Low Compression causes..
 

Have a low compression cylinder at #2 on a 2.7L (w/CIS) Apart from a piston ring being chipped or broken which is what I expect - what else might cause this condition? 15K on a rebuild (apparently/supposedly).

Doug C

Valve seating issues: Poor valve / seat re-grind or too thin a margin on the valve face, weak spring pressure, chunk of carbon on the valve.

You should do a leak-down test and this will tell you what's happening.

I feel ignorant asking this but, can a leakdown test be performed while the engine is not installed in the car?

Doug C

It should be at operating temp, so yes ideally it should be in the car. Try not to sweat too much here. Just find out what's causing the issue. It may be something simple.

Have you checked the valve gap? A valve to tight could also cause low compression.

Do a leakdown or have it done for you. Engine does not have to be in car or be at operating temp. THe leakdown will identify the source of the low compression, ie valves or rings or both. Do the test and go from there.

THe problem you identified is a gross problem, ie low compression. Do not let the relatively small differences between a hot leakdown and a cold one prevent you from doing the test that will still tell you whats wrong with the engine.