83 300SD Fuel Pump Down? - Mercedes-Benz Forum
 
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#1 (permalink) Old 07-14-2007, 05:48 PM
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Unhappy 83 300SD Fuel Pump Down?

Couldnt get my car started..along the way I did this:

disconnected the "out" hose from the pump and was able to squirt out diesel fuel
using the prime button till the cows came home. When I crank the car over, nothing comes out of the hose. Im thinking the pump is toast yes? Should add that I have a new fuel filter on...

Last edited by Tom Yaz; 07-14-2007 at 05:50 PM.
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#2 (permalink) Old 07-14-2007, 05:59 PM
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Not necessarily – I’d make sure the fuel system is not leaking fuel or air. Then check the fuel tank strainer is not clogged. How many miles on your car? Recent work?
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#3 (permalink) Old 07-14-2007, 06:14 PM
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No recent work...268K miles. Fuel tank strainer? Your talking about that small inline filter under the hood? That has been replaced as well. On the air leak possibility, wouldnt I have problem hand pumping if there was an air leak?
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#4 (permalink) Old 07-14-2007, 06:19 PM
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No, there is a fuel screen in the tank itself...likes to clog esp. on higher mileage cars.

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#5 (permalink) Old 07-14-2007, 06:34 PM
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Thats a new one for me...didnt know there was one..If I cant get it resolved under the hood, I will have to get my mencanic to look at that..If I had a air leak or the tank filter you mention is clogged, how can I pump sucessfully by hand but not by cranking? Shouldn't I have problems hand pumping as well?
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#6 (permalink) Old 07-14-2007, 06:45 PM
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When cranking the engine, the diesel injection pump cylinders pump the fuel into the injection lines/injectors at about 2,000 psi. When you use a hand pump, you are only pressurizing the fuel to several psi to purge the air out of the system. Be glad that nothing comes out of that disconnected hose when cranking the engine.

Loosen the bolt on top of the main fuel filter. Then hand pump the fuel until you see it coming out of the filer housing under the loosened bolt. Tighten the bolt and try cranking the engine.
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#7 (permalink) Old 07-14-2007, 07:20 PM
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Hi P100, you got me a bit confused now..What I am "hand pumping" is the primer button on the lift pump. If the lift pump is not supposed to be "lifting" fuel out its exit hose to the fuel filter when cranking,("be glad nothing is coming out") then what is it good for?

When I said "fuel pump" I mean the lift pump, not the injection pump....

Or am I misunderstanding something?

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#8 (permalink) Old 07-15-2007, 06:07 AM
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The fuel supply pump only runs at a few psi. The injector pump delivery pressure is the high pressure side.
The tank screen is a prefilter to keep the big cunks out of the fuel lines. ie: rust and debris in the tank. You would think that if the tank screen was plugged or had a cracked fuel supply line that the primer pump wouldn't draw fuel either. Where is the supply pump located on your car? Under hood or in the tank? It makes a difference if the pump sucks or pushes the fuel through the line. A leak on the suction side will draw air, cause the pump to cavitate and only leave some reisdue at the spot of the leak when the pump is not running. A leak on the preasure side will leave you with a big puddle. Also a leak on the suction side makes it difficult for the pump to prime itself.
Also have you checked the fuel pump relay? It should be loacated under the hood with the fuse panel. When the canister filter was changed during priming the system did you open the bleeder screw on the injector pump? injector pumps do not like air in any amount.
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#9 (permalink) Old 07-15-2007, 06:28 AM
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There is no external fuel pump on OM617 (I think), the fuel pump is in directly connected to the engine. You are disconnecting the line leading "to" the pump, so that's the reason nothing is coming out. But it should suck air
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#10 (permalink) Old 07-15-2007, 06:31 AM
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Quote:
Originally Posted by isc62
The fuel supply pump only runs at a few psi. The injector pump delivery pressure is the high pressure side.
The tank screen is a prefilter to keep the big cunks out of the fuel lines. ie: rust and debris in the tank. You would think that if the tank screen was plugged or had a cracked fuel supply line that the primer pump wouldn't draw fuel either. Where is the supply pump located on your car? Under hood or in the tank? It makes a difference if the pump sucks or pushes the fuel through the line. A leak on the suction side will draw air, cause the pump to cavitate and only leave some reisdue at the spot of the leak when the pump is not running. A leak on the preasure side will leave you with a big puddle. Also a leak on the suction side makes it difficult for the pump to prime itself.
Also have you checked the fuel pump relay? It should be loacated under the hood with the fuse panel. When the canister filter was changed during priming the system did you open the bleeder screw on the injector pump? injector pumps do not like air in any amount.

Yes we are talking about the fuel supply pump. It is located under the hood. It is mechanical and doesnt use electricity. Yes I agree that if I can manually prime (suck) just fine then it would follow it should also lift mechanically. Hence the issue. I have disconnected the outlet line so that the injection pump is out of the equation. I am priming into a can right now. I can fill up the can with hand priming, but cant do it mecahnically (cranking). So I am thinking the lift pump is malfunctioning
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