Interpreting compression results.

The results you have do not show a definite fault (or even a possible fault). If you think you have a leak then test for it with a block testing kit which checks for combustion gas in the water system, and/or an airline connected to each cylinder in turn, which can show you where the air is going. Adding oil to a cylinder is inconclusive, usually, as many pistons have cutaways that the oil sits in.

mrcheerful

Thanks very much for the helpful suggestions

Andy

The message from Andy Pandy contains these words:

Nearer "never" than "always", I'd have said.

I think most people would be quite happy to have an engine with only a 10psi difference on one cylinder, especially if it's done a few miles. A 10% variation would normally be the start of an indication of trouble and even then not serious trouble. 30 or 40 psi down and clearly there's a problem but 10 psi wouldn't show up as a noticeable loss of power or any driving fault. There's a detailed guide to compression testing on my website.

-- Dave Baker - Puma Race Engines

Dave Baker (Pumaracing(NoEmails)@aol.com) gurgled happily, sounding much like they were saying :

Hell, yes. I've got a two-pot sat outside with a 150psi difference between the two...

Andy, the results dont look worrying. one item though - did you repeat the test? are the results repeatable? i liked to test cylinders 1 to 4, warm up engine again and then do 4 to 1 again to cancel out lower compression due to engine cooling by the time you do the last one.

i found it was helpful to position the gauge where i can see it so i give equal starter cranks for each cylinder test.

Yes I did repeat the test several times and I agree with you that the bad cylinder improved as the engine cooled. The reading is a little lower immediately following a run.

I still have my doubts as to this one, as I have had a slight but steady water loss for sometime now and I therefore decided to investigate before the thing gave up completely.

I'm going to take Mr Cheerful's advice and get a block test done ASAP.

Thanks for the additional observations.

Andy

Within 20% is the figure I remember.

Adding oil to the top of the piston will increase the compression ratio and give a higher reading. It's really a test if a *very* low reading - if it comes up to near normal it means faulty rings. If it makes no difference valve or gasket problems.