The previous owner of my 1999 XK8 overheated the engine from a destroyed water pump belt.
Now two cylinders are low on compression.

 

The question is how low ...and how does it run?

 

one cylinder is zero the other 30 psi. Engine does not run very well.
Am thinking about alternatives. Pull the heads. Pull the engine?

 

Mmm. if one is zero, you might have timing issues & a bent valve or two.

I'd pull the spark plugs on the affected cylinders and poke a borescope in there to take a look:- certainly before you start the engine again.

 

Are the two cylinders on the same bank ? If they are you could pull the head on that bank take off the sump and pull out the pistons. Examine the bore etc and replace or re ring the pistons and or re hone the bores ..KISS dont spend silly money .... of course if you are not a mechanic your wallet is blown like the cylinders.

 

To my superficial knowledge, the "standard" failure mode of overheating our engine is loosening of one or more valve seats and associated valve damage, i.e. cylinder head work. Have you tried to eliminate the possibility of worn rings? I believe the normal technique is to add some engine oil through the spark plug hole and re-do the compression test. The cold/thicker oil is supposed to temporarily seal the rings and help you determine if there is a valve issue.

Might be worth checking the oil for evidence of coolant, or any type of white smoke on startup pointing towards a failed head gasket.

Best of luck, keep us posted.

 

+1

Even as a DIY project, a Jaguar V8 engine rebuild is very expensive in parts alone. Scope the probable cause of the low compression before throwing any money at it.

Here in the UK used Jaguar engines are usually easy to source and few of us would consider a rebuild other than for the technical challenge and satisfaction of doing it. I know it is not the same in the US and finding a replacement used engine there is much more difficult but rein in the natural enthusiam to get started until you are sure the engine is recoverable.

Graham

 

the two failed cylinders are the farthest rearward in each bank. Double trouble!!

 

When I got my first XK, it had a dropped valve seat, rear most on the drives side... The SCOPE is a great idea and you can get one on Amazon for very little money. It will plug into you phone charging port and you can run it down a sparkplug hole, as Michael says, and try to get the thing to point UP at the underside of the cylinder head... Takes some wiggling, but it's possible. As Mertz says, the failure is often dropped valve seats.

One might have fallen (the 30psi cylinder) and then got KNOCKED back into the head crooked,,, and now isn't sealing properly... Hence low pressure.

Another way you can take a peak (IF it's an intake valve seat that has failed) is remove the 10 bolts holding the intake, undo and remove the few bolts under the throttle body and coolant hoses (all this is most likely in your future anyways) and look down the intake holes. My missing valve seat was very obvious this way. Top of cylinder was all dinged up,,, cylinder wall was fine, head was toast. Heads are plentiful and can be had complete, on eBay... I like "playground par" on eBay, and NOMA - I think they are called.

NOTE - running the car this way will destroy the CATs and potentially start a fire. There is no compression BUT there is gas and spark and raw unburnt fuel will be pouring into the exhaust OR UP into the intake plastic. More, the (I think) the O2 sensors will be reading LOTS of oxygen,,, for which she will accommodate by adding more fuel in certain circumstances. All I'm saying is be careful

 

This is true.
The oil will improve sealing around the rings but will do nothing to improve sealing around the valves. Any improvement in compression means ring issues.
No improvement means valves.
Definitely pull the spark plugs, compare them to each other to see which one or two seem different and do the above mentioned compression check.
This is a Nikasil engine, right?

 

You bought this right knowing that it had engine damage so you must have bought it cheap...If you got it cheap enough the money you put in should be recoverable in the final value.
Certainly you will need the heads off and can get the pistons out if necessary without removing the engine.
Its my nightmare what you are going through. I often think of what engine/transmission I could throw in if the engine blew...Could I get a Rover 3.5 in there ?

I have done,
1929 Humber 9/28 with a Suzuki engine.
1980 Range Rover with Triumph 2.5 ( Brilliant ) and another with Ford 3.5l 6 cyl diesel ( junk)
And many kit cars

 

A small zip tie can be very useful for persuading the camera to stay looped around back on itself and not spread out inside the cylinder. Though I havent pulled plugs on my xk yet so not sure how big an aperture we have to work with lol.

 

Check your valve clearances, especially the one with zero compression. If you have excessive clearance the valve is not seating properly. Most likely cause would be a loose valve seat or bent valve. I'd put my money on loose valve seats from overheating. If you have excessive clearance the only way to repair it is to take the head off. Sometimes you can visually see that the valve bucket is not all the way up like it should be.