I have a 2004 Mini Cooper base and did a compression check yesterday and found that cylinders 1-3 were all about 190 and cylinder 1 was about 120. The car is showing no other symptoms though. Specifically, there are no codes being thrown, no oil and coolant mixing (in the oil or in the coolant), i’m not Burning oil or losing coolant. The car is maintaining temperature perfectly. It’s not blowing an inordinate amount of white smoke. I’m not getting anything unusual (smoke or noise) out of the valve cover cap. I’m not getting air bubbles in the coolant.

Before anyone asks why I checked the compression if all was well, I was doing regular maintenance and changing out the plugs and since th car is approaching 100k miles, I thought I would do a quick check.

Anyone have any ideas?

 

Going to move you to the Stock Problems/Issues area.

Next step would be a leak down test. Could be rings, gasket of a valve.

 

I would suggest squirt in a table spoon or so of ~20w40 engine oil into the cylinder. This will seal up the loose rings or badly scored cylinder wall. Do this only on #1. Run the compression tests again. This should help you narrow down if the problem lies in with the piston/ring/cylinder, vs the cylinder head. I think more likely than not you have a valve problem.

 

Not sure which of your cylinders is off, as you mentioned in your post that cylinder 1-3 were @ 190, and cylinder 1 was @ 120... Is cylinder 4 the potentially 'bad' cylinder here?

Irregardless of which cylinder's compression is lower than the three others; During your recent change of spark plugs -- was the plug of the 120psi cylinder different, appearance-wise, from the (3) 190psi plugs in any way? I would think that if you have a ring problem in that cylinder, the 120psi plug would've looked different from the others.

 

Short of a leak down test to confirm, the usual culprit is burnt exhaust valves on the Coopers, which requires the head to come off for repair.

Wet test it as said above, see what that comes back with first though.

 

I second reading the plug colour will also tell you something too. Additionally there has been one recent case of low compression which turned out to be a broken exhaust valve spring. Say to yourself, I am lucky and pray really hard.

 

Sorry for the confusion. Cylinders 2-4 were 190 and cylinder 1 was 120. When I changed the plugs, there wasn’t anything different about the plug in cylinder 1.