Hi everyone! Great to be part of this forum.


A couple of weeks ago, I got an 85 300SD W126, Automatic, the California turbo model. It's showing 384K miles, but the engine is obviously fresher than that. No paperwork, no history. The car is very straight and rust free, and a lot of stuff works. The dash needs some help- ignition switch area is broken and a temporary switch has been cobbed in. It may well have overheated at some point in it's history- the coolant pressure cap was down to bare brass- all the rubber gone. Drove it 6 miles and shut it down before the temps went into the red. Got a pressure cap, tightened the fan belt. It also had a sticking brake, and a couple of other little issues- but I drove it 350 miles home.



Got it to the shop and pressure washed it, changed oil, adjusted valves, did diesel purge, fuel filter replacement, replaced return lines, got fan clutch working (so greasy it wasn't sensing temps), new valve cover gasket, 1 motor mount, lots of little things. What I've done has made big improvements- the car now kicks quite well, sprays gravel, chirps tires, driving quite well.


Here comes the 'But...' There's a noise, and a miss. You cannot hear anything except the usual OM617A rattle out under the hood. Back at the tailpipe, you hear a miss, probably an injector not firing very well. But in the drivers seat, in time with the miss, you hear a quiet dull thud. Cracking injector lines has not yet revealed which cylinder it is- you can't hear it under the hood. Each injector line affects the idle, as if they were all firing. An infrared camera shows each cylinder top at 216 - 220 degrees F. Oil pressure is excellent. Timing chain has virtually no slack, and blow-by is minimal. Sometimes I catch a little blue smoke from the tailpipe, sometimes it's very clean.


There's another occasional oddity: operate the pump linkage, and it runs one way up to about 1200 RPM: perhaps a little rough and erratic. Then continue to increase throttle very gently, and between 1200 and 1500 there is no control. Something suddenly changes and it's at 1500 RPM. I cannot get it to stay steady at say 1300 or 1400 RPM. From 1500 up, it runs very smoothly, no missing.



I'm hoping it's an injector, but it's kind of a bigish thud. Will get a stethoscope on the bottom end looking for rod knock. Will pull oil sump, check for metal (none in the old filter).


Any suggestions are much appreciated.

Side question: one motor mount?

Check the balancer center bolts and the four small bolts for tightness. Try to get a look at the flex plate with a borescope for cracks and look for the exhaust hitting the body or worn mount exhaust bushings at the transmission.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

@ Shern, yeah only the right side motor mount. The drivers side isn't great, but it's not shot. I'm trying to avoid spending extra dough in case this engine is shot.

@ Sugar Bear, those are good ideas. Thanks.

Also check air cleaner mounts and bracket.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Sounds like two separate issues to me.

The miss could easily be an injector —spray, pop pressure, etc.
A while back, I remember reading something about hydraulic knock. Hazy on the details but something to do with a dribbling injector causing delayed ignition and a partially firing cylinder. If you can get your hands on a pop tester, think it’d be worth confirming the state of your injectors.

The RPM thing is weird and points to an issue within the pump, like the governor or some other binding, gunk, etc. Sadly, on most forums the pump is a black box and operating knowledge beyond the basics is rare (I certainly don’t have it). You ran a purge, so less likely the latter. I’d come back to this one after solving the knock.

In addition to Sugarbear’s suggestions, I’d head straight to the injectors.

Let us know what you find.

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1981 240D 4sp manual. Ivory White.

Minimal blowby means a sound engine. See a youtube test. With crankcase well sealed and oil cap tight, block Valve Cover vent. If engine shuts off in > 10 sec, little blowby. Crankcase pressure builds until it pushes out the vacuum shutdown valve.

Others describe a bad injector spray as "nailing", like hammer taps, not a thudding sound. I have a pop-tester, which also lets you judge spray pattern.