OM603 Compression Test - PeachParts Mercedes-Benz Forum

DieselPaul · #1 09-28-2010, 11:00 AM

So, I have a motor that has been sitting for 6 years that I'd like to compression test. I was told it came into the yard running. The car was full of oil when I got it, right to the max line, the turbo wasn't too grimey, it was full of coolant (green) but there was no water/oil mixing. I took the injectors out a month ago to try the in my daily, but I put rags in the injector holes. I take it I need to dump something in the cylinders to loosen things up. What do you guys recommend? What is the procedure to compression test a motor? I've never done it before, I bought the Harbor freight compression tester, it says it has the mercedes thread.

jkubica · #2 09-28-2010, 12:09 PM

Quote:

Originally Posted by DieselPaul

So, I have a motor that has been sitting for 6 years that I'd like to compression test. I was told it came into the yard running. The car was full of oil when I got it, right to the max line, the turbo wasn't too grimey, it was full of coolant (green) but there was no water/oil mixing. I took the injectors out a month ago to try the in my daily, but I put rags in the injector holes. I take it I need to dump something in the cylinders to loosen things up. What do you guys recommend? What is the procedure to compression test a motor? I've never done it before, I bought the Harbor freight compression tester, it says it has the mercedes thread.

Hi
I'd just shoot a good dose of WD40 in each cylinder, rotate the engine through several revolutions by hand to confirm that nothing is stuck - hold or tie down the stop level to cut off fuel - and then spin it with the starter until oil pressure starts building up.

Using a remote starter switch (about $8) under the hood makes it easier.

Hook up the tester and give the engine 8 or 10 spins and record the results. The absolute numbers on a cold compression test are not so important as is the differance between cylinders - which shouldn't exceed about 45lbs.

If you have suspect numbers, its time to get the engine running, adjust the valves and do another compression check at operating temp.

Sometimes a low cylinder reading can be the result of stuck piston rings. Some members have used a Marvel Mystery Oil soak to free up carboned rings.

Good Luck,
Joseph

babymog · #3 09-28-2010, 12:21 PM

As WD-40 is mostly solvent, it will wash the lubricant from the cylinder walls, I'd choose something else such as the afrementioned and often suggested Marvel Mystery Oil. A spray that I use for coating parts for storage is Slick 50, it is in a red/black can and has a lot of tenacious oil, the only Slick 50 product I've found to be useful. I use it for spraying open cylinders, camshafts, etc. and it stays on the surface to prevent corrosion for a long time.

DieselPaul · #4 09-28-2010, 12:34 PM

The motor is out of the car, it's sitting on a palette and I don't see a block heater. I know my numbers will come up a bit low. I'm just looking for evenness of numbers. If I get 5 marginal and one 0, it's probably bad. I've got a can of stabil fogging oil I'll give the cylinders a spritz of that. I'll buy some marvel oil to loosen up the rings.

DieselPaul · #5 09-28-2010, 03:56 PM

Should I fill the pan with some used oil or will it be okay just turning over the few times by hand?

Aquaticedge · #6 09-28-2010, 04:43 PM

if the pan is empty I'd put oil in it. should never be run or turned without

jkubica · #7 09-28-2010, 06:11 PM

Quote:

Originally Posted by babymog

As WD-40 is mostly solvent, it will wash the lubricant from the cylinder walls, I'd choose something else such as the afrementioned and often suggested Marvel Mystery Oil. A spray that I use for coating parts for storage is Slick 50, it is in a red/black can and has a lot of tenacious oil, the only Slick 50 product I've found to be useful. I use it for spraying open cylinders, camshafts, etc. and it stays on the surface to prevent corrosion for a long time.

Hi
I agree that WD40 has low lubrication quality. However, in the case where an engine has been sitting up a long time and may have rust in the cylinders, Using WD40 or Kroil can loosen the rust and prevent damage (such as a broken piston ring) when the engine is first rotated.
Joseph

babymog · #8 09-28-2010, 10:07 PM

Fair enough. And not to offend, but I have found WD-40 to be a poor penetrating oil, would prefer the Kroil that you mentioned for that purpose, ... WD-40 has only been useful as a starting fluid and for cleaning in my garage (and there are better fluids for both of those purposes also).

DieselPaul · #9 09-28-2010, 11:26 PM

I sprayed it with fogging oil, let it sit, sprayed it again, then turned it a turn or two, then added more oil, the turned it over again and sprayed some more oil in it before I left. The motor doesn't have any injectors in it, so its not really compressing anything but I can't decide if the motor is too easy to turn over. But it isn't locked up, doesn't make any weird noises. The cam looks fantastic, the chain doesn't look worn and isn't obviously loose. When you turn it over you can hear it blow air out of the cylinders.

I left my compression tester in cincinnati, I'll have to go get it this week.

scottmcphee · #10 09-28-2010, 11:39 PM

sounds ready to compression test

(make sure you don't have too much oil dumped into the cylinder you're testing for fear of hydrolock)

Jeremy5848 · #11 09-29-2010, 12:31 AM

Quote:

Originally Posted by jkubica

If you have suspect numbers, its time to get the engine running, adjust the valves and do another compression check at operating temp.

No valve adjustments on an OM60x engine!

babymog · #12 09-29-2010, 11:18 AM

Oil in the cylinders will also mask bad rings (wet compression test) as oil will not leak past the rings as fast as air will.

Quote:

Originally Posted by scottmcphee

sounds ready to compression test

(make sure you don't have too much oil dumped into the cylinder you're testing for fear of hydrolock)

DieselPaul · #13 09-29-2010, 12:51 PM

The fogging oil is pretty thin, I'm only doing 2 second bursts, not like filling the cylinders. I'm picking up my compression tester tonight.

barry123400 · #14 10-01-2010, 12:53 AM

The reputation of the seller is very important with an engine like this. Bad wreckers sell more boat anchors sometimes than good engines. They have no concience about a customer installing a bad engine and having to pull it out again.

An engine on the ground is a risky proposition. Running when the car came in can mean anything. If you take it at least have an agreement for full refund if a dud.

I would rather buy a smashed car or any cheap car that I could critically examine the engines operation before using it.

DieselPaul · #15 10-01-2010, 12:58 AM

I paid $600 for a complete car. I didn't buy it hoping the motor was good. I was looking for a spare IP, spare turbo, fuel tank, and an uncracked head casting. I got all that and much much much more. I am just curious as to whether or not the motor is usable as it sits for the S350 I was looking at.