Big Worm, as for the anticipated compression, it should be in the neighborhood of 150 psi. The JTIS just says that the compressions should be within 75% between the highest and lowest cylinders.

As for other ideas, a test that I would do is to bring cylinders 1,2 and 5 (as you numbered them) up to TDC one at a time. Then, using some compressed air, fill the cylinders via the spark plug holes to about 90-100 psi. THen, putting your ear near the the intake (remove a vacuum line), the exhaust pipe, and a breather point on the engine, see if you can hear any air coming out. If you hear it coming out of the breather point, then you have bad rings, if you hear it coming out of the intake, you have a bad intake valve or a bad head gasket. If you hear it coming out of the exhaust, then you have a bad exhaust valve or a bad head gasket.

Since you have 3 cylinders that suddenly went to zero, there aren't too many things that can cause this. ONly 2 come to mind right off the top of my head: 1) engine was overheated and damaged the head/head gasket, or 2) oiling issue and since those cylinders are furthest from the oil pump, they were starved of oil and now damage has been done to the rings/bearings. It may be advisable to do a detailed inspection of the dipstick to see what the oil looks like and possibly even do an oil change (checking for metal shavings). Having the timing jump a tooth doesn't make sense since you have some cylinders near normal. Even if a cam shaft broke for example, that would mean a cam shaft broke on each side of the engine at the same time (that is either really, really bad luck or engine abuse from over revving).

Hopefully this gives you a starting point to figure out where the problem lies. If you need someone to bounce ideas off of, let me know. I fought a similar problem with my Ford Expedition. But, in my case I had the timing on all the cylinders off due to installing the timing chains wrong. But I can relate to the confusion you are experiencing.

 

Yes, that.

 

Doing a basic compression test should be pretty simple then, as well as knowing that it isn't a valve cover gasket, jumped timing or valve seals. An engine is an engine when it comes to basic stuff like this. Don't over think it as this point or presume there's something unique because it's a Jag engine.

 

Probably not rings. Never heard of a Jaguar with ring problems since the old Nikasil days? With the year model car of your car I would look at timing chain Tensioners. It sounds to me that one side has already broken and the chain has skipped a tooth or two. This is an interference engine so the valves will hit the pistons.

First take the cam cover off the side that has no compression and inspect the timing chain and tensioner. I would NOT drive the car until you take a look. Maybe even crank the engine a bit with it off to see if anything is jumping around. If you don't know what you’re looking at it helps a bunch to remove the other side at the same time and compare the action.

It sounds like you have no internal engine damage yet. Is there any noise when running or cranking the engine? Inspect for oil leaking past the cam covers too, it's very common on the 2000 model. Again coils and plugs should be changed if you’re that deep into the engine. They are probably contributing to the mis-fire too!!

Look here for info on the Nikasil problem and the different variations of timing chain tensioners.

http://www.jag-lovers.org/cjw/
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