Mercedes OM 636 hesitation

[gorb]

Anyone help with a strange problem? We have a Westerly 33 with a Merc OM636 42HP diesel. Until the big cruise this summer, the donk has run perfectly.

However, we now find that when hot (running for an hour plus) the diesel will suddenly drop 200 revs. It will stay like this for a minute or so, then return to cruising revs of 2000rpm. Very occasionally revs wil drop to 1000rpm and only return to cruising revs after a few minutes or after adjusting throttle up and down a few times. Despite all this, the engine runs perfectly at (its) selected revs. Just that the engine decides revs - not me!

It's definitely NOT fuel - this has been checked and rechecked. There is no smoke, black or otherwise. Air filter is clean and well fitted. There is no air in the fuel lines - they have been checked and bled.

After each incidence, engine returns to normal running and the fault may not recur for a few minutes or, more usually, a few hours. To repeat, the engine runs smoothly at all times, with no hesitation, smoke or other unusual behaviour - just the drop in revs.

Could it be a governor problem in the high pressure pump? Any suggestions very welcome.

 

[bdsweeting]

Do you have a rope cutter fitted ?

 

[gorb]

Thanks. Rope cutter fitted and the prop is clear - forgot to say that we checked that idea. And this has now gone on for some time.

 

[bdsweeting]

Drat !

 

[HenryB]

I have the same boat and engine and the only times that I have experienced similar problems have been when the fuel filter has been blocked. The man who probably knows more about the OM636 than most is Manfred Princz at Westfield on 01524 791968 I have found him most helpful. He rebuilds them and stocks (or can get) all the spares. Try giving him a call.

Good luck,

Henry

 

[Brian_Moffat]

Yup - definately fuel flow - or rather - lack of it. Have you had a look in the big steel tank that you keep it in? Is any fuel getting past the gunge on the end of the wire wool?

Have 'ad two of them thar little critters this year already - same problem!

regards

Brian

 

[Rick]

[ QUOTE ]


It's definitely NOT fuel - this has been checked and rechecked.

[/ QUOTE ]

Beg to differ, and suggest it is definitely fuel - something is restricting fuel supply like a blockage at the tank outlet, or maybe a crimp in the air vent, but it is something stopping fuel causing the issue. I'd start investigations in and around the tank.

 

[gorb]

I know it sounds like fuel - but I can't think of anywhere else to check in the fuel system! Fuel tank is clean - as it happens, it was out last winter and thoroughly steam cleaned after an inspection hatch was cut in it. Glass bowl filter is fine and all cartridge filters were changed last winter.

Should say that this has gone on for about 50 hours running (we had a 12 day cruise with no wind up here apart from 4 hours sailing!) and remeber - the problem only starts when the engine is hot!

Definitely sounds like fuel - I agree. But where else to look?

What about governor problems - anything known?

 

[capsco]

Are there any rubber hoses in the system, that may be partly collapsing under suction???

 

[tincouch1]

om636

Did you ever find the problem?

 

[davey]

Mercedes OM636 Running Problems

These engines are normally fitted with inline fuel injection pumps. This means that you have one pump plunger for each cylinder. Could it be that one of the pump plungers is sticking or getting air into it? Please also note that inline pumps (often called Simms pumps in the UK) need lubricating oil in their cam box. Not many people know that as they assume that either the fuel oil or engine oil provides the lubrication, Nooooooo! Good luck with solving the problem.

 

[SHUG]

Are there any rubber hoses in the system, that may be partly collapsing under suction???

A problem I had was that a small piece if the rubber hose lining had come adrift. When the revs went up and the fuel flow increased the bit of rubber drifted to the nearest constriction and restricted the flow. When the revs went down the scrap of material fell back and the fuel flow was restored....... and so on and so on. Took about 3months to track down.
I have just noted that the OP was in 2006!!!!!!.
It must be fixed by now.

 

[BR34]

I'm looking for a heat exchanger for one, for not too much money...
My engine has a vertical air inlet which prevents an air filter due to lack of space. I would really like the horizontal type that most of the marinised versions have fitted.