the best bit of advice so far Bob, just enjoy the car! The XK8 is my first Jag so I've no experience of ownership of one before. I remember as a callow youth being given a lift in a 420G and thinking: so this is what a Rolls Royce must be like!

Given the V8 was an engine design departure from tradition for Jaguar, why-o-why did they not bow to conventional wisdom and drive the camshafts with toothed belts like the rest of the world! I wonder if the all-new 5 litre V8 in the next generation XF will retain this piece of nonsense.

[mike020150]

As someone previously responsible for one of the UK's largest fleets of vehicles from small cars to HGVs, I can honestly say that cam belts were one of my biggest problems with all manufacturers. Please don't persuade Jaguar to use them more widely or I'll feel like abandoning my lifetime admiration of the marque! Mike.

[bobwnm]

I totally agree Mike! toothed belts are the invention of the devil. Good only for hanging things up in the workshop!
Even when all the tensioners had given up on an XK engine it would still run & give service, albe'it a bit rattly!!!
Bob

[Billroot]

I am suprised that you have found toothed belt driven camshafts are bad. My experience of years of company cars is that I have never had a problem. I bet that if the Jaguar V8 had had these, this forum would contain virtually no discussion on them except for the need to ensure they were changed every 60k or so. It seems to me that if >90% of other modern cars engines use tem (my guess) they can't be so bad?

[PrideofLyons]

I think the true reason is, probably, cheapness, ie an accountant's solution rather than an engineer's.