Author Topic: M42 low compression cyl4 (Read 5555 times)

Cobrakiller · « **on:** September 15, 2010, 12:44:27 PM »

Hi, I'm new to this forum.
I am havin some issues with my 91 318i.
First, I bought it with a cracked head that was leaking coolant outside the engine.
Bought a used head and had it cleaned, inspected and pressure tested. Everything checked out great.
Honed the cylinders and installed new rings because before it was torn down it had low compression in cylinder 4.
So now it's back together with all new gaskets and
seals.
Still has compression issues in cylinder 4 what's goin on?!?!
Could a falty lifter cause low compression?
I'm stumped any help would be great.
Thanks.

nicknikolovski · « **Reply #1 on:** September 17, 2010, 06:13:07 AM »

Have you also done a wet compression test? Anything replaced in the cyl head? Was your timing done correctly. How much compression (PSI) did you get on cyl 4 compared to the others?

Cobrakiller · « **Reply #2 on:** September 20, 2010, 08:00:49 PM »

I would get 30psi cold and 180+ on the others then put some oil down there and I got around 150psi cold. Checked the timing many many times. Seems that the rings aren't sealing. But I don't understand why everything looks great.
I Replaced all the lifters too. I'm ready to crush this car with my jeep. Lol
Any ideas?

DesktopDave · « **Reply #3 on:** September 20, 2010, 09:07:50 PM »

You can get good results with a leak-down test too. Before you decide to tear that motor down (again), get it tested by a pro. I always go for a 2nd opinion. How far did you hone the cylinders? Did they need oversized pistons? Any unusual wear or scuffs in the piston lands?

Lifters only take care of valve lash between the cam and valve or rocker arm - they can't hold a valve open by themselves. No way that'd cause a serious leak unless you had a bent valve or worn out guide too. But you had the head checked, that's unlikely.

Cobrakiller · « **Reply #4 on:** September 20, 2010, 09:42:54 PM »

I didnt take much off at all. let me add that the motor had this problem before the motor was ever torn down.

DesktopDave · « **Reply #5 on:** September 20, 2010, 09:49:06 PM »

That is odd. I've heard all kinds of stories about the cylinder head cracking, profile gaskets dying, oil leaks galore...but not one complaint about the bottom end. They're very durable motors. I'd still go for a leak down test.

Cobrakiller · « **Reply #6 on:** September 20, 2010, 09:56:02 PM »

I have heard the same things.. Im gonna do the leak down tomorrow.

Cobrakiller · « **Reply #7 on:** September 22, 2010, 11:39:54 AM »

Ok so the valves on cyl 4 never close all the way when the cams are installed. But when there not the valves seal perfectly. Keep in mind the head was inspected and tested by napa. What the heck.

nickmpower · « **Reply #8 on:** September 22, 2010, 12:45:11 PM »

hmm, sounds like a stuck lifter? try it with just one cam bolted on so you know what side it is

Cobrakiller · « **Reply #9 on:** September 23, 2010, 01:07:34 AM »

That's what I thought. But was told that a lifter wouldn't cause a valve to stay open.
It's one of the intake valves.

KenC · « **Reply #10 on:** September 23, 2010, 07:03:25 PM »

If it's stuck in its fully extended stage, it will keep the valve open.