Hi Everyone…

This is my first posting. I would really appreciate some kind intuitive thoughts on this problem I’m having with my W123 300D. Forums like this are great since when something catastrophic happens to your own car, it can be harder to diagnose than someone else’s…biases and prejudices get in the way, or just panic…kinda like an MD diagnosing a family member!

Anyway…my 300D is a three-owner car, only 180K on the clock with all service records, owned by the proverbial little old lady who willed it to her nephew who soon put it up for sale. I recently did a diesel purge, changed all hoses, never been run on anything but diesel. It always starts right up even in cold weather; minimal blowby, good idle, but occasionally idled rough after a high speed sustained run, but later smoothed out. Never did a valve job yet and didn’t take compression until now with current problem.

I was driving at a sustained 70-80MPH on the highway the other day when I noticed a trail of black smoke. Stopped and idle very rough, obviously missing on a cylinder. Got home, cracked the fuel lines, number 5 cylinder next to firewall dead. Was hoping just a stuck open injector. Waited a week for some tools from MerSource (buy all parts here though), and pulled all injectors, all pintles looked the same, number 5 popped at 1800 PSI, good spray. Compression was zero. Did not adjust valves, but cam was opening valves, so should have seen something. Before pulling injectors and valve cover, oil cap sitting loose sat fine…no blowby. No water or oil in coolant. Did not feel ANY pressure blowing out of that cylinder when cranking. No mechanical noise or anything is rattling about. Compression was approximately 340 on 1, 2, 3, but 4 was 200 and 5 zero. Sooo…here’s what I’m thinking, appreciate any thoughts…

1. Can’t believe a burned valve, but probably the case. The valve rocker gap is loose so no valve was hanging open to burn a seat, but as these are sodium filled to act as a heat pipe, if they don’t seat I know they will burn. Diesel combustion temps are higher than a gas engine but more work is done making them more efficient so actual exhaust exiting should be lower…unless a weak spring allowed hot high velocity gas past seat and toasted it.

2. I don’t have a slide hammer to get precombustion chambers out to inspect with a bore scope…hopefully soon. I don’t think I’ll see a hole in a piston or anything like that, otherwise there would be a heck of a lot of blowby. And probably rattling if a valve head dropped and poked a hole in a piston.

3. Adjacent cylinders make me think head gaskets burn through between 4 and 5 perhaps? Wonder why 200 in 4 and zero in 5…if burn through I would have guessed zero on both. Exhaust pipe ‘dollar bill’ test reveals ‘suck back’…classic valve leak t4est. Also, wet compression test didn’t improve anything on number 5, still zero.

4. One research paper I read indicates that more deposits form with rich condition causing debris in and around valve seats leading to burning. Engine oil was not likely the cause. Their conclusion was that “ valve failures can be attributed to the deposits forming on them. The control of lubrication oil dilution by uncombusted and partially combusted fuel is more critical to valve deposit formation than the oxidative stability of the lubrication oil. The maintenance (lubrication oil contamination), operation (adjustments of air intake, fuel supply and coolant flow), component supply (manufacturing processes detrimental effects), and design (prime mover correctly sized for the task to ensure its runs in its efficiency envelope), are critical to maintaining problem free equipment”

5. Loose cam/rocker condition would only cause a low compression…engine ran well despite this. Too tight is worse of course…’ turbo charged engines sometimes sodium filled valve stems are used. These valve stems then act as a heat pipe. A major cause of burnt valves is a lack of valve clearance at the tappet; the valve cannot completely close. This reduces its ability to conduct heat to the cylinder head via the seat, and may allow hot combustion gases to flow between the valve and its seat’.

So I’m thinking a bad head gasket and a burned valve…does everyone agree? I kick myself driving over 45 minutes at a high speed, but had to keep pace with traffic. I wonder if the LiquidMoly diesel purge several weeks ago loosened up some debris that got caught in the valve seat. I guess we all have to be careful. I n retrospect, I would after buying a diesel make sure all injectors were spraying correctly, since a rich condition appears really bad for these engines. I would also be interested to know if anyone has heard of a loss of compression with a low mileage car, or head gasket or burned valve…this all seems really surprising in a well maintained car with no blowby. I hope I can get by with a rebuilt head from Metric Motors…they are about $1500…I was stunned a long block from them is $10K! That’s crazy and really scares me. (Same price for a W113 engine). Makes me want to try to do new sleeves pistons and bearing in the car after I pull the head and if needed, but I’m not thrilled about that prospect. Didn’t sleep much the last couple of days worrying about all of this.

Appreciate any thoughts…thanks in advance!

You may want to perform a cylinder leakage test. This test injects compressed air into the cylinder when the piston is at TDC and both valves are closed. If you hear air escaping through the exhaust pipe you have a bad exhaust valve; if you hear air escaping around the intake system - bad intake valve ; air escaping out of the oil fill hole on top of the valve cover worn or broken piston rings. One sign of a blown head gasket is coolant in the oil - the oil will look like cream.

Thank you...that's a good point...I did look for anything in the coolant after the engine was stopped, and when I ran it the following day...nothing...and nothing out the tail pipe but black smoke when revved due to the dead cylinder....so I don't think if a blown head gasket it is near the coolant passages, just possibly between cylinders...then again, not having done a valve lash adjust, the 200 reading on number 4 may be just due to intake not opening enough and long enough. Have you ever head of a hole in the piston of an OM617 before? I guess I would also be surprised broken rings at this mileage...I would think the wet test should have shown some increase even with broken or stuck rings, yes? Number 5 is a totally dead zero PSI...

Sounds like it might have a hole in the piston. The only thing that pops into my head is a broken glow plug falling into the cylinder or the tip of a calve falling off and doing the same. Zero is very low. Bad rings would not do that.

If its a hole in the piston there would be massive blow by.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Yes, zero is very low! Doesn't the glow plug sit inside the precombustion chamber, which itself is made of stainless? I was just thinking about what you said about the head of the valve breaking off...I'll try to get a slide hammer to get the precombustion chamber our so I can poke a scope in and look...I'll check the rocker for any clearance at all..if the valve head broke off, the stem should pop up with spring pressure up against the rocker and cam lobe. I didn't hear any rattling around while cranking...have you ever seen a hole in a piston on one of these diesels? I'd be really interested to know what could do that...

Possible reasons for a hole in the piston could be:

Timing chain jumped and the valve and top of the piston collided

Valve head broke off and punched a hole in the top of the piston.

Thanks for the reply. The chain is solid, no slop....really impressive double row chain too. Can't imagine why they went to a single row in the gas V8's in the eighties...
I'm thinking that maybe there could in fact be a hole if a chunk broke off the valve, punched the hole, and dropped into the crankcase...things are spinning and moving about a quite a clip at 4 grand RPM...then there would be no rattling about.The chunk out of the valve would also explain the suck-back at the exhaust pipe, although I don't know why there is no crankcase pressure occurring. I'm still stunned this could happen in a low mileage well maintained car...

Holes are definitely a possibility, have you drained oil to see if you have metal in it?

Here is a pic of the engine I'm finishing up a rebuild on.



Got it for free off of Craigslist. The guy said it just died while driving around 70 on the freeway. This one never restarted though.

Is all your overboost protection stuff still functioning?

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Milan Brown 1979 240D, rebuilt OM617.952 turbo diesel, rebuilt 722.315 transmission (my only daily driver)

Instagram: @maximed93

Thanks for that tip...I didn't think about draining the oil to see what comes out...I'm stunned by the photo. I guess these diesel 'tank' engines aren't so rugged after all. You bring up a good point...the other day when I was replacing air cleaner mounts, I noticed a hose had dry rotted and fell off between the turbo compressor high side and what appears to be the waste gate on the exhaust side. I just replaced the hose. If that wasn't working for a long time, I could imagine the waste gate freezing up, so reconnecting the hose would do nothing...if that was the case, the boost at 80MPH might have been too high, and temps went through the roof, leading to a catastrophic failure. Everyone, if haven't checked your diesel, make sure that waste gate works so you don't have what likely happened to me happen to you! I'm going to check that gate tomorrow and I'll report back.

Hopefully by well maintained that includes an annual adjustment of the valves. Good maintenance on a MB diesel should include regular oil & filter changes, regular replacement of the fuel filters, maintaining the cooling system and an annual valve adjustment. Just a guess but it sounds like the valves in your car may not have had a regular adjustment.

Yes sir, well maintained means all that EXCEPT valve adjustments...I could see that affecting performance but not engine failure. I've just been reading a few old posts about the waste gates on these cars sticking, and the hose from the compressor to the waste gate deteriorating. I have a bad feeling there was too much boost, and the gate was frozen and the electronic cutout failing...someone mentioned that it would have been easy, inexpensive, and a good idea if Mercedes had added an overboost 'idiot' light on the dash...would have been simple to sample manifold pressure and add a simple closure switch for such a light. If it turns out I have melted piston(s) and whatever that support an overboost possibility, I'm going to design and add such a warning light...this is nuts to lose an engine over something like a broken hose and/or stuck wastegate! Again, something to add to the list we should all be on the lookout for when buying a MB diesel.

And there you have it.

I don't follow...I must be missing something...if the gap on a valve was too tight, it may not seat...not seating could affect heat transfer and allow hot gases to escape leading to a burned valve. Too loose, and performance would be down, as not fully filling the cylinder with enough air would make the mixture too rich. But I'm having trouble understanding how that could lead to zero compression and do so so quickly...the loss of compression was a rapid transient event while at about 4K RPM on a 30 minute run on the highway. The rockers are intact and the cam is opening the valves on number 5. So other than engine performance, why do you think not getting to a valve adjustment would cause catastrophic engine failure if they were loose but not tight?

Leak down test is your friend. You probably don't need the tool kit. Set the cyl to tdc. Hook up something to pump compress air either through gp or injector. You may be able to ascertain why cyl has 0 compression.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

Don't guess, don't randomly remove pre chambers,don't randomly remove the cylinder head, don't randomly replace parts.

DO a cylinder leak down test and listen for where the air come out. You do not need a leak down test gauge set because we don't care about % numbers at this point. Pull the check valve from the compression tester fitting, set cyl 5 to TDC both valves closed , pump in about 90 PSI of air then see where it leaks out. I'm voting for a burned exhaust valve.

An exhaust valve can burn and still have good lash. A hole in the piston would result in lots of oil out the exhaust / crankcase blow by. Whatever failed on the engine likely was already failing prior to the extended run.