M50 engine R&R

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Bill - 03 May 2007 16:54 GMT

This is a new thread from posting on May 2, to avoid confusion (for me!)...

> Bill, I know of a person who has a M50 engine in a 1994 525is. It has
> been sitting in his garage for a year because the engine will overheat
> if he turns it on. He took it to a dealer before he placed it in his
> garage and here is the report from the dealer:
>
> 'Started vehicle cold. Found running rough for first few minutes
> beforte warming up. Perfromed short test, No faults found in DME.
> Removed DME to check for corrosion or moisture . ok. Check ignition
> coils for cracks or defects. ok. Reoved spark plugs. Performed
> compression test. Compression test failed #6 cyl first time but passed
> second time. During compression test, noticed oil spraying out of cyl
> #3. Looked inside and noticed top of piston head covered in oil.
> Performed leakdown test ..results:
>
> 1=10 2=12 3=35 4=17 5=12 6=17
>
> cyl #3 leakdown not acceptable. Heard air coming out of dipstick tube
> during #3 leakdown test. Vehile requires engine rebuild or
> replacement."
>
> Now, I am very skeptical about the conclusion that the engine needs
> replacement. My question is: would a gasket change take care of the
> oil leak problem here in cylinder 3?

-----

First guess is the rings on #3 have failed, thus the oil. You didn't mention
smoke from the exhaust or anything about coolant, so I can't be more
specific. Either way, I'd remove the head. If the rings need replacing, it
can be done on #3 without removing more than the head and oil pan (I've done
it), i.e. the engine can stay in the car. This will allow for direct
inspection of all the cylinders, walls, and the bottom of the head and
valves. You'll be able to replace nearly anything you need to at this point.
New: head gasket, head bolts, rings #3 (they come as a complete set and I
might just do them all to be safe since it's open), valve cover gasket.
Check the timing chain guides or belt and tensioner, intake-to-head gaskets
if removed. Maybe do the water pump and gasket. No need to replace the
engine. They are built pretty well and this should be rebuildable easily.
You will see the cause of the problem when the head comes off. Unless the
rings gouged the cylinder wall or ruined the piston top (doubtful), this
should be basic R and R. You may need the cam holding tool for the head
removal. Time consuming but not hard, and uses basic tools. Get access to a
Bentley manual first. Oh yeah, what about coolant system?

Bill in Omaha
'86 535i

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Jeff Strickland - 03 May 2007 17:33 GMT

> This is a new thread from posting on May 2, to avoid confusion (for
> me!)...
[quoted text clipped - 28 lines]
> smoke from the exhaust or anything about coolant, so I can't be more
> specific.

Please allowo me to add a tidbit here.

The #3 Cylinder's Exhaust Port is a known area where a crack can occur. This
crack happens between the valve seat and the adjacent water
jacket/galley/passage (chose the term you like best, they are all functional
equivelents to me for the purpose of this discussion). (Bill, the OP did
mention noticable "white smoke," which is normally steam as opposed to
smoke, so there is a very real possibility that there is a crack in the
head.)

All I am suggesting is that your only trouble might be a crack in the head
and the resulting failure of the gasket in the immediate area of the crack.
Whether you need rings or not, I can't tell. But, you have to remove the
head as Bill says in order to understand what is happening.

You (the OP) needs to do the stuff that Bill suggests, but while doing it be
sure to inspect the head in the area of the exhaust valve seats and the
adjacent water galley. I'm not certain if there can be cracks at other
locations or not, but the head on my M50 motor definitely was cracked where
I talking about.

Having said all of that, I have to wonder if you could have a leak-down
result that comes solely from a crack at the exhaust port.

Either way, I'd remove the head. If the rings need replacing, it

> can be done on #3 without removing more than the head and oil pan (I've
> done
[quoted text clipped - 17 lines]
> Bill in Omaha
> '86 535i

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