Compression vs Leakdown tests

[FiveOJester]

I'm currently very interested in a '95 530 that is at a local dealership. The car currently has 120k miles on it and looks to be in exceptional shape. I had a local BMW shop do an inspection and they passed it, citing a few minor issues like the guibo, rotors, etc. Nothing major. The shop also did a compression check and found the cylinders to be in the range of 155 to 165 psi. The mechanic said that brand new they are 170 psi and everything was good. At the time I didn't realize that there is another test called a leakdown test that is often performed. Is this necessary to perform or are compression numbers good enough to ensure that the engine won't take a dump on me anytime soon? I plan on keeping the car until at least 200k.

I also just got a Carfax and found out the dealership had been lying to me. They claimed it was a single owner car owned in Monterey, CA. Carfax showed me it was originally leased in New Jersey for 3 years and then brought over to California. I know that the East Coast had the higher sulfur fuel which created that whole nikasil controversy. This has me a bit worried. The car idles a little rough at 600rpm once it warms up, however not any rougher than most other cars I've driven. Turning on the A/C which kicks up the idle to 750-800 rpm smoothes it right out.

Do I need to worry or does a smooth idling 120k mile motor with good compression and an unknown leakdown percentage have a good chance of lasting?

Thanks!
-Dave

[632 Regal]

mine doesnt idle very smooth either, if the Nikasil issue bothers you then have a leakdown test performed or you wont sleep at night. A leakdown test is performed with each piston at top dead center...thats where the Nikasil chaffing developes.