XJS Compression Test
 

Ok,
I finished my compression check. . These were my results: 1A-203, 2A-198, 3A-200, 4A-200, 5A-201, 6A-205, 1B-205, 2B-204, 3B-200, 4B-198, 5B-204, 6B-204. Couldn't attach movie file, but have movie file if interested. Tested after warming up engine. Removed A/C compressor and throttle pedestal so I could get at everything. Also removed pickup coil from ignition amplifier and removed fuel pump relay ( #3 ) from socket in trunk. I believe these readings show that the engine should be ok, but comments are welcome. I believe I posted this originally in the wrong location...sorry, I'm new.

The numbers look good to me !

Cheers
DD

This sounds fantastic, what year is the car?

I tested mine and got all from 120 to 122. Later I remembered that I did not open throttle when I cranked. I think this might have restricted some of pressure buildup? I will re-test soon, with non-Chinese tool.

My 95 XJS 4.0 coupe tested at 175+-3 for each cylinder cold. At the time it had 52800 miles on it, has 54600 now. I did a pre purchase inspection on a 95 4.0 convertible which I did not purchase. It was in the 150 +- 10 range hot with 65,000 miles

I actually had the throttle pedestal unbolted, so also did not have the throttle all the way open, but I think that will have an affect on how much you have to crank it. You have to crank it until it no longer produces any increase. Having the throttle open all the way will allow it to get up to max pressure faster. You also have to make sure the spark plug adapter is screwed it all the way, then you screw in the compression tester. Do not use the "hold in" type as they won't work with these high compression engines. I also think this engine is good, but removing it to patch up all the oil leaks. The "AllDataDiy" manual does help ( Autozone ). You have to go online to access the manual. It's more specific to the year vehicle you have and you have to pay about 25 dollars/year to keep up the subscription. It told me what the pressures are supposed to be and all the other pertinent info as well. Thanks for the replies.

Just re-tested and got 188 and 180 on two front cylinders.

Surely having the throttle open will make no difference to the compression readings as the engine shouldn't be running whilst you do it?
You stand a good chance of getting badly electrocuted off the plug leads if you are not careful with a running engine.
The compression test is just to see how much pressure the cylinder can generate and is best done cold as that is the worst it is going to be.

True, but if one really wants to see how much their motor worn, run test first and then add little bit of oil in cylinder and run again. If reading second time is 10psi+ higher, then there is some wear on cylinder walls.

Thanks for the response. I actually removed the pickup for the coil, so that wasn't a problem. I just ran the engine, so the test was done when the engine was hot. I also disconnected the relay to the fuel pump before engaging in the process. Although I didn't do the oil check, I assumed that since my readings were so high that it was unnecessary to pour the oil down the cylinder ports.