Recently entered the W123 diesel world by buying a cheap 1981 240D 4 speed on CL. Relatively new to these OM616s, would greatly appreciate any wisdom you Mercedes experts have to offer.


The car's odometer shows 109k miles, but has been broken for some time. Previous owner estimates closer to 200k. It was cheap due to the engine needing a valve adjustment, according to the PO. The car was in decent shape otherwise, not much oil leakage, decent interior. I test drove it briefly and clearly could hear a tapping coming from the engine, which I assumed to be the valve train. It was directly proportional to RPM. The more load I put on the engine or higher RPM the louder and faster the tapping got. It almost sounded like really loud injectors.



Decided to buy the car and adjust the valves and pray that's the sole reason for the tapping. I live about 3 miles from the buy site and after some thinking decided to drive it home on surface roads rather than opt for a tow.


Drove it back and it seemed to be handling it fine until about 1000 ft from my house. Had to stop at a stop sign and felt the engine struggling to idle, wanting to die. I kept it going by breezing through the stop and putting the pedal to the floor, but there was one more stop sign before my driveway, and had to stop here because of a school bus dropping kids off. When I slowed this second time the engine stalled. No warning lights came on during the drive and oil pressure was fine. No overheating either. Tried to restart and the engine barely cranked, sounded like a dead battery. Decided to just push it home and figure it out from there.


Got it home and decided to jump it and see if i could it started again (maybe the battery really was dead, dying alternator I didn't know about?). Got it started, but quickly shut it off as the original tapping sound had now morphed into a much louder and more menacing tapping/grinding sound.


Worried I had caused some other internal engine damage by running the car without adjusting the valves, I did some inspecting. First I did the valve adjustment, which turns out did need to be done. This also revealed that at least the engine wasn't seized, crankshaft turned fine. I also dropped the oil pan to look for metal shavings thinking maybe something in there got shredded somehow. Oil was pretty old but no shavings found. Pulled the glow plugs thinking that may give some clue about what's going on (not sure if this is as effective a diagnostic procedure as it is in gas engines). There was a small ring of black soot on each one about halfway up the pencil.


Wanting to avoid starting it and doing compression tests and all that until I know exactly what's going on for fear of damaging the engine even more. I'm doubtful that it was only the valves that caused the stalling and subsequent tapping/grinding. Maybe I exacerbated some pre-existing problem or caused a whole new one. I know there are about a thousand things that could cause a stall, but before going down that rabbit hole of checking fuel, ignition, and compression, I'm hoping this can be narrowed down a bit by someone who knows much more about these cars than me.



Any tips on things I can look for that might give me a clue to what happened? Would love to get this baby on the road soon. Any advice would be greatly appreciated.



Thanks!

I believe these OM61x engines have a weakness in the vacuum pump drive or the oil pump drive that can cause the noise, but I don't think they would explain the stall.

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M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

How far off was the valve clearance? If its just a little out of spec, chances are that's not the culprit.
Has the car been sitting a super long time?

I'd go after a fuel system related prob first for stalling. Maybe a plugged filter or gunk in the fuel? There's a clear inline filter near the primer pump. Might be worth shining a bright light into it for inspection.

The first cylinder rod bearing may need examined unfortunately. At least it is fairly easy to access. Usually there would not be a grinding noise though.

The vacuum pump does have bearings in it. It the bearings go, then the ball bearings can get into the engine. I'd pull it off and see. I think a rebuilt one is roughly $300. Also, for stalling, there is a check valve on the vacuum pump. It the hex part on top. Has a tube going back to the fire wall.

The PO is just guessing or " The internet says. . . " , this is similar to " Car does not run, needs something simple " , " Does not run, needs $ 0.50 fuse " , " bad head gasket , parts are $ 20." Of course the cars are being sold way below market.


Bad valve adjustment won't cause a grinding and a short trip won't damage anything.

You really need to have a person with engine rebuilding experience to look at it. The engine might be locking up due to failed connecting rod or main bearings. The test is to turn the engine by hand and see how tight it is.

Seek professional help before blindly throwing parts at it.

Loss of power on Diesels first thing to do is change the Fuel Filters and bleed the air out of the fuel supply system. Look at the plastic fuel filter and see if it is clear enough for you to seen inside of it. A lot of black gray particles inside mean that you likely have something growing inside of the fuel tank and that is what is causing the filters to plug up.

There is also a fuel tank screen that can get plugged. The check for the fuel tank screen is to have 1/2 tank of fuel and swap the positions of the fuel return hose and the fuel inlet hose where they come off of the tubings at the fender well.

As mentioned the vacuum pump issue can be an engine killer. You will have to start and idle the engine but probe around with a Mechanics Stethoscope or my favorite a piece of 2 foot long broom stick with my ear on the other end and see if you can locate where the sound is coming from. If it is coming from the vacuum pump don't run it any more till the vacuum pump is fixed.
Some people have changed the vacuum pump ball bearing so you may or may not need a new pump if the bering is no good.
You also need to inspect the timer gear face for wear when the vacuum pump is out.

If the fuel tank drain is plugged you will pull a vacuum inside of the fuel thank. If you remove the fuel fill cap the open tank fill tube will become the tank vent. Meaning if that cures the issue you know the tank vent is the problem. However, people have said if you drive like that with a full tank fuel comes out of the fill tube.

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Attached picture of plugged fuel tank screen.

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84 300D, 82 Volvo 244Gl Diesel

A lot of good suggestions. But a tap that becomes a real knock is a concern. Most the common issues will not produce a grinding sound either.

Although I am a believer in cleaning out the fuel system from end to end before putting one of these in regular service. Shortly after acquisition.

I also I agree it is better not to start it. Yet it might become required to find the source of the noise. Your description for example could just be an alternator seizing up. Is the belt still on the air conditioning pump? On the 616 engines ninety percent of the time if there is an internal bearing issue it is the first rod bearing.

The more I thought about this the more I agree with the checking of the vacuum pump. When the engine stalled out on the next start something that was already getting bad just got really worse.

Thank you all for the replies, there are some great suggestions here.

It seems the vacuum pump is the next thing to check. I’ll take a look at it today and post an update.

So far I have adjusted the valves, and in doing so confirmed that the crank turned smoothly, so at least there is no engine seizure yet.

The valves were pretty out of spec. The majority of them were tight enough that I couldn’t get the feeler between the rocker and cam, and the rest were way too loose. I agree with most of you though that this probably wouldn’t cause the grinding or the stalling.

I’ve also replaced the fuel filters and did an oil change, just to cross those off the list. The car definitely needed it, the inline prefilter was super clogged. That alone could be the stalling issue. Haven’t tried starting it yet. If the vacuum pump checks out I might just try cranking it to see what happens.

Thank you barry12345, I’ll also check the first rod bearing as well, that’s a good idea. The belt is still on the AC pump. Is it possible some bearing went out on the AC, power steering, or alternator? Might explain the grinding, not sure about the stalling.

Again, thanks to everyone for their input.

I would take a really good look at that vacuum pump. It could be the cause of the noise and also the stalling - no vacuum and the engine shuts off, just like turning the ignition will kill the engine with loss of (no) vacuum.

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Junqueyardjim
Christianity, if false, is of no importance, and if true, of infinite importance. The only thing it cannot be is moderately important. C.S. Lewis



1983 Mercedes W123 240D 4 Speed 285,000 on the road with a 617 turbo, beautiful butter yellow, license plate # 83 240D INDIANA

2003 Jaguar Type X, AWD. beautiful, good mileage,
Mom's car, but I won't let her drive it!

No vacuum to the shut off actuator will prevent the engine from shutting off, not the other way around.

When you turn the key off, you open a valve in the ignition switch that provides vacuum to the shut off actuator. Unlike a gasser, these engines do not require vacuum to run. They don't even make vacuum on their own, hence the vacuum pump.

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

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Also, just out of curiosity, is it normal to have two of the little inline prefilters installed? Here’s a picture of the fuel filter setup the PO had. Why have two?

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Thank you all for the great input. Lots of good advice so far.

So far what I’ve done is the valve adjustment, fuel filter change (both inline and main filter), and oil change. No actual engine diagnostics or anything like that yet.

The valves were fairly out of spec. Most of them I couldn’t even fit the feeler between the cam and rocker, and all were out of adjustment somewhat. Still I agree with you all that this shouldn’t be the reason for the knocking/grinding and stall.

The vacuum pump is a great idea, I’ll check that out today and report what I find. Thanks also to barry for the first rod bearing, I’ll try to take a look at that as well.

Could a bad bearing in some fanbelt component be making the sound? AC compressor, alternator, etc? If so how would I check that? Just turn each component manually and listen?

Or just remove the belt. The engine can run for a few minutes easily without the water pump function. Of course you can hand check them all with the belt off as well. Plus you have less front engine noise with the belt off to help find the noise. Even the fan creates noise.