I don't want to step on toes here or guide you in the wrong direction because NBCat is 100% correct and you shouldn't assume anything. I'm just saying the carbon buildup CAN cause a valve to hang. I'm not sure how susceptible these engines are or are not to dropping a valve seat but I understand they are quite prone to the timing chain/tensioner issue.
As soon as you discovered the lack of compression in that cylinder, a tear down was imminent to further diagnose. Anything else is pure speculation and is getting you no where.
Good luck and keep us posted. There's some smart people here that know these engines and are willing to help along the way.

 

Just finished doing an inspection of the dead compression cylinder # 8 with an endoscope camera. Hanging down is the seat from an intake valve. It actually covers part of the spark plug hole with the valve open so it wasn't hard to see. While I've build a couple dozen or more Chevy and Ford engines in my lifetime I have never opened a DOHC Jag V8. I watched a YouTube video and I know the work. It''s the special puller for the balancer and timing that I would have to buy. Then I'm in the Metro Vancouver area in Canada where a used head of needed is pretty scarce and I don't buy Ebay anymore after a few rips. So I just wanted to close the thread with the question answered as what was the problem and thank all for help. Unless you have a good usable driver's side head for a 2002 4.0 NA.

 

autozone rental balancer puller and the included m8x1.25 bolts will take it right out

you can buy the pulley holder to get the crank bolt loose and all the other needed timing tools for like 40 bucks.

https://www.ebay.com/itm/9PC-Timing-Tool-Set-Fit-For-Jaguar-Gas-3-2-3-5-4-0-4-2-4-2-V8-Chain-/333725623764?mkcid=16&mkevt=1&_trksid=p2349624.m46 890.l6249&mkrid=711-127632-2357-0

 

Do you know if a head from a 2004 4.2 will interchange? I understand a lot of parts do.

 

it will but the combustion chamber is different

 

Thanks. Looks like I chance the dropped seat can be repaired by a head shop or on the hunt for a 4.0 head here where one probably doesn't exist.

 

you can buy new valves from fcp euro. the seats are like 25 bucks each

 

Sometimes they will bang the top of the Pistons.

 

The above photo:
This is the intake runner from an AJ26 head specifically but, I’m pretty sure the AJ27 intake runner is the same.


This second photo is the intake runner from an 05 4.2 XJ.

I didn’t look into the combustion chamber differences (if any?). Not to step on anybody’s toes but, I don’t think getting one 4.2 cylinder head would be the ideal situation for you. I can verify that the later 4.2 mls head gaskets will work on a 4.0 block and are truly superior to the earlier head gaskets.

I have 2 thoughts. 1) You can likely get the 4.0 cylinder head repaired. I would HIGHLY recommend getting the other head sent off for a valve job so they’ll both have a fresh valve job, new valve stem seals, valve adjustment and machined heads for a slightly higher compression ratio, it’ll run great. 2) if you get a 4.2 head, I’d HIGHLY recommend getting them both but, who knows if the factory 4.0 intake manifold will bolt up and seal to the 4.2 heads.

 

you need to drill a couple new holes for the 4.0 manifold to fit the 4.2 head.

repairing the head seems like a no brainer as long as it’s just a dropped seat

 

Hello picture number 17 was for information only it’s a 2000 XJ8 with a broken intake valve. What caused it was a broken timing chain. The valves got loose inside the head and banged on the piston so I lost the motor because the cost of replacing the piston to plus the cylinder head Could have gotten a hotspot because of the beat marks after it was rebuilt.

 

I posted another photo of an XJ8 where the valve seat had dropped down. What caused it was the car overheated.

 

All that carbon will definately cause hot spots. It can actually start to glow red, cause pre-ignition/detonation, and all sorts of related issues including causing the valve to not hit the seat the way it should. Venting the crankcase breathers back into the intake/combustion chamber is the sole cause of all that carbon and it's 100% preventable.
A simple Google search will give you all kinds of results all telling you exactly the same thing and more.
Could it have caused this dropped seat? Absolutely.
How easy is it to prevent this sort of carbon buildup? It's as simple as installing a catch can and/or venting the crankcase to atmosphere and not back into your engine. The benefits are more than just the prevention of that carbon buildup too.