# Mercedes OM636 diesel will not accelerate



## agmfabr (Jun 20, 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


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## NCboatrx (Dec 2, 2010)

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.


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## klem (Oct 16, 2009)

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?


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## smp (Jul 29, 2010)

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.


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## agmfabr (Jun 20, 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 ******* 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


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## jrd22 (Nov 14, 2000)

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.


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## NCboatrx (Dec 2, 2010)

agmfabr said:


> 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.


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## agmfabr (Jun 20, 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


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## klem (Oct 16, 2009)

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.


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## agmfabr (Jun 20, 2011)

Thanks Klem. Yes I agree. I have been trying for weeks to get this guy to bleed this fuel system but this "******* 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


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## agmfabr (Jun 20, 2011)

OK, I'll try this again. Took the boat to the lake. Bled everything I could find to bleed in the fuel system. This didn't help anything. The engine will push the boat around the lake in idle but will not accelerate. I checked all the linkage. I removed the linkage and operated the throttle by hand. Nothing!! The engine never made an attempt to rev up. Put the engine in neutral, works great. Changed props, nothing. Took the props off, works great. The prop turns freely in neutral. No smoke of any kind in any throttle position. When the trans is in gear the throttle makes no change in rpm whats-so-ever. It is as if there was no throttle at all. The only time the engine changed rpm was when we pu the outdrive in gear. The rpm dropped a short time and then back to idle speed. Removed the linkage to the cutoff and operated it by hand and made no difference. The engine never even labors as if it is trying to accelerate when you operate the throttle. Nothing! Well, I'm not sure what else to try. If the governor and throttle work in neurtral it should at least make an attempt to aperate in gear. This thing works in every position except GO! Back to the drawing board. Get a bigger hammer I guess. thanks for listening. Dale


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## MacGyverRI (Nov 14, 2007)

Sounds like a bad injection pump, possibly all gunked up inside. Try some Power Service fuel conditioner to loosen it up, it might help. Change the fuel filters. There's also a strainer near the primer pump that might need cleaning.


Those engines were popular in Thermo King and Carrier trailer refer units and are very long lasting BUT are made for idle and high speed only on a 4 cyl. refrigeration compressor. Not multi position RPM's


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## agmfabr (Jun 20, 2011)

Hi MacGyverRI, thanks for the input. With this being said, you may be right about the gunk inside and I'll see if I can get him to find the fuel conditioner. I have also gone thru the primer pump and it is working ok and the fuel filters are new, although that doesn't really mean that they are good. But even if the engine were only made to operate at idle or high speed it stands to reason that the rpm would change when the throttle were moved. Right? Or am I thinking wrong? Thanks, Dale


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## MacGyverRI (Nov 14, 2007)

agmfabr said:


> But even if the engine were only made to operate at idle or high speed it stands to reason that the rpm would change when the throttle were moved. Right? Or am I thinking wrong? Thanks, Dale


Governors can sometimes work differently on stationary engines w/ power curves.

Go w/ a good conditioner 1st (double dose) and run it under a load for at least a few hours to loosen things up.

Checking the pump timing is another option if this doesn't work. That will be a lot of typing on my part to make it easier than the manual reads.  I've worked on lots of these engines through the years, they last forever. (use 15-40 Shell Rotella T)


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## sawingknots (Feb 24, 2005)

just a guess,but i think the governer isn't functioning,when the engine is off the govener should open up wide open as it starts it should return to a more normal position depending on where the hand throttle tells it to,as you give it more throttle the governer should also increase it also senses a increase in the engine load and opens up tp compensate,try manually opening up the governer by hand,if it will open up and the engine/boat increases speed/rpms,the governer is your problem if not the problem is somewhere else.a govener is simply device made of weights and springs,the springs pull the throttle open ,the weights depending on the rpms counteract the springs,centrifically


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## MacGyverRI (Nov 14, 2007)

sawingknots said:


> just a guess,but i think the governer isn't functioning,when the engine is off the govener should open up wide open as it starts it should return to a more normal position depending on where the hand throttle tells it to,as you give it more throttle the governer should also increase it also senses a increase in the engine load and opens up tp compensate,try manually opening up the governer by hand,if it will open up and the engine/boat increases speed/rpms,the governer is your problem if not the problem is somewhere else.a govener is simply device made of weights and springs,the springs pull the throttle open ,the weights depending on the rpms counteract the springs,centrifically


Do not take the pump apart to play with the governor, If it wont straighten out w/ conditioner, have it rebuilt.

If you do need to take the pump off, make sure it's on the #1 compression stroke and at the index pointer on top of the flywheel housing cutout for #1, degrees FB (Fuel begins) BEFORE you take the Injection pump out. There's also a master spline on the pump where it bolts to the timing case and timing it going back in isn't really that hard but it must be re-timed.

The geyser of fuel squirting out of #1 w/ the barrel spring and valve removed and pumping the crap out of the primer while someone turns the engine over w/ a breaker bar slowly will be messy when re-timing it... "Fuel begins" when it actually stops the geyser squirting, but that's another very messy issue 

HTH


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## agmfabr (Jun 20, 2011)

Geez MacGyver, that sounds like loads of fun. I don't mind getting my hands dirty but I'm not sure I want to take a bath when I do it, Ha! We were talking today about changing the injection ump if he could find a rebuilt. I have looked at the manual, or at least the partial one I got off line with a couple of pages missing. Anyway, he is getting to the point that he would rather change it than find the problem with this one. He's not a very patient guy and I really haven't helped much I'm sure. Just don't know that much about these deisel things. I have learned a great deal about them though since I have started trying to help with this one. With all the online stuff I've read and having an old pump to open up (not much help) and all the ideas and help from all the guys here. I just don't know where to go from here. Wish I at least had a decent manual. I'm not sure how all this governor rods and control and the throttle all work together. Can't find a IPB of how this stuff is suppose to work together. Well, onword and upward! Thanks guys. Dale


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## sawingknots (Feb 24, 2005)

you can manipulate the governer without taking it apart as it operates with a set of springs and weights,just use your hand [while the engine is running and the boat is underway]to increase the throttle/rpms if need be just disconect the throttle cable,your problem could even be in the throttle cabe! BE VERY VERY CAREFUL NOT TO TOUCH OR GET TANGLED IN THE MOVING PARTS WHILE THE ENGINE IS RUNNING


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## MacGyverRI (Nov 14, 2007)

11th post; "I removed the linkage and operated the throttle by hand. Nothing!! The engine never made an attempt to rev up."

There isn't an external governor, it's inside the pump. This is a 4 cyl. Bosch injection pump.


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## sawingknots (Feb 24, 2005)

good,now you can know that,you now know your problem is in your fuel supply or the injection pump or possibly the injector,i am assuming that theres not a lot of black smoke from the exhaust right?if not i'm going out on a limb and say its your injection pump,yup most governers are contained internally, sometimes you can isolate the problem by comparing the fuel output [spit]from the lines at the injectors,you could even have one or more cylinders not firing due to no pump presure ,lol now you have me interested,plz report back


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## agmfabr (Jun 20, 2011)

Hi guys. Thanks for the come backs. Yeah, I've played with all the levers and gizmo's externally by hand with no results. It is a bit frustrating but I have this bad habit of I love a challenge and this has so far been a real winner. It would be more fun if it were actually mine and then I could really get my hands on it. But, then the wife wouldn't see me till the thing was running or over the side of the mountain, ha ha. I love it! MacGyver, I don't know when or if this guy will buy the conditioner you told me about. I gave the name of it to him so now it's his turn again. I have access to an old pump and I have taken the back end off of it to see what makes it tick but it really needs some work to get it to work smoothly. Laid out in someone's yard for years. I don't see where this throttle actually does anything except put tension on a large spring that connects to a bar that runs down the back of the housing. Nothing inside this thing moves much. Isee the governor and the part that closes and opens it operated by the shutoff lever but the rest of the stuff don't move much. Will have to get a bigger hammer I suppose,Ha. Maybe I can get the rail to move smoother. Doesn't look like there is much to make that move either. Thanks guys. Let me know if you think of anything else I can do. You have been a great help. Dale


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