Compression test meaning

Have a question about how to regard compression readings on 76 xj12 when one cylinder is at 117 when others are around 127. Put oil in it and it went to 127. The book says that any difference over 5 psi means problems that need taking care of. Is this right?

You’re fine with that difference. The reality is the V12 could be dragged along at half the usual compression in multiple cylinders and run just fine, not optimal of course, but would get you down the road.

Don’t sweat it, just enjoy the car!

What’s the history of the car, is it one you have been driving or has it been sitting? It may be that the cylinder in question has sticky rings that could loosen up with driving.

Yes it has been sitting for many years because it has developed a no start condition after driving down the road to the store or something and having to wait an hour for it to cool down enough to allow start again to get back to the garage where it remains to this day. That s been about 20 yrs now in the garage and driving on Sunday to church and back a few times which is 3 miles away because it won’t run right. Cannot find anyone to work on it So I’m going to address it in the spring when I return from Florida

Dan,

maybe you should just look out for a church that is located a bit further away - some forty miles of highways and byways with some twisty bits along that way would make your car a great runner, I bet …

In fact, my car is doing a lot worse currently, being taken out only once every four weeks, but at least driven until thoroughly warmed up …

To answer your question: a ten percent range is usually considered ok. It sounds strange to me though that 11 others are “around 127” - that sounds almost virtual! … Are you sure your tester is working correctly? I would have expected maybe 6 with a range of 5 and the next 4 within a range of 10 and maybe 2 within a range of 15 … Assuming that everything else is fine though I’d just try to run in the engine carefully, but with some commitment. Maybe you take it on a motorway and - on the fully warmed up engine - keep it in second for a few minutes to just let it exercise at >4000RPM. Only afterwards address that hot start problem.

Good luck

Jochen

75 XJ6L 4.2 auto (UK spec)

Hi Dan, 10% difference is still okay, if oil helps it means the rings seal a bit less but again just keep driving. Driving longer distances and hard regularly.
Church further away, lol. But you can take a 50mi shopping round or other kind of trip.

I thank you guys for getting back to me on this matter. I’m confirmed in my guess that the engine is probably fine and the ROM is over kill when it says 5 psi difference off the others is time to start thinking about taking things apart. I mean 5 is hardly any difference or percentage! Anyway the car has only 94k miles. So I m leaving the engine alone. I am going to need your help this spring in tracking down why it’s not running correctly and am happy to have discovered this forum

We are standing by, Dan - a more precise description of ‘not running correctly’ would be helpful. It’s not conceivably related to the slightly low compression in one cylinder - which should be unnoticeable…

And welcome on board!

Frank
xj6 85 Sov Europe (UK/NZ)