Jaguar Repair: 85 Jag #5 cylinder varying low compression issue, mass air flow, air flow meter

Question
I have a 1985 Jag XJ6 vandenplas with 4.2L(144,000 miles). I did a dry compression test with mass air flow (or throttle plate closed) and all the 6 cylinders read 125 lbs except the #5 cylinder back from the front of the engine(#5 read 50 lbs).After a couples days of thinking and reading about this in all experts (your answers in particular) I rechecked the # 5 cylinder again dry with the throttle plate open and the compression meter read zero.So I tried the #6 cylinder again to see if the meter was acting up.#6 now read 150 lbs compression which was in range (135-165 lbs).If I put the #5 cylinder up on tdc on compression stroke so the valves are closed and just take a blow gun in the sparkplug hole and listen for air escaping from the throttle plate or exhaust will that tell me if the problem is an exhaust or intake valve issue?Do you think the valve or valves are stuck or bent or possibly a ring is stuck or broken?A mechanic told me that the head was cracked.Will pouring marvelous mistory oil in this cylinder and then either letting it sit a while or turning the engine over free a sticking valve?The car was sitting and not run since 2006.The car did not run then much later after  I had a Jag mechanic seal the valve covers down so they did not leak oil in 2006 so I'm reluctant to remove them unless totally necesary.If I removed the head ,how much would you estimate total parts would be for like the head gasket set and valve seal set?How much time would it take a mechanic with moderate chevy engine rebuilding skills to remove the head or is this something only a Jag mechanic can do correctly?Any suggestions as to what tests or repairs are needed to correct this cylinder would be greatly appreciated.

Answer
Hi Jim,

Be sure you are at TDC of the compression stroke and pumping air into the plug hole is a good way to check for valve leakage. Then open the throttle and open the air flow meter flap and put a hose like a vacuum hose in one ear and use the other end as a probe to listen for air hissing in the intake and then go to the exhaust and do the same. If you hear hissing you should remove the cam covers and use a feeler gauge to check valve clearance in the dead cylinder. They should be close to .012" to .014". If either valve has excessive clearance then that valve may be stuck (unlikely) or more likely bent. Another possible is that the adjustment pad (shim) has jumped out of place and it held the valve open. This would be indicated by no clearance at all.

A Chevy mechanic can do a valve job on a Jag but should purchase a shop manual first and go a step at a time because without it there are too many traps to fall into. Even a good mechanic who does not know Jaguar can get into a lot of trouble without a manual.

I can help also but get the manual first and I can walk you through the process. This is if it is necessary but you are not at that point yet.

ID which valve it is first and then check valve clearance then get the manual. Then contact me again and refresh my memory on this project. Very unlikely that a gasket or crack is the cause.

Howard