Pardon my ignorance but I am a bit more familiar with pushrod V8's rather than dual overhead cam setups. What I am wondering is do all cars with overhead cams or dual overhead cams have issues with timing chains and tensioners or are the problems we keep talking about (funky tensioners that fail) limited to Jaguar? I thought that even the American pushrod V8's have timing chains from the crank to the camshaft so why don't I hear about problems with those engines? Is it the fact that the chains are longer with the overhead cam setup and that there are more chains once you have dual overhead cams? Sorry if this is engines 101 for some but I am a bit mystified.

Thanks,

Doug

 

I think it has something to do with the actual design and materials on the initial series of AJ engine. In the quest of maximizing weight-savings, the initial generation utilized timing chain tensioner made out of plastic rather than metal.

Due to factors such as overheating, poor maintenance and others, the plastic components start experiencing stress fractures. I am not an expert mechanic but this info is widely available:

Newsletter from North Georgia Jaguar Club:
http://www.ngjc.us/Newsletters/MAR-06.pdf

A post of a specialty Jag-shop from the Bay Area:
http://www.jagservice.net/jaguar_v8_...tensioners.htm

I took mine to a specialty shop in Austin http://www.classicjaguar.com/
for an oil change and the shop manager gave me the 411 with both types of components (metal and plastic) laid out in his office.

Hope that helps.

BTW - if anybody is a fan of classic Jaguar (who isn't really?) - Classic Jaguar has several E-types on restoration at their shop in various stages. Those are real work of art!

 

Yes--I was well aware of the problems with the earlier models of tensioners on the Jaguar engines but if this type of device is routinely used in engines with overhead cams, is Jaguar the only manufacturer that tried to re-invent the wheel by playing around with the design? As a layperson (but an engineer), it just seems obvious to me that you would definitely NOT want to be using plastic in an environment that is subject to heat, fumes, and vibration. I guess this was limited to Jaguar otherwise we would be hearing about similar problems with many other manufacturers.

Doug

 

The push rod American engines had a chain about 3/4" wide and the sprockets were only a few inches apart. They would get sloppy and a little out of time with wear but seldom heard of complete failure. Since most of the engines of my earlier life were non-interference types, if the chain did fail you just replaced it and kept driving.

Had a '80 Volvo V6 that had single overhead cams and very long chains from the crank. Common problem on that car was the tensioner shoes wearing and/or the oil pressure controlled tensioner failing. The problem has been around for a long time.

The really terrible thing about the V8 and plastic tensioners is that Jaguar sold a weak design on an interference engine.

 

I had not heard that term before (interference or non-interference engine). What does this refer to?

Doug

 

An interferance engine meens the valves will hit the pistons when something breaks. Non interferance will never hit. And doug, pushrod v8's???i heard you were more familiar with flathead fords
its an issue on nissans as well. And was also an issue with timing chains on v8's of yesturyear too...dont you remember that far back? Lol i had to think back...the cam gear teeth were coated with plastic and for the same reason with time/poor maintenance, the teeth broke off, then lots of chain slop and sometimes breakage. But it depended on the engine design wether the valves hit or not

 

I guess I picked up that "pushrod" thing from all the self-effacing articles I would read about Corvettes when I owned one. It seemed that virtually every car magazine writing a review of the new C5 and then the new C6 would have a comment on how GM chose to stick with the proven "pushrod" design rather than go the way of overhead cams like the other exotics out there. It got to the point that every time I read something about the LT series of engines there was always some apology for the fact that these were pushrod engines.

Doug

 

AAwww Doug, you missed my stab...flathead fords are WWII era

 

No--I got that stab but ignored it since I had heard plenty about those flathead engines in my younger hotrod days (still do occasionally hear about them)!

Doug

 

The issues regarding secondary tensioners specific to Jaguar have a lot of causes more than just plastic parts which were chosen for weight reasons. The Valve in head design is a complex series of tradeoffs. Changing the entry angle of the intake/exhaust valves affects lots of things including head shape, squish patterns,symmetry etc.

Wihout going into an extremely long dissertation on valve train design, Jaguar accepted some tradeoffs for performance with a certain entry angle at the sacrifice of other benefits, one of which was they ended up with an asymmetrical intake/exhaust valve train pattern. Essentially, this means there is not a perfect balance between intake and exhaust spring forces as the crankshaft turns through two full revolutions. This causes a "bobbinlike" affect with the secondary tensioners as an overbalance of valve spring force first "pulls" the secondary chain forward, then a counter-overbalancing force actually "pushes" against the normal rotation in the secondary chain. Again, this is slight, but causes an excessive exercise of the secondary tensioners. Eventually, as the tensioners become exhausted, the secondary chain will begin to play "crack the whip" (I think that's what we used to call it) with the secondary tensioners with ultimate failure.

All this kind of "evolved" as I understand it. Valve angles changed even in the last alpha designs of the AJV8 while the plastic tensioners were already chosen for weight. By the time the engine reached Beta testing, the asymmetrical valve angles were finally chosen, but beta did not produce engough MTBF to show the plastic secondary failures...thus the "surprise field failures", TSB and subsequent metal redesign.

 

Great discussion of this popular subject!

Always wondered how such a catastrophic failure issue made it through development and into 4 years of production.

 

Forgive the ignorant question..but did Jag change the tensioners in later years? and if so when?

Thanks

 

August 13, 2001.

My '02 was built before that date and had the plastic tensioners. Look at your manufacture date on the driver side 'B' column.

 

My B pillar manufacture date says 08/01. No day given. I always thought it was safe as an 02. Has almost 130k with narry a rattle. Every day driving may be the trick to keeping kitty purring...

 

Tomorrow may kill your engine.

Pull a valve cover and then you can sleep better or spent a few bucks and sleep better.

The engine build date will be before the vehicle assembly date.

August 13, 2001, a very good date.

 

Thank you Test Point for that information ..much appreciated