Bad turbo seals would not cause low compression.

I think I'm leaning in the direction of pulling the head and shipping it to Metric for a rebuild. But if I do that, will it be very evident what the problem is? Will I be able to identify a burned or holed piston with the head off? Or will I get everything buttoned back up only to find out the engine needs to come out?

Also, is the Haynes manual a good enough resource to guide me through the r&r?

Thanks everyone for all the good advice.

My guess...
 

My guess is that you have worn valve guides and a couple bad valves. I'll be amazed if it's a hole in a piston.
Some of the smoky/not smoky tests are different on a turbo than on a NA or a gasoline engine. Decelerating with a gas engine with the throttle closed builds up a lot of vacuum in the intake manifold, and can suck oil down past the valve seals and into the combustion chamber. On a NA diesel (unless it's an old old Bosch throttle-body) you only have slight vacuum, caused by air filter resistance. On a turbo you go from neutral to a lot of positive pressure in the manifold. On a boat engine (or other stationary) you can load it right up (and create boost pressure) while you stand there with the rocker cover off, and sometimes see oil blowing up away from the valve seals. Sure sign of a bad valve guide.
I'll also be amazed if a cam lobe could wear enough to screw up a compression test. But there's a first time for everything.
Particularly with a turbo, exhaust valves are the ones that burn. You might possibly be able to hear air leaking out past the exhaust valves if you listened with a stethoscope, but equally maybe not.
You also might try rigging an air compressor hose to blow pressure in with the engine stopped, just before TDC for each piston. If it's a holed piston, air will come whistling out the oil cap. If it's an exhaust valve, you'll hear it whistling in the exhaust manifold, and if you hold a tissue over the exhaust pipe it will blow it. Clamping your hand over the exhaust won't indicate, because if any other piston's valves are in overlap, pressure will escape into that ex. mfld, into the cylinder, out the intake valve, and back into the intake mfld. Puzzling anamoly.
Lots of cars and trucks these days have more complicated heads than an old Mercedes, and at least in my rural backwater there are two shops where they're accustomed to stellite face/sodium filled valves, and would not be challenged by doing a valve job on a 5cyl Mercedes. Whether they'd be cheaper than sending in for a reman is another question. Certainly the mailorder approach allows quicker turnaround on the job.
So what do people say? Always plane the head? Always replace the head bolts?

I'd think twice about putting money into a head rebuild with those compression numbers and that mileage. It seems likely to me that the rings are reaching the end of their useful life also. I'd consider rebuilding the engine but more likely, I'd look for a good used engine to swap in.

I'm not sure what your experience level is with rebuilding motors, but a hole in the piston should be pretty obvious.

A quick test for leaky valves after you remove the head is to turn the head on it's side, fill the ports with cleaning fluid, and blow compressed air around the valves on the piston side. Bad valves will show as air will leak past the valves and appear as bubbles in the cleaning fluid. If your compression is down to 100, this should easily yell you if the valves are bad. Most machine shops will check this for free; find a local one and get to know the machinist.

If the valves are ok, the next place to look would be at the pistons/rings.

I'm not that familiar with the OM617 engine, but I'd lean towards something fishy on the head before condemning the bottom end. It could be piston or ring related, but it will be bery hard to tell without pulling the head. I think you'll have to yank the head to start with, have it checked out, and go from there.

Unless this car is a showpiece, and/or money is no concern, I wouldn't go with a Metric rebuild. You should be able to get a good used 617 dirt cheap nowadays. It might take a little work to bring it up to snuff (i.e., you might have to do the timing chain on the used motor) but in the end it would likely be several thousand dollars savings.

Just my $0.02...

:kid:

Dave, I'm not sure if you're referring to just the head or the complete engine.

If it's the complete engine, I'm in agreement.

However, to rebuild the head with all new valves and valve guides is approximately $600. at Metric. This is not exactly overpriced and the workmanship is top notch. They will also convince you to replace prechambers at a cost of another $300

But, the results that I received were nothing short of outstanding. The SD runs like the proverbial clock.

Dave, thanks a bunch for the input. Brian, you too. An expert I am not, and I didn't even stay in a Holiday Inn Express last night ;-), but I have two strong feelings: 1, that my poor compression is due to valves, and 2 it's probably because of the PO driving around with the timing chain way out of spec. Wish I had just done the head when I did the timing chain. Oh well.

If a head rebuild likely eliminates my smoking problem, then I'm tempted to do it. I can do all the labor and wouldn't mind putting $900 in for the complete job including prechambers.

Does a head rebuild, from what you've seen in your experience, usually help out all the compression numbers? I'd love to see all the pots above 300.

What's funny is that the car will start down to about 25f w/out plugging in and it idles great on Diesel with Cetane 45 (does get a little rough with the Cetaine 40)--no nailing that I can detect, but again not an expert on detecting noises.

Thanks again, guys.

a badly worn cam could be not letting a full charge of air in if the intake lobes are so worn it is not staying open long enough.

i suggest you take the valve cover off and have an experienced machinist look at the camshaft. you can look as well, if it is off that much it may be pretty easy to see.

with compression figures so low i would think the performance would be terrible. rough idle too.

i suspect the compression test. it may be giving you bad readings.

i wouldn't pull the head just yet.

tom w

With the cold weather performance that you have indicated, I'm suspicious of the compression test. It's sometimes difficult to get a properly running 617 going when it drops below freezing.......with four cylinders, it would be next to impossible.

You may very well be suffering from burnt valves due to PO neglect. If the valves are ignored for sufficient time, they will eventually fail to close and will quickly burn the edges.

Naturally, it's impossible to guarantee that the head is your issue. But, suffice it to say, that you'll be very pleased with the performance of all new valves unless the compression is truly way below par due to worn rings or scored cylinders. I don't see that as probable in this case.

but look at the cam first.

i agree it is a long shot but worth checking.

pulling the head is a lot of work. i wouldnt want to be around if you did that and the machinist informed you the problem was just a couple of worn lobes on the cam!

in a turbo engine the intake pressure would also mask this, i think.
but it would show up on a compression test at 3.5 rpm.

tom w

Sorry for the confusion - yes, I meant it's not worth shelling out for a rebuilt long block 617. However a head rebuild, as you described, would be fine. I would NOT shell out for new prechambers, I consider that a waste of money unless they exhibit some signs of failure (cracking, pitting, etc) which would be quite rare, IMO.

The idea to check the cam lobes is excellent. If the cam looks perfect, then it's time to yank the head.

:kid:

Apparently, the shaft that the ball sits on will loosen in the walls of the prechamber over time. After 175K, the shafts were not rigid in the walls anymore and the likelihood of failure in the next 100K is significant.

The question is exactly how long these shafts would last. The extra $300. was worth the peace of mind in my case.

I'd inspect them first. if they are loose compared to new ones, it wouldn't hurt to replace them. A failure of the spherical pin is quite rare... I've only heard of this a couple of times. Maybe it affects the 61x engines more than 60x.

:wacko:

First, thanks a bunch for the help everyone. My ONLY issue with this forum is that folks don't update their thread to show the resolution, so I will definitely do that. It seems like I'll find a thread that looks like similar issues as mine, but then it just stops with no explanation of a fix.

I'm going forward with head removal, but first I'd like to do a leakdown test, just to be sure I know what I'm up against. If it's bottom end, then out with the motor! It seems like if I get into the valve cover a leakdown could be a small investment in time.

To do a leakdown, don't I need to measure each cylinder on each stroke--intake and exhaust? With the valve cover off could I just shoot air into each cylinder as I observe the valves in a synced closed position? Or if I take rockers off, wouldn't that insure that every valve is as closed as it's gonna get and start shooting air from that point?

If I find that leakdown shows burned valves, what are the chances at that point that a cylinder could have both burned valves and bottom end problems?

Also, is Haynes a decent manual to use for head r&r? Or is there a better manual out there. I have decent mechanical experience, but I've never been that far inside a diesel or a Benz.

Thanks for help.