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 can’t keep up with use. The fuel pump doesn’t seem bad either. I’ve crank it over without starting - I can get diesel coming from the top screw of the filter.
I haven’t 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 haven’t 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.)
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).
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.
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.
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
Didn't realize I supposed to have a filter at the fuel pump until this email! I guess it isn't truly needed. (I'll address the missing filter down the road.)
Having basically the same issue, to find air leaks I hooked up an electric fuel pump before the valve, hoping to find fuel coming out somewhere.
Had already blown out the line from the valve and into the tank with compressed air.
Bleeding was pleasurable with the pump doing the pumping rather than changing fingers every couple of minutes on the hand pump. First filter, fine. Second filter...nothing.
Seems like lucky me gets to drop the tank and find the pin hole in the pickup tube. But just to be sure, a hose into the filler neck and to the pump will be tried first.
Either way, the electric pump will likely become permanently installed. That way there should never be a need for it again.
i had problems on a 617
it turned out to be the manual priming pump .it was letting air in but not leaking diesel out .
new Bosch primer pump fixed it .
in the processs of finding the problem i replaced all the worm drive hose clamps with the type MB use ,
full circle wrap around overlap with a small nut and bolt . they seal much better and do not damage the hose .
also if you are chasing a diesel leak , degrease and have everything dry , start up and sprinkle talcum powder around .
any leaks will be very obvious .
it is a bugger to clean up , but worth it to find any leaks .
Thanks Kerry460. Do you have picture or link to the hose clamps you are talking about. There are no hose clamps on my fuel hoses. Also, I'm thinking since I'm in to this now, I might just replace all the fuel hoses. Do you know the size?
I bought a repair manual from EI, but the chapter I really need (fuel system) is missing!
When you had the pickup assembly out of the tank did you check the in-tank hoses VERY CAREFULLY for leaks and pin holes ?
I have a dual tank setup on my 416 DoKa. When I switched to the front tank and it was hot outside, I had fuel starvation symptoms that were not cured by external pre-filter cleaning or when running on the rear tank. After much f-ing around with the transfer switch and other components, it turned out to be a cracked line on the pickup assembly inside the tank that was pulling air.
Have you tried replacing with a factory gasket? If the original lasted as long as it did, the replacement should last at least as long?
This is not for a SEE, but for my Unimog (406.120, ymmv), looks like the gaskets are available separate from the strainers for both the in tank strainer and the on frame:
I have obviously not searched hard enough to find one. But I'll try again.
Thanks for kicking me in the butt, tkv000, I need that every now and then to keep going.
Took a stab at the fuel/air issue on my SEE today. Tried feeding the system with a hose (first from the tank, then from a can) with the electric pump. Air coming out at the bleeders, but it didn't even fill up the strainer. Apparently the pump never managed to get primed.
Blew (by mouth) into the line from the tank, no noticeable restriction and bubbling in the tank. Hooked the system back up to normal, the electric pump still in line before the valve. Now it did fill the strainer.
The good news is that it probably saves me from pulling the tank, the not-so-good that the fuel never makes it to the bleeders, for some reason. This might force me to think.
Sure looks like you may have crap in the stock mechanical fuel pump assembly. I bought a non OEM one off of Ebay from a German parts distributor for pretty cheap when I was chasing down my fuel system gremlins but never installed it as once I got the air leaks solved the stock pump worked fine. I expect your pump was partially plugged at one point and all your diagnosis work may have sealed off whatever free area you had. Of course you could just jumper over the stock mechanical pump and run the outlet of the electric pump to the inlet to the filters and see how that works to isolate back to the fuel pump assembly.
Sorry about the slow update to the thread I started ... Many late winter/spring ranching chores and I had to follow up on all your excellent suggestions. Since the last post I have replaced the gasket/filter in the pre-filter, nearly all of the fuel lines, and and the fuel pump along with a new primer pump. (By the way bel-betric is excellent.) I am still having the same problem. Prime the fuel system, run the engine until it starves of fuel - about 10 minutes.
Now I think I have been misunderstanding the long vent tube at the top of the tank. I assumed it was to equalize tank pressure by letting air in, but on the advice of a friend I intentionally plugged the end with electrical tape. Primed everything, fired up the beast and waited ... waited ... drove it for 10 min waited ... waited. It never starved of fuel. I turned the engine off after 40 min.
Can someone explain that top tube to me. Does it have a check valve to let pressure out? Do I need to replace it? Honestly, I thought it was just a tube.
Okay, that's odd. Your engine ran better with the vent plugged?!?
I thought that the vent was just that, a simple vent. On my FLUs, some have a hose on them, some are just the bare fitting, and I've read about people having issues because the vent was plugged by liberally applied CARC paint.
In my trials and tribulations I have naturally loosened the cap a few times to ensure that the tank isn't under negative pressure, but woulda never thunk to create it on purpose.
Plugging the vent made something work harder, which helped make it work better??
Well, at least you must've got rid of your air leaks. That much I think it proves.
Ok, not so fast. Truck ran yesterday as mentioned, started right up today, twice, quick drive, then idled for awhile ... then stalled out. Couldn't restart. Didn't have time to look at the problem, but I believe lack of fuel.
As I mentioned in my last post, I thought the tube was a simple vent, which still makes sense to me. I will try tomorrow with vent open, repriming etc. If that isn't the problem, then we are left with the pickup tube. Not much else left.
We might have the problem solved. I have been running the beast on/off for two days without a hiccup. Can't say what the final fix was, but I really appreciate the help from forum members. Thanks!
To recap: replaced nearly all the fuel hoses with clamps (including a kludgy splice from a previous owner), replaced the fuel pump, replaced the prefilter and a new gasket, cleaned tank, replaced fuel filters.
Inspired by WyoMog's success, I finally took the time to take another shot at my SEE's fuel issue(s).
Removed a plug at the filter housing intake (where the diagnostic line used to be hooked up) and pressurized the tank.
No fuel came out, despite trying a couple of times with increasingly higher pressures.
Then I tried the primer pump, and fuel gushed out. Hmm. Put the plug back in and tried bleeding in normal fashion.
After only three or four pump strokes (instead of the normal 30 or 40) it became firmer and fuel was coming out of the bleeder like never before.
Before, this SEE would run great for 15 to 20 minutes, then sputter and die. Now it runs for at least an hour under load.
Now I'm not sure which is worse; having the fuel problem, or having fixed it (maybe) without knowing how, or what was wrong.
That is how I felt! Not exactly sure why it is running, but it is. I've been using the beast daily without a problem. I'm beginning to think that it might really be fixed.
Hope it keeps working for you FLU farm.
(By the way, just so that there isn't any confusion the top vent pipe is open and not tapped over.)
Well, in my case it now runs much better, but still not right. Found that out during a test drive today.
Now, if lowering the rpm for a while and let it catch up, it'll run fine again...for a while.
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