Hi everybody - greetings from Wales, UK.
I have a 1973 Unimog 421 with the standard OM616, with a big, new problem that I cannot fathom and has got me seriously worried, as this vehicle (believe it or not!) is the basis of a part-time forestry business I run - from timber winch to delivery vehicle - it does the lot!
Have alook at her in action in happier times:
The current engine was fitted about 3 years ago as an as-new 'short engine' (professionally re-engineered barrels, new pistons, conrods, new crank bearings etc), a skimmed head with new valves etc. It also has a newly reconned injector pump and plugs. It has run perfectly since fitting, and hasn't missed a beat, and has done very few hours work since.
Last weekend I had to do some welding to the cab floor and removed the injector pump to get access. When i refitted the pump it A. would barely start, and B ran like a dog once it did. I soon traced this to some crap in one of the injectors and have now sorted this, but before i got to this stage thought i'd double-check the timing delivery...
The important bit: As I was barring the engine over (in the correct direction) to set up the injector timing there was a dull 'crack' noise from the engine which i noticed at the time but though nothing of.
BUT - after getting the timing set up, the reconned plugs fitted and firing it up for the first time:
A. the starter motor is noticeably MUCH louder and rougher-sounding than normal - like the noise is resonating through the whole engine, and it occasionally jumps off the ring-gear when I'm cranking it over
B.the engine barely turns over - and ONLY on a fresly charged battery, even though the battery is in A1 condition, as is the starter motor.
C.The engine is cranking over irregularly, like 3 pistons are easy, and one is hard/sticky, and it's pausing on the one.
Once it fires up it runs like a dream, as normal.
It feels to me like i've bent/broken something, or mashed a shell bearing on the crankshaft that's now putting a huge load on the s/motor, but i don't understand how, or what i've done just by barring it over?
The only other clue is that I had done an oil/filter change just before the 'crack' occured and had to top up the oil by about 4 litres afterwards to get it to the right volume, so there is an outside chance it could've been caused by low oil level., but having said that it would've had a minimum of 5 litres in it before top-up, so the oil wasn't that low
I'd be really grateful for any advice or solutions anyone may have.