Compression Test (the saga continues...)

Hey guys,

I am the guy with the smoking 220D.

I was told by my mechanic that I most likely had a bad injector, because the car was squirting unburned diesel smoke. He thought that it might be stuck open, thus injecting on all cycles.

His theory was wrong though, because I systematically replaced every injector and found no improvement.

Thanks to helpful members of this forum, I began to think about the motor assembly itself. I was idling the car, and began touching the injectors. Sure enough, as predicted by a distant uncle, one of the injectors wasn't heating up. The #4 cylinder doesnt produce a complete combustion as far as I can tell, because the injector never heats up and the line doesn't "hit" like the other combusting cylinder's diesel lines. Damnit.

This means one of two things: either #4 has lost compression, for one reason or another, or the injector pump is not providing the correct fuel pressures to that injector at the appropriate time.

Before diving in to the injector pump, I am going to investigate the cylinder and head by running a compression test. I need help though ... what adapter size should I get for a diesel compression tester? Lots of companies offer the compression tester for $90 or so, and various adaptors for extra monet. What # adpator works with my injector size? Any good model compression tester I should look for?

After I test compression, I will repair as needed. I suspect a bad valve. Could it be that the valve is just waaayyy far out of adjustment?

Any advice on the pump? Do these fail regurally? Any common issues/fixes?

Thanks for any more help guys!