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Old 06-06-2020, 05:15   #1
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Air in injector pump will not dissapear

Hi cruisers,
I hope that some of you might be able to provide me with advice to fixing my Mercedes OM636.
Back story:
While playing around with my Separ 2000 water separator. I accidentally opened the bleed nipple without first shutting of the diesel supply, resulting in a big amount of air being sucked into my fuel line.
I blead it and the motor was able to start.
However every single time I started the motor I had to first bleed the injection pump + filter.
Now I am unable to get the motor running after bleeding and I believe it is due to an airlock in the injection pump (but I might be wrong)
#1 + #3injector spurts out diesel just fine #2 + #4 very little or nothing.
I have pumped the small hand pump like a mad man and there is no more air coming out of bleed nipple at injection pump and filter, but still it will not start.
Could this be an injector fault?
Any tips of what to try next?
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Old 06-06-2020, 06:27   #2
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Re: Air in injector pump will not dissapear

Let's back up and look at the problem. Fuel is not getting through two injectors. I suggest that you loosen the fuel lines at the injectors themselves and crank. If fuel sprays, then it's time to remove the injectors for service, because in the disruption you've managed to clog them. If fuel does not spray out, there is a blockage farther back, so try loosening the other end of the fuel lines and see if fuel is coming from the #2 and #4 pumps.

Overall, I'm more suspicious of a mechanical blockage than I am of entrapped air.

Trace it doown, and good luck with it. You will soon be your own diesel mechanic. For reference and conceptual help, try Nigel Calder's Marine Diesel Engines.
Never let anything mechanical know that you are in a hurry.
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Old 06-06-2020, 20:17   #3
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Re: Air in injector pump will not dissapear

Thankfully this model Mercedes has been around for so long that it holds few surprises. I would be suspicious of the lift pump, a couple of types were fitted. The early engines had a pump with a knurled plunger knob that needed to be unscrewed for priming and tightened down so the engine could run. The later engines had a self returning plunger with a plastic cap and required no special technique other than pumping the plunger.......both types often leaked on that plunger as suction leaks, no fuel drips.
I had a look inside an old Separ and thereís no obvious reason that leaving the bleed nipple loose would cause anything but air to get into the system. Did you bleed both ends of the injector pump, there should be 2 bleed screws. Iím thinking thereís air in the system and if I was doing the hands on work, my next move would be to get a container of clean fuel in the cockpit then run 2 hoses down to the engine establishing a gravity feed.
Itís very important to consider the possibility of sticky injector pump plungers and have a way to cut off the intake air supply in case the engine tries to overspeed as a response to a stuck plunger.
Oh, the reason I have a spare Separ in a drawer is that itís very hard to get the square filter element ....but easier to just buy a Racor 500.
Iím hoping for air in the system....Pete.
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injector pump, pump

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