A very very sad 300SD - low compression
 

The '80 300SD I picked up for $500 a couple weeks ago still hasn't started. Now I know why. I pulled all the injectors to run a compression check, and the results are not good. Cylinder by cylinder, here are the results - 1 - 175 psi. 2 - 50 psi 3 - 70 psi 4 - 150 psi 5 - 300+ psi (maxed out my cheap guage). These results are with a cold engine, as I couldn't warm it up if I wanted to.

My question is - what are my chances that a head gasket will solve this problem? 1%, 10%, 20%, 50%? I figure I have nothing to lose now, so I'm going to pull the head off this weekend and have a look.

Anyone else have experience with compression this low?

Steve

Dear Jetmugg:

Unfortunately, I think your chance is only 1%. You have to pull the head off to take a look.

Eric

Are the valves adjusted or at least checked? Numbers that low sound like valves not closing.

Jettmug,

I think you can do a wet test (add a table spoon or so of engine oil to the cylinder above the piston, repeat the compression test) and then a leak down test (connect a source of compressed air to the injector threaded hole and pressurize the cylinder, then listen for the leakage one cylinder at a time to track down where the air is leaking to) to further identify the problems. I know there are members of the forum that find a wet compression test scary (it is theorized the oil you pour in to the cylinder could result in an undesired ingintion of the oil during the compression test) but my local dealership's mechanics have told me they do wet compression tests on Diesels routinely and there is no suggestion in their factory instructions that this may be dangerous.

The wet test should tell you if the problem is a head/valve/head gasket problem or a cylinder wall to piston ring sealing problem(the added oil will temproarily improve the piston ring to cylinder bore seal, so if the compression numbers go up, the problem was with the piston ring to cylinder bore interface). Then a leak down test can tell you if the problem is a particular valve or the head gasket/head leaking. If this does not narrow it down suffciently you will have to take the head off anyway.

I agree you should check the valve lash and timing first though, and make sure the valves are not tight.

Good luck and I hope this helps. Jim

I second adjusting the valves. Also if I remember from your
first thread, I would make sure it was in time. Check you
Bar engine around in proper rotation and line up the timing
marks on the cam. You will find them behind the cam gear.
Then look at the harmonic damper to see if how much off
from 0 it is.

Let us know.

Thanks for the good advice.
 

I appreciate all the good advice. I will be running the valves, doing a wet compression check ASAP, and a leakdown test. As far as the timing marks, what do I line up the timing mark on the cam gear with? Does it sit level with the edge of the head, or is there some other pointer/indicator that the cam gear should line up with? I'm a pretty good gear-head, but this is my first time with an MB diesel.

Steve

Line up the notch in the thrust bearing on the back side of the cam sproket with the little bump on the driver's side of the front cam tower. You need to be fairly careful to get the bump centered in the notch.

Peter

definitely check that the valve lash is roughly equal cyl to cyl because you said "The '80 300SD I picked up for $500 a couple weeks ago". Some people's ability to understand machinery simply cannot be underestimated. Besides, it's easy to check and it almost seems improbable that compression that low could happen gradually.

WOW! STOP! HOLD YOUR HORSES!!!!!!!!!!!!!!

Do NOT do a wet compression test on a diesel!!!!!!!!!!!!!

The compression chamber is tiny and the oil can serve as fuel and cause the engine to start. It may not cause bodily injury, but it will blow out your compression gauge for sure.

Since you have already determined that the engine has low compression, put the compression gauge away. If you want to determine the source of the low compression, do so with a cylinder leak down test.

With pressure applied to cylinder, listen for escaping air; if from the tailpipe, it's exhaust valve, if from air cleaner, it's intake valve, if from oil filler hole, it's ring/piston/cylinder.

Good luck,

I would pull the cam/valve cover and look for bent or broken lifters.... any sign of recent work on the car? Sounds like there is/was some mechanical problem to me (bent lifter, valves, something like that). I can't imagine it would have ever run long enough for the compression to get that low just from wear on parts, headgasket might be a possiblity I guess.

Larry, Did you read this ?

" I know there are members of the forum that find a wet compression test scary (it is theorized the oil you pour in to the cylinder could result in an undesired ingintion of the oil during the compression test) but my local dealership's mechanics have told me they do wet compression tests on Diesels routinely and there is no suggestion in their factory instructions that this may be dangerous. " -----JimSmith

No I did not read that. I have known about the risk of wet compression testing a diesel for a long time.

If you're going to do it, make sure you don't use too much oil. Remember that this is a tiny combustion chamber and oil does not compress. Beyond the danger of the engine running is the danger of too much oil not compressing and causing damage. Be careful.

Have a great day,

This is much more risky on a high compression (21:1) diesel than a more typical American 16:1 diesel. Much less room in the combustion chamber, more heat.

Not only can the oil ignite, blowing up the gauge, but if you put too much in, it can blow the head gasket out if you fill the piston chamber area -- it ain't gonna go through the flame holes fast enough.

Peter

Maybe this is a vote for the leak down test... where the effects could be tested of sealing up the rings but without the danger....

Don't need to seal the rings for a leakdown -- air will escape from bad valves even if the rings are shot.

Peter