Low[er] compression cylinder #6

[berndt]

Just got around to using the compression tester I bought a while ago. Most of the cylinders read surprisingly high:

1. 180
2. 155
3. 175
4. 180
5. 170
6. 115

As you can see #6 (back of engine) is considerably lower. Also, there is a pretty solid oil leak coming from the back of the engine. It ends up dripping from the around the transmission drain plug (it’s motor oil). Lastly there are occasionally some small “swirls” on the dip stick. I’ve actually not seen milky oil when draining the oil.

Anyway, I’m wondering if this could be a head gasket? Or rather, what does it sound like to you guys?

Generally, the car runs pretty darn well, but I need to start tackling the oil leaks at some point and wondering if there is a bigger issue lurking.

Here is a pic of the plugs:



Thanks in advance!


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[95BMWIC]

You have three signs - low compression and oil leaking down the back of the block and trace coolant in oil.

I think you got it - head gasket.

I have a write up (as do others) on this procedure. If you are somewhat competent and have tools -you should be able to handle the removal and install. You will need the head inspected and leveled -cleaned and new valve seats - ask the machine shop to inspect and recommend.

[berndt]

Thanks. I will be looking at write ups. Question I now have is bimmer heads head, or find a machine shop. I’m in the Bay Area so any shop/labor is extremely expensive.

I should note that my original post was a bit wrong. I have NOT seen coolant when draining the oil, but have found swirls on dipstick. That said, pretty sure this is the culprit.

I’ll ask local shop about inspecting and leveling regardless. But I like the idea of a hot cam and chip while I’m at it :).


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[e30davie]

First put a.bit of oil in cylinder 6 And try the comp test again. If its not a significant improvement its a headgasket or valve issue. If it is a big improvement then it could be rings but with all your other numbers it is highly unlikely.

Next is head off And drop it at the machine shop. Ask for an acid clean and a report. And hopefully theres not much wrong with it.

New gaskets all round and plonk the head back on, maybe a timing belt too for good measure and keep on trucking:)

Edit: cylinder 2 is suspect also....do the oil test above on all of them.

[berndt]

Thanks for the info! Will do wet test next weekend and report. I get a little blue smoke on acceleration, which I understand is generally valve seals. Hence, I’m thinking just getting a rebuilt bimmerheads head. Of course this will depend on the results of the wet test. But I assume the bottom end will be good. It’s only got 170k miles and has been well maintained.

I like working on this car so much better than my e39 v8!


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[wilholl]

It wouldn't hurt to check the valve adjustments while you are at it

[berndt]

Can a bad valve adjustment affect compression? I adjusted them maybe 5k miles ago and nothing seemed out of whack. The engine does clatter a bit, but assumed normal for m20.


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[wilholl]

Usually not unless the valves are so far out that they are not closing properly.
I just thought I would suggest just in case

[TobyB]

Can a bad valve adjustment affect compression?

Yes. But you'd have to be pretty tight to give you what you're getting on #6.

Personally, I would do a leakdown test, both cold and hot. This will give you
a better sense of where the cylinder's leaking. It very well could be a head gasket,
but with only one low, it should also be puking coolant. Or vomiting out the crankcase.

Since it isn't, I'd expect a leaking valve or two (#2 might be having trouble as well).

Absent other symptoms, try to figure out what's wrong before you yank the head.
Compression tests are not the most reliable way to make sure you have an engine.

t
has had junk under a valve freak him right out...

[berndt]

Thanks! I finally got around to doing the wet test today. Strangely, the "dry" number today on #6 is ~158 which is much higher than the first time. The wet number was ~165. The gist of it is, I'm probably going to take it to a mechanic to diagnose both the compression and the source of the oil leak.

[digger]

sounds like the original test was an error. the leak is probably just the rubber rocker shaft plugs