A Jaguar owners group reports engine failure due to timing chain tensioner failure:

“you are at risk of a failure of the camshaft secondary timing chain tensioner which, in the worst case, can lead to catastrophic engine failure and a replacement bill of $10,000…¬† The January issue of Jaguar World Monthly magazine describes the failure “of those brittle timing chain tensioners” as common and proceeds to lead the reader through the relatively demanding repair. It appears the plastic housing of the tensioners become brittle with age so the failure is just starting to show up on the early V8’s built. Accumulated mileages of 50,000 to 70,000 have been mentioned. It is not clear if or when Jaguar might have changed the plastic material whose aging is at the heart of the problem.

The tensioner pushes on the bicycle-type chain between intake and exhaust camshaft sprockets to keep the slack side of the chain from whipping. With the wear and stretching of higher mileage there is more slack. When the tensioner fails the chain slack may allow the chain to jump one tooth on a sprocket and then the exhaust valves open and close later than normal. Performance will suffer. But if the chain jumps two teeth or more the rising piston at the end of the exhaust stroke hits the exhaust valve which is still open, and that instantly is the end of your engine. Tooth jumping happens especially as the engine kicks into life when you start it. Jaguar has redesigned the secondary tensioner 3 times since V8 introduction. Technical Bulletin 303-30 dated 12-98 recognizes a rattling noise caused by tensioners at cold start and notes a revised design with noise dampener clip added. This change commenced with engine No. 98102106XX and will be installed on earlier engines under warranty.

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