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Old 27-03-2011, 09:05   #1
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Varying RPM . . . Gettting Worse Every Few Days

Hey,
Hoping someone can help out with our issue with our engine. The engine is a mercedes OM636 and the RPM started varying by several hundred RPM when attemping to run the engine at 1000RPM in nutruel. Oddly when put in gear the problem when away.

A few days later trying to run the engine at 1500RPM in gear and the problem of the RPM varying wildly happened again. Today the engine must be run at 1800 or above to stop this happening.

Other facts:
  • The problem only becomes evident once the engine has been running for few minutes
  • The problem is much for likely to show is self when the engine is not in gear.
Ideas(for google):
  • I need to ensure no air in fuel system
  • Ensure no leaks in fuel system or vaclum hose and connections
I will be checking the above tomorrow and will post a update.

Any thing else that needs to be checked?

Kind Regards

Jarvis
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Old 27-03-2011, 13:11   #2
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Re: Varying RPM.. Gettting worse every few days

Aloha Jarvis,
I have the same engine and it began "hunting" when the overflow line back to the tank from the fuel filter became restricted. Yes, check all the lines starting from the inside of your fuel tank all the way to your injector pump and back to your tank. Then check the injector lines to you injectors. If that doesn't do it then you may have a governor problem in your injector pump but I wouldn't mess with that until you have checked out everything else.
Have you changed fuel filters recently? You are very lucky to be where you are and have access to the OM636 guru at OM636 Diesel Engine Spares
kind regards,
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Old 28-03-2011, 04:09   #3
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Re: Varying RPM.. Gettting worse every few days

Cheers for your imput i will check that over. Out of interest what you use as your cruising RPM? I normally run around 2000RPM.
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Old 28-03-2011, 17:39   #4
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Re: Varying RPM.. Gettting worse every few days

Pretty much 1800.
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Old 30-03-2011, 03:55   #5
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Re: Varying RPM.. Gettting worse every few days

Quote:
Originally Posted by SkiprJohn View Post
Aloha Jarvis,
I have the same engine and it began "hunting" when the overflow line back to the tank from the fuel filter became restricted. Yes, check all the lines starting from the inside of your fuel tank all the way to your injector pump and back to your tank. Then check the injector lines to you injectors. If that doesn't do it then you may have a governor problem in your injector pump but I wouldn't mess with that until you have checked out everything else.
Have you changed fuel filters recently? You are very lucky to be where you are and have access to the OM636 guru at OM636 Diesel Engine Spares
kind regards,
The lines back to the fuel tank and incoming lines to the secondary fuel filter are clear. The primary filter was completely clogged with sludge, this has been replaced. I learnt diesel does not taste good as i was able to suck diesel through the lines, both incoming and return lines.

When i went to bleed the fuel system I noticed the manual lift pump button was not pumping fuel so i could not bleed the system. There is fuel in the line that connects to the lift pump as i sucked the fuel through getting a mouth full of diesel.

So the engine runns yet hunts really bad now, from 500rpm to 2500rpm.

Is it possible the lift pump is faulty and causing the hunting or air bubbles in fuel system?

Is it possible to buy a service kit for a lift pump?

Kind Regards

Jarvis
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Old 30-03-2011, 20:17   #6
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Re: Varying RPM.. Gettting worse every few days

Aloha Jarvis,
Your lift pump could be the problem but suspect air in your injector lines. I'd give those folks at the link I sent a call. They will help you with a kit too. I have an extra lift pump around here somewhere but mailing it to you would cost more than contacting those folks at the link.
Once you have the engine running did you go to see what levers are working on the injection pump? Can you control the hunting at the throttle control at the injector pump by hand? Sometimes a very loose mechanical connection at the throttle can do what you are describing.
Have you tried bleeding the air out of the fuel at each of the injector's while the engine is running?
I truly hope it is something simple. If you believe that it is the governor inside the injector pump don't try to fix it yourself. They are very touchy pieces of machinery.
Again, try that link I forwarded to you. They have been very helpful for me. You are blessed to be so close to them.
I had the problem of hunting after I installed my injector pump but it was because I had not set the idle properly and had it too low. I think the factory recommends about 600 rpm idle but I'd set it a little higher.
kind regards,
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Old 31-03-2011, 08:54   #7
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Re: Varying RPM.. Gettting worse every few days

Quote:
Originally Posted by crazyhorse77 View Post
...
There is fuel in the line that connects to the lift pump as i sucked the fuel through getting a mouth full of diesel....

Jarvis
Next time slip a small round balloon over the end of the tubing before you suck on it. The fuel will be caught in the balloon. It will save wear'n tear on your dentine!
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Old 31-03-2011, 09:36   #8
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Re: Varying RPM.. Gettting Worse Every Few Days

the balloon works great..used it as a teenager when I routinely siphoned gas out of my Dad's company car on weekends...those mid-70's Pontiac Bonnevilles had huge gas tanks. Thanks for reminding me of a fond memory...drove my Dad nuts.
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Old 24-04-2011, 13:50   #9
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Re: Varying RPM.. Gettting Worse Every Few Days

Cheers for the suggestions, it turned out to be a hole in government diaphram. A new one was ordered from UK to morocco where we were. 9 hrs of customs, five different offices to get authorizatoin later, much bad french speaking and we had our part to replace. 1 hour to change and one happy couple of sailors!

The Adventures of Croc Bones
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Old 24-04-2011, 15:56   #10
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Re: Varying RPM.. Gettting Worse Every Few Days

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Originally Posted by crazyhorse77 View Post
it turned out to be a hole in government diaphram.
A hole in what?
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Old 25-04-2011, 18:28   #11
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Re: Varying RPM.. Gettting Worse Every Few Days

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A hole in what?
Holes in lots of folks's diaphragms have caused problems, especially when the problems grow up and want to go to college, interfering with your cruising plans!

Jim
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Old 25-04-2011, 18:35   #12
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Re: Varying RPM . . . Gettting Worse Every Few Days

rofl.... sorry -- some of the answers are rockin--sounds like an airleak .... gooodluck.
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Old 25-04-2011, 18:35   #13
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Re: Varying RPM . . . Gettting Worse Every Few Days

Governments are black holes, sucking up all they can get.
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Old 25-04-2011, 18:40   #14
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Re: Varying RPM.. Gettting Worse Every Few Days

Quote:
Originally Posted by Jim Cate View Post
Holes in lots of folks's diaphragms have caused problems, especially when the problems grow up and want to go to college, interfering with your cruising plans!

Jim
Good one. LMAO
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Old 25-04-2011, 19:06   #15
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Re: Varying RPM . . . Gettting Worse Every Few Days

Very good indeed Jim, LOL
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