Mercedes OM636 diesel will not accelerate

agmfabr · #1 (**permalink**) 07-12-2011

My neighbor's boat has a Mercedes OM636 with a Bosch fuel injector on it. The engine starts and idles and accelerates fine out of the water. When he puts the boat in the water with the prop on it the engine will not accelerate. It will not even try to accelerate. When he pushes the throttle forward the engine just idles. I'm a mechanic but I am not familiar with these diesels, the Bosch injectors or the govenors that controls the rpm. Can anyone out there help me with this problem? Can't find any info. on the web.

Thanks for your assistance. Dale

NCboatrx · #2 (**permalink**) 07-13-2011

If the engine performs well in neutral but not in gear, it makes sense that there may be a problem with the transmission or cutless bearing.

klem · #3 (**permalink**) 07-13-2011

Have you gone through the easy stuff yet? Is the engine getting good fuel (check/change the filter and look at fuel quality)? Is the shutdown on the IP returning fully? Is the throttle actually working correctly? Will the prop spin freely?

What version on this engine is it? Is it one of the ones with a small turbo? If so, some use an AFC. If the line to the AFC is leaky or the diaphragm is ruptured, it will limit power although the engine will get above idle.

If the engine won't go above idle, it is unlikely to be injector related as it would have to be several injectors at once. It is possible that something is wrong in the IP but those units are pretty reliable.

Are you seeing any smoke? If so, what color is it?

smp · #4 (**permalink**) 07-13-2011

Not sure if this will be totally helpful but you mentioned you couldn't find much searching google... OM636 is used in a few of their cars. You might have some luck finding more info on that motor here, I would imagine the fuel injection setup would be the same:
Benzworld.org - Mercedes-Benz Discussion Forum - Search Results

Good luck.

agmfabr · #5 (**permalink**) 07-13-2011

Hi guys, thanks for the replys. I talked with the guy again this morning and he staits that: The outdrive has just recently been completely rebuilt. He can put the outdrive in a barrel of water W/O the prop on it and it will accelerate just fine to a high rpm. When he takes it to the lake and puts the prop on it will not accelerate????? This model of engine does not have the turbo on it. As I understand it this unit was at one time a refer unit in a truck and then was converted to the boat use. He says the guy he bought it from, now passed away, used the boat but it has set unused for several years. I don't know what all the simple basics are for this engine and the guy prides himself on being a Redneck so it is possible that all he has done is put fuel to it. Hope this helps a bit and I appreciate the help

Dale

jrd22 · #6 (**permalink**) 07-13-2011

As Klem pointed out, more information is needed. You say that the engine accelerates on land (you are supplying cooling water somehow I hope) but in the water with the prop on it doesn't even try to increase rpm. When on land did you put it in forward, or were you in neutral, when it accelerated? Will the engine accelerate in neutral when in the water? If the trans. or prop shaft were locked up the engine would try to accelerate and you would see black smoke from the exhaust, same if the prop was heavily fouled and you would hear it laboring. The lack of any effort to accelerate seems like a cable or linkage problem to me. I would check that the throttle arm on the IP was actually moving when in gear and the throttle was moved forward.

NCboatrx · #7 (**permalink**) 07-13-2011

Quote:

Originally Posted by agmfabr

Hi guys, thanks for the replys. I talked with the guy again this morning and he staits that: The outdrive has just recently been completely rebuilt. He can put the outdrive in a barrel of water W/O the prop on it and it will accelerate just fine to a high rpm. When he takes it to the lake and puts the prop on it will not accelerate????? Dale

Thus the prop is too big for the engine or has too much pitch. Speak to the prop supplier.

agmfabr · #8 (**permalink**) 07-13-2011

Ok, well I'm back again with more info. The guy says that the boat will idle around the lake all day but will not accelerate and that the boat shop told him that there are only two props that fit this setup and he is useing the stock prop on the boat. Says there is a power prop, which he is useing, and there is a speed prop that he has but is no in use. Yes we are using a barrel of water to run in out of the lake. With that said, I did have him start the engine, put it in gear and try to accelerate with the prop on,out of the water momentarily and the engine accelerated!! So, it will idle around the lake but will not accelerate and it will accelerate with the prop on on dry land?? Is there a govenor problem here?? Thanks for the help guys. Dale

klem · #9 (**permalink**) 07-13-2011

If it won't accelerate, it is almost always that there isn't enough fuel getting to it or there is too much load.

Too much load could be a problem with a malfunctioning power transmitting component (bearing etc) or a significantly oversize prop. Either way, there should be significant black smoke coming out of the exhaust at full throttle when it will not accelerate.

Too little fuel won't be apparent when the engine is not loaded. However, when the engine is loaded (trying to accelerate), it won't do it. Common symptoms include the ability to rev it all of the way up in neutral but no power under load and no smoke. The likely culprits are fuel filters, bad fuel, throttle linkage, shutdown, etc.

It sounds like someone who knows how to listen to an engine and look at the exhaust needs to go with him to the lake and see it really under load.

agmfabr · #10 (**permalink**) 07-13-2011

Thanks Klem. Yes I agree. I have been trying for weeks to get this guy to bleed this fuel system but this "redneck truckdriver, I know all about diesels" mentality is hard to get through and since I cant get a wrench on the thing because it's not mine. I think I have convinced him now though. With enough talking and show & tell he finally is convinced this needs to be done. Tomorrow morning we are out to the lake with me on board so I can play with it myself---not that I know anything, but like you I think there is air in the system. Will see what happens next. Thanks for your assistance and hanging in here with me. See ya tomorrow. Dale