what should the compression be?

jag. x type

 

*****, the compression on these engines (assuming everything is fine) should be in the 150-165 psi range. Along with that, all cylinders should be reading within about 15% of each other (ie, within about 20 psi between the highest and lowest). If you have only 1 cylinder reading low, odds are it is either a head gasket problem (look for milky oil, oil in the coolant, or oil leak from the head gasket area) or a bad valve (proven using a leak down test, if you want to know how to do this, let me know, pretty easy to do once you get a special fitting). If you have 2 cylinders side by side with low compression, then you most likely have a bad head gasket or a cracked head.

If all your cylinders are running low, then you have bad rings in the cylinders due to age/wear. You can prove the rings are bad by adding a little bit of oil to the cylinder and then repeating the test. If your pressures come up, bad rings. If they don't, then you probably have buildup on the valves and that is causing your leakage.

 

THERMO, I have good and bad news. your tip on how to check piston rings worked great, my psi check before was showing 60psi on mostly all cylinders , sprayed wd40 down the spark plug holes and it came up to 90psi. The Bad news,looks like it need rings or r&r engine. Great tip.

 

*****, that is why we are here. Normally they recommend using something more like motor oil or stuff they use to initially build motors. It is a bit thicker and tends to allow a better seal. But, as you found out, even something as thin as WD-40 can affect the compression.

Just out of curiousity, how many miles are on the motor? I keep track of how many miles people are getting out of their motors and common problems with our motors.

 

I don't know for sure, but have any of those engines (2.5 and 3litre) been of the "nikasil*" type? The V8 petrols in XJ8s were, up to about 2000, and if you've been running on high sulphur petrols, the nikasyl coating can be damaged. BIG problem in UK, so much so you can't sell nikasyl engined cars here for buttons. BMW has had the same problem for the same reason. Nikasil and alusil coatings on ally cylinder walls are a german idea, a plating of silicon carbide in a nickel 'grid' about 5 to 10 thou. thick directly onto the ally cylinder walls as an alternative to cast iron liner. After 2000, Jag. went back to liners.
*If you have a nikasil engine and it fails the compression test, don't waste money doing any work on it. You can't, it's not worth it. If you keep the car, find a good second-hand engine that's reasonable and replace the duff one. Nikasil's ok so long as high sulphur petrols haven't been used. You can tell the condition if you get the engine hot and test for a powerful "wind" as you carefully lift the oil filler cap just a little way. This is blow-by down the sloppy fit between pistons and cyl. walls. If you want to get shot of the car, or struggle on as it is, try one of those oil thickeners by Wynns or S.T.P. They really do reduce oil consumption, but it's a 'last-legs' method.
Leedsman.

 

Leedsman, to the best of my knowledge, the Nakasil engines are only in the upper end cars with V8's. They are not in the X-Types with the V6 engines.