Hello Mini community,
Wondering if anyone might have some insights to my issue?
I picked up a 2005 S as a project knowing a shop diagnosed #4 cylinder issues.
I went thru the engine (rebuilt head, new rings, bearings, etc.)
Also did the clutch, axles, etc since it has 135K miles.
The cam and rockers looked good so they were used as is.

Got it back together and running and noticed it was missing and determined it has no compression in #4 cylinder.
Pulled the head, took it back to the shop where they verified the head is as new.
The #4 piston top and combustion chamber were clean, where the others had some small carbon evidence of firing.
Reinstalled the head for a leak-down test, all cylinders are about 12%.
Intake and exhaust have no blockages.
I've had several local mechanics and auto enthusiasts look at it and we're all still scratching our collective heads.

Any chance you've maybe seen this issue before? If not, any ideas for a fix?
This is too nice a Mini to not get back on the road.

Many Thanks in advance,
Pat

 

How did you determine #4 has no compression when all 4 cylinders have 12% leak down?

Perhaps you have a fuel delivery problem to that cylinder which would cause misfiring and lack of carbon build-up.

 

Hi mrbean,
The compression test was done with everything assembled and cranking the engine over as normal with the compression gauge attached to the spark plug holes (1-3 good, 4 not).
The leak-down gauge also attaches in the spark plug hole but the engine remains static and test is done only by attaching the compressor to the gauge (all 4 the same reading).
BTW - The autozone code reader also indicated an issue with #4

I guess I could try swapping the injectors around to see if that changes anything.

 

I would be concerned about a microscopic crack in the wall on cylinder 4. Was the block inspected?

 

If it's not the cylinder wall (as cooper48 suggested), maybe a problem with the intake manifold?

 

I'm also assuming the head was rebuilt with new valve seals. Yes?

 

My question is really why would a compression test show a bad cylinder and the leak down did not. This is confusing because in my experience leak down tests often catch problems that a compression test does not.
When you say #4 is "bad", what does that mean as far as pressure readings? Was #4 a few psi lower than the others or was it ~100 psi lower?

edit:
A problem with a cam lobe and/or tappet could cause a low compression test but not show up on a leak down test. I would pull the valve cover and start poking around in there.

 

Sorry for the confusion, by "bad", what I really should have said is nearly zero, where #1-3 were around 140 if memory serves. So I pulled the head expecting to find an obvious issue with either the piston/rings or valve train. But everything there looks normal. So I reinstalled the head and did the leak-down test only to find all cylinders basically equal. This is why I'm reaching out regarding what has us all mystified.

I should also clarify, the head is currently off and on the bench with the cam and rockers installed so the cam can be rotated to see that all the valves are operating correctly.

Thanks again for any/all input!!