Overly 'tight' OM616 will barely turn over - Mercedes-Benz Forum

 
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post #1 of 5 (permalink) Old 01-24-2011, 05:57 AM Thread Starter
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Overly 'tight' OM616 will barely turn over

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:

Background:

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.
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post #2 of 5 (permalink) Old 01-25-2011, 03:21 AM
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And if you turn the engine over by hand via the crankshaft bolt, does it also encounter a tight spot? I hope you didn't use the camshaft bolt to turn the engine over, because the chain can skip a tooth if you do that.

A crankshaft bearing isn't likely to be damaged by turning the engine over by hand, it is more likely a ring gear of the flywheel/starter motor issue.

I would pull the starter motor and inspect the starter motor and flywheel and ring gear.
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post #3 of 5 (permalink) Old 01-26-2011, 02:46 AM Thread Starter
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Originally Posted by Govert70227 View Post
And if you turn the engine over by hand via the crankshaft bolt, does it also encounter a tight spot? I hope you didn't use the camshaft bolt to turn the engine over, because the chain can skip a tooth if you do that.

A crankshaft bearing isn't likely to be damaged by turning the engine over by hand, it is more likely a ring gear of the flywheel/starter motor issue.

I would pull the starter motor and inspect the starter motor and flywheel and ring gear.
OK - thanks for that.

I guess I need to remove the plugs and turn it over to check for tight spots.

I don't understand how I've damaged the starter motor/ringgear by barring over by hand though, but will also check motor.
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post #4 of 5 (permalink) Old 01-26-2011, 05:09 AM
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You don't need to remove the plugs to turn the engine by hand, but it makes it easier.

The two things (turning the engine by hand and damaged ring gear) don't have to be related, they just happen to occur at the same time. A damaged crankshaft bearing is just as unlikely.
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post #5 of 5 (permalink) Old 02-03-2011, 04:33 AM Thread Starter
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Originally Posted by Govert70227 View Post
You don't need to remove the plugs to turn the engine by hand, but it makes it easier.

The two things (turning the engine by hand and damaged ring gear) don't have to be related, they just happen to occur at the same time. A damaged crankshaft bearing is just as unlikely.
Ok

Removed starter and stripped - armature shaft has sheared at the business end - you were right!

Also removed plugs and turned engine over a few times - perfect - no tight spots.

Many thanks for the advice - I'm back in business!

Also amazed I could buy a new armature in the UK for £45 instead of having to buy a complete unit for £170...
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