Fuel System Problem - Mercedes-Benz Forum

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post #1 of 40 (permalink) Old 03-17-2017, 10:01 AM Thread Starter
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Fuel System Problem

Hi all,

I could really use some help! I've run out of obvious ideas. I have a '78 416 mog that has the following problem.

After priming with the handpump and bleeding at the filters and at the port on the rail, I can start up the beast. It runs fine for 10 min or so and stalls out. It sounds like it is running out of fuel. This is confirmed because the filters have been emptied of diesel - I have to pump the hand pump many times to get diesel to come out of the bleed screw of the filters.

The problem started on a cold day (well below 0F) winter day. I thought that the problem was gelled diesel. I treated the diesel and changed the filters. All seemed pretty good for a couple of more hours of run time, before the unimog stalled again. Changed filters again. Cleaned the small mesh filter just down stream of the tank. It was pretty gunked up.

Same problem persisted. Emptied and cleaned fuel tank. Put in fresh diesel. At this point, I can reliable hand pump the diesel into the filters and out the rail, start it up and have it run smoothly for about ten minutes.

To me this suggests that 1. filters (now changed twice) are fine. 2. Injectors are fine. 3. Fuel flow canít keep up with use. The fuel pump doesnít seem bad either. Iíve crank it over without starting - I can get diesel coming from the top screw of the filter.

I havenít tried to bleed the injectors, because the engine runs smoothly for 10 mins. I think that I will change the filters again. Not more than 1/2 an hour of run time has passed through these filters, but they havenít been changed since I cleaned the tank. But before do this again, I think I need more brainpower on this problem. Ideas/suggestions greatly appreciated. (Sorry about the long post.)
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post #2 of 40 (permalink) Old 03-17-2017, 10:43 AM
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Sounds like an air leak to me. Hard to find because you're dealing with a vacuum.

I'd carefully work your way down the line from the pump to the tank. You may find a split on the hose where it meets a fitting, perhaps a cracked fitting or maybe the little rubber gasket in the prefilter is leaking.

I've suffered this before and got lucky and found a split in a line. Cut out the offending piece and was back to the races (slow races mind you, it's a Mog) in no time.

Some folks put an electric fuel pump in the tank which will show you the leak, as you're under pressure but also speed up the priming time (which you're pretty good at now, but bet you wish you didn't have to do it again).
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post #3 of 40 (permalink) Old 03-17-2017, 10:44 AM
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Your fuel cap may not be venting! Or your pickup tube in tank may hav a booger in it! The screen in tank may be plugged-- all causing fuel starvation-- there may be black fuel bugs in tank that plug everything up. I had similar problems with an 1977 BW 416 diesel and had to have everything boiled out at a radiator shop to clear it all out of the system. --- anyway sounds like fuel starvation. Plus u might replace the little fuel plunger pump. It may have died on u.
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post #4 of 40 (permalink) Old 03-17-2017, 10:52 AM
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Not sure if a 416 uses the Mercedes plastic fuel lines line the 419 SEEs do but just in case.

The plastic fuel lines of that era have a known issue where the loosen up on the barbed fitting upstream of the fuel pump. They may not leak under pressure but can leak under vacuum. The method I used that I saw on a Mercedes forum is to cut a short piece of fuel hose and slit it lengthwise to allow it to be slid over the hose barb, then put a worm gear type screw clamp over it.

The other spot for air leaks on SEEs is the external fuel strainer gasket either missing or no longer resilient. SEE specific is also the hose connections on top of the fuel tank.
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post #5 of 40 (permalink) Old 03-17-2017, 12:40 PM Thread Starter
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Originally Posted by Britbike2001 View Post
Sounds like an air leak to me. Hard to find because you're dealing with a vacuum.

I'd carefully work your way down the line from the pump to the tank. You may find a split on the hose where it meets a fitting, perhaps a cracked fitting or maybe the little rubber gasket in the prefilter is leaking.

I've suffered this before and got lucky and found a split in a line. Cut out the offending piece and was back to the races (slow races mind you, it's a Mog) in no time.

Some folks put an electric fuel pump in the tank which will show you the leak, as you're under pressure but also speed up the priming time (which you're pretty good at now, but bet you wish you didn't have to do it again).
You might be right. I haven't put in much serious time looking for leaks. Yup, pretty good at priming now!
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post #6 of 40 (permalink) Old 03-17-2017, 12:45 PM Thread Starter
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Originally Posted by tlane View Post
Your fuel cap may not be venting! Or your pickup tube in tank may hav a booger in it! The screen in tank may be plugged-- all causing fuel starvation-- there may be black fuel bugs in tank that plug everything up. I had similar problems with an 1977 BW 416 diesel and had to have everything boiled out at a radiator shop to clear it all out of the system. --- anyway sounds like fuel starvation. Plus u might replace the little fuel plunger pump. It may have died on u.
Top venting tube is clear. Cleaned pickup tube, screen, and tank ... Seems line from tank to first small mesh filter is ok. I think next step is to start at that small inline and head towards the engine. Thanks.
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post #7 of 40 (permalink) Old 03-17-2017, 12:47 PM Thread Starter
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Quote:
Originally Posted by peakbagger View Post
Not sure if a 416 uses the Mercedes plastic fuel lines line the 419 SEEs do but just in case.

The plastic fuel lines of that era have a known issue where the loosen up on the barbed fitting upstream of the fuel pump. They may not leak under pressure but can leak under vacuum. The method I used that I saw on a Mercedes forum is to cut a short piece of fuel hose and slit it lengthwise to allow it to be slid over the hose barb, then put a worm gear type screw clamp over it.

The other spot for air leaks on SEEs is the external fuel strainer gasket either missing or no longer resilient. SEE specific is also the hose connections on top of the fuel tank.
Yes my mog has plastic lines. I hadn't really considered leaks there, but I guess those lines harden over the years ... I'll add it to my list. Thanks.

Keep the ideas coming. I need to look at this through someone else's eyes.
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post #8 of 40 (permalink) Old 03-17-2017, 01:06 PM
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I fought this for awhile and never found a distinct culprit but expect it was a combination of things from sitting for so long. I rigged up an outboard motor fuel tank with a squeeze bulb on the line and hooked it up to the outlet line of the external fuel strainer. I set the tank up higher than the fuel pump and used the squeeze bulb to prime the system along with the primer pump. (Note my rig had the old style screw in type primer pump that did not work very well. I bought the new style plunger style from EI). With the boat tank the engine would run fine but once I switched it back to the fuel tank it would quickly start sucking air. I would find one possible leak and then hook the plastic tank back up, get it running correctly and then would switch it back to the fuel tank and have ti stall. I ended up pulling the sending unit and clamping every fuel line plus replaced some plastic with diesel rated fuel line. Note the recommendation is when replace the plastic fuel line cut it off so a short length is covering the barbs on the connectors. Standard fuel hose slips right over and there is enough texture of the barbs through the plastic to hold tight when clamped with work gear type clamp.

The primer pumps are an issue, EI sells a filter kit for SEEs and they threw in replacement primer pump in with the filters
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post #9 of 40 (permalink) Old 03-18-2017, 09:30 AM
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Dr. Embedded had a similar problem awhile back with his 416, he traced it down to the hand pump.
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post #10 of 40 (permalink) Old 03-18-2017, 12:10 PM
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Quote:
Originally Posted by tlane View Post
...your pickup tube in tank may have a booger in it!
I've had this happen. There was some debris INSIDE the screen on the pickup tube which would eventually get stuck in the tube, shutting off fuel flow. I diagnosed the problem by carrying a piece of hose which I could connect to the hard line coming from the fuel tank. When the engine became starved for fuel I'd install my piece of hose and blow the fuel backwards into the tank. There would be initial resistance, then the debris would loosen, and I'd reconnect the fuel lines and be on my way.

To remedy the problem I blew shop air through the fuel lines into the tank. I did it outdoors because the air bubbling through the fuel created a small cloud of diesel fumes, because I'd removed the fuel cap so the tank could not become round like a balloon. I wouldn't smoke while doing this! I hoped that the air pressure would make the debris rattle around on the inside of the pickup tube screen, until it wore down enough to pass through the screen, which would also make it small enough to pass through the pickup tube. It worked, and I haven't had the problem return.

Another cause for your problem could be a pinhole in the pickup tube which is not a problem until the fuel level drops below the hole, in which case you'd be sucking air and not fuel into the filters.
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