Faint

Hello everyone/5series forum members. I just picked up a 2008 535xi E60. I'm new to the chassis, but I also currently own a Topaz e46 330ci... I obtained this e60 knowing there was something mechanically wrong. The previous owner stated that it had a rod knock, but upon multiple inspections/tests I heard nothing that even remotely compared to rod knock. With this being said, I compression tested every cylinder and 4 was the weakest link with only a measly 25 psi. I pulled the intake off and inspected the intake valves and they are very gunked/built up. Making sure cylinder 4 was at TDC, and the intake valves were closed I filled the cylinder with a valve safe cleaning solution... From here I took compressed air and pressurized Cylinder 4 and when I did I receive compressed air creating bubbles seeping past the seated valves, enough to make it look as if the solution is boiling/bubbling.. My question is on a stock untouched/unmodified N54 what are the common failure points? I am waiting on my borescope to hopefully get an idea as to how the piston looks... Do rings die on these commonly? Shouldn't I have a higher compression number than this even if the valves aren't seated all the way and has anyone else ever had this occur or happen? I'd hate to tear the engine apart just to determine the valves have gunk on them. I don't think the walnut media blast will clean this valve, because I believe it is in the actual seat that the build up is. Anyway, I hope someone can shed some light on the situation, and hopefully I posted this thread in the right place.

Thanks,
Faint

NoQuarter

Problems with rings are very rare - almost never discussed on the forums over the years.

You started with diagnosis info but you have not stated how it runs. What are you diagnosing besides something the previous owner mentioned?

JuniorB

Pull an oil sample, have it checked out, it will show metals in the oil if it's a rod knock. Don't drive it just yet, you need to confirm it's not, and be able to repair if it is.