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Jaguar Repair/compression


As much as I am attracted to this vehicle; the issues I keep discovering a giving me second thoughts.
I decided the state of the engine would determine my further commitment.
I did a compression test with the following results, cylinders back to front:
Is this an aged but otherwise healthy motor?
I hope not to tire you with thank you,

ANSWER: Hi David,

Those readings are not any reason to discard that engine or car. Your tests needed to be done with the throttle wide open. Then right away put about two teaspoons of engine oil in that plug hole and run a second compression test. The second test called a "Wet Test" will be higher then the first. Both must be done with the throttle wide open. The "Wet Test" should not be more then 20% higher then the first "Dry Test".

Then warm up the engine oil by running the engine for a while after the coolant temp is up. then shut it down and remove the oil idiot light sending unit from the lower side of the block and put a fitting in to accept a hydraulic oil pressure gauge. (not expensive from any auto parts store)
Start the engine and note the oil pressure at idle. It should read from 25 PSI up to 35 PSI. Providing you have the correct weight oil in the engine. (10W x 30 up to 20W x 40) then set the RPM at about 2000 RPM and you should see about 45 PSI to 60 PSI.

If all the readings are ok your engine is ok.

Let me know the results of all these test.


[an error occurred while processing this directive]---------- FOLLOW-UP ----------

I did the wet test after the dry but was racing my hurry I could have possibly poured more oil into the cylinder than necessary...would this have skewed the results?
For example the first cylinder that I accurately measured out a tablespoon of oil and poured it in through a funnel at the spark plug hole read:
Dry 130
Wet 150

The other cylinders wet were all up around 165-185...unfortunately I am writing this at the library and do not have the exact recorded numbers at I read more on the net most suggest 2 teaspoons of oil...I started at 1 tablespoon and more...
I also neglected to crank each cylinder twice to spread the oil on the piston before performing the wet compression test...
The vehicle's exhaust is smoke...and the high wet reading would imply some oil passing the rings into the combustion chamber...correct?
In short, should I retest?
As to the oil pressure, I will perform that as soon as my tool arrives...

Hi David,

It is not unusual to see results like that. You didn't say if on the dry test that you held the throttle wide open. But your figures are not far out of line so I would guess your engine is ok.
One thing about the XJ-6 is that it can't handle much exhaust back pressure. Back pressure would come from either a partially stopped up CAT or one of the mufflers. There is an easy test. Remove  one of the caps on the side fittings on the exhaust manifold. Do one at a time and put a small piece of hose on the end of your fuel pressure gauge hose and force it into the port on the side of the exhaust manifold. You can't leave the short hose in long because the heat of the exhaust manifold will soon burn the hose. But you don't need to, as the test only takes a few seconds. Have someone start the engine and rev it up a couple of times. You must not see more then about 1.5 PSI. If you have a problem you will see a quick 5 to 7 PSI on a quick rev up. Do both exhaust manifolds. GM claims their cars can handle up to 3 PSI but I tested a lot of British cars and none can handle over 1.5 PSI.

Are you having any drivability problems or were you just worried that the engine was tired and worn out?

The oil pressure test is a good test of the engine bearings. The dry and wet test is for the combustion chamber and the back pressure test is for the CAT and mufflers. The only other test is the dye (available from any auto parts store) that you put in the radiator and run it and look for a color change is for a leaking head gasket.


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Howard M. Fitzcharles III


Jaguar from the XK 120 to XJ-6 ser. 3 1987


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

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