FFuel Problem with my U1300L, need advice - Mercedes-Benz Forum
 
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post #1 of 10 (permalink) Old 05-23-2019, 08:16 AM Thread Starter
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FFuel Problem with my U1300L, need advice

Hi I'm having a fuel delivery problem with my ex-German military U1300L. It is powered by a non-turbo OM352 engine. When I start the truck it runs fine for about 5 minutes then the RPM drops from 700 to 500 RMP then she stalls out. The only way I can get it to restart is re-priming the pump by hand then she will restart but it goes back into the same cycle of drop in RPM and stalling. What I have done so far is I drained the tank, removed the tank. Blew air through the line from the tank to the fuel filter. There did not seem to be a blockage. I put the tank back on refilled it. Changed the fuel filters. re-primed the filters and right back to the same problem. Has anyone seen this before, if so I could sure use advise on getting back on the road. Thanks Dean Hansen from Maryland [email protected]
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post #2 of 10 (permalink) Old 05-23-2019, 10:10 AM
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Sometimes the prime pump letís in air.
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post #3 of 10 (permalink) Old 05-23-2019, 10:10 AM
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When it dies, are the main filters full or empty?

Generally speaking, if there are fueling issues and no blockage, you have an air leak. Hard to find since its a suction system.

Do check the draw tube in the tank as crud can fill that up but leave all the lines above clear. Might explain why you can pump enough fuel with the primer pump but there is not enough flow to keep the engine running.
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post #4 of 10 (permalink) Old 05-23-2019, 10:29 AM
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Quote:
Originally Posted by Dean Hansen View Post
Hi I'm having a fuel delivery problem with my ex-German military U1300L. It is powered by a non-turbo OM352 engine. When I start the truck it runs fine for about 5 minutes then the RPM drops from 700 to 500 RMP then she stalls out. The only way I can get it to restart is re-priming the pump by hand then she will restart but it goes back into the same cycle of drop in RPM and stalling. What I have done so far is I drained the tank, removed the tank. Blew air through the line from the tank to the fuel filter. There did not seem to be a blockage. I put the tank back on refilled it. Changed the fuel filters. re-primed the filters and right back to the same problem. Has anyone seen this before, if so I could sure use advise on getting back on the road. Thanks Dean Hansen from Maryland [email protected]
If I had to make a guess, knowing nothing else of what is going on, I would look at leakage into the fuel lines. Basically, if there is a point where suction can draw in air to the line, it can build up until it air locks the mechanical pump, which is downstream from the tank and most of the lines. There have been some good posts on this in the past, and one comment that comes up regularly is to use GOOD, fuel hose-appropriate clamps, and NOT the generic hose clamps from Home Depot. See Bel-Metric, or maybe Summit Racing for the fuel-injection rated small hose clamps. I have re-done my entire fuel delivery system with Parker Push-on hose and their dedicated fittings (very clean install), but there are a number of terminations that required conventional barbs and clamps. Using stuff from Bel-Metric has worked flawlessly.

Have you also cleaned/ blown out the return lines? I am unsure how back-pressure might affect things, but it can't help.

Another thought is to check the VENTING of your tank. If the vent is clogged, you will sooner or later pull a vacuum in the tank that is too great for the suction power of the mechanical fuel pump to overcome. I would expect that to show itself as a much shorter time if one starts with a full tank/ small air volume. Around here, mud-daubers (wasps)are copious, so when I re-did the fuel system, I did new vent hoses for both tanks, leading them to a SS "box" at the center post just behind the cab. This vent box is above the fording depth, looks down, and has a bronze wool filter filling the open end, to keep out bugs and mud.
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post #5 of 10 (permalink) Old 05-23-2019, 12:01 PM
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A problem that I have heard of is a pinhole leak in the fuel tank take-up tube, but if you are working with a full tank, that should not be the problem.

I agree with BloodyGerman - the Primer pump can pull air into the system, which is hard to detect. They are not expensive - I'd get a new one ordered, and at least eliminate that as a possibility.

The fuel tank venting issue, mentioned by photobldr is a good tip.

I have some kind of micro mud wasp around here, and any small round hole outside will often find itself neatly sealed up
with a flush mud plug. Very neat job, no smears, no drips, just a plug.

The ground terminal hole in a wall receptacle, idle jet wells on a dirtbike carburetor , you name it, the little mud masons are attracted to any dark hole that is about 3/16ths or less....You'd think that with a whole forest of nooks and holes in the tree bark all around here for days, they wouldn't bother with manmade holes...but Noooo...

Last edited by TRUKTOR; 05-23-2019 at 12:15 PM.
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post #6 of 10 (permalink) Old 05-23-2019, 02:06 PM
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Originally Posted by photobldr View Post
Another thought is to check the VENTING of your tank. If the vent is clogged, you will sooner or later pull a vacuum in the tank that is too great for the suction power of the mechanical fuel pump to overcome. I would expect that to show itself as a much shorter time if one starts with a full tank/ small air volume. Around here, mud-daubers (wasps)are copious...
This is where I would start.
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post #7 of 10 (permalink) Old 05-24-2019, 08:27 PM
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Check the filter in the little glass bowl under the hand pump. If it's blocked you get exactly these symptoms.
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post #8 of 10 (permalink) Old 05-24-2019, 10:35 PM
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post #9 of 10 (permalink) Old 05-25-2019, 10:55 PM
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Same problem with my 1250 ran a new fuel line from tank to pump all is good so far !!!
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post #10 of 10 (permalink) Old 05-26-2019, 02:21 PM
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Add a backup 24V electric fuel pump. You can use it to fully prime the system during service (should not be needed for operation, unless using it in backup mode).

You can also use the electric pump to find fuel system leaks. It’s just handy, and cheap.

C.

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