Mercedes OM312 help/information wanted

[chrisisbd]

Our boat in France has an ancient (1950s/1960s) Mercedes OM312 engine.

 

We recently went out to the boat for the first time in 18 months (due
to Covid etc.).  Much to our surprise while it was dirty outside and
needed a thorough clean it was mostly OK.  In particular the solar
panels have kept both Leisure and Starter batteries in good condition.

 

The engine even started fairly easily but immediately tried to rev
itself to pieces so I had to close it down immediately.  I tried a few
more times but it did just the same.

 

So, presumably, the governor has got stuck.  Does anyone here know
if likely failure modes?  I'm pretty sure it's a centrifugal governor
and I do have a manual for the engine but if I can go back with a few
pointers it would be helpful.
 

[Tony Brooks]

I suppose make sure the "throttle" lever ob the engine actually moves with the control in case the cable has snapped.

 

It seems it first went into production in 1947so might have a pneumatic governor, but the illustration I found did not seem to suggest it has the sort Simms built in the UK. I can't understand the illustration I found because it's in German.

If there are any pipes running from the inlet manifold to the large end of the pump furthest away from where it mounts on the engine, then it might be pneumatic, but I suspect not. If its not pneumatic, then it really has to be centrifugal.

 

If it stops on the cable then you must be pulling the rack  to no fuel so It's unlikely to be stuck (hence me wondering about pneumatics) and I can't think of anything other than a stick rack that would cause a centrifugal governor to rev up like you describe.

 

If it turns out to be pneumatic then the diaphragms can dry out and go hard but the fact you can stop it but again suggest the rack and thus the diaphragm is moving.

 

If it is pneumatic then they use a depression caused by a throttle butterfly to "suck" the rack towards the no fuel position, so an air leak on any air pipes or a split diaphragm would give the symptoms you describe. I note from the illustration that it seems there is an oil filler on the large end of the pump, but I have no idea what it is for. I have never worked on such engines.

 

 

 

 

 

 

[chrisisbd]

Well it turned out to be easy to fix.  I came back to France and the boat yesterday and took a look today.

 

I simply lubricated everything as it should be (both the governor and the injection pump have their own oil and dipsticks which I hadn't realised before), disconnected the cable connection to the throttle control and waggled it back and forth to free  it up as much as possible, and it worked!  🙂   It started very easily and settled down to a nice steady 700rpm tickover.

 

Phew!

 

So I now have some time to do some more general maintenance.

 

[Tony Brooks]

I did think it might have been a stiff governor after a long shut down but discounted it because I though the bob weights would develop enough force to free them.

 

Anyway, thanks for the feedback.