Mercedes-Benz Forum banner

1 - 16 of 16 Posts

·
Registered
1980 U1300L
Joined
·
13 Posts
Discussion Starter #1
I own a U1300L Ambulance-to-camper conversion, with the OM352 normally aspirated engine. It came with a (roughly) 36 gallon (135 liter), passenger side mounted, galvanized steel tank. I decided I wanted to have more fuel capacity, and fitted two Freightliner M2 50 gallon aluminum tanks. I used the Pollack 6 port electronic switch to route the two tanks' fuel flow to the engine.

Once I purged all the air from both tank lines through to the fuel filter, the truck ran fine while idling in the driveway. But, while driving, it died every few blocks, and took forever to get restarted . . . only to die again after traveling a few blocks.

Has anyone else had problems with tank/line conversions? Before the tank conversion the truck ran great.

Probably my most perplexing concern is about the original plumbing. The original supply and return hoses were 8mm plastic, attached at the engine with banjo fittings. Since both the Freightliner tanks and Pollack switch came with 3/8" supply and 5/16" return lines, I had new 3/8" and 5/16" diesel grade hoses made to go from tanks to switch, and switch to engine, replacing the banjo fittings at the engine with 90 degree, metric thread, crush-washer fittings. One mystery was a third line (about 5mm hose) that runs from the top back of the engine, just in front of the removable service panel in the cabin, to the top of the original fuel tank. I included a photo of the attachment point for this "mystery" hose. The truck ran OK with it open. It dripped a little fuel after I turned the truck off. So, I plugged it with a bolt that stopped the dripping, and the truck still ran OK (at idle speeds). You can see the end of the line with the bolt plug in the bottom of the Pollack switch photo.

I feel the truck dies because of fuel starvation and/or air in the line. I also do not know what to do with the small third hose that was attached to the top of the original tank with the supply and return hoses. I am not even sure of its purpose. I am out of ideas. Any suggestions?
 

Attachments

·
Registered
1991 1250L Doka Unimog, 2002 ML320
Joined
·
1,183 Posts
Mine is not military. It has the lines you speak of and an extra one going to the heater under my rear seat. I think the line you questioning might be the vent going to a dry area for venting.....
 

·
Registered
1976 416.141
Joined
·
350 Posts
Hello,
That 3rd mystery line at the back of the engine is the fuel return line from the injectors, it should return to the tank, I can't help you with the all the other plumbing except to say try using fuel injection hose clamps, the worm drive type you are using can damage your fuel line and allow air to get into the system.
Rick Montez
416.141
 

·
Registered
1980 U1300L
Joined
·
13 Posts
Discussion Starter #4
Thanks Rick and Chas.

I kind of thought the line might be a vent since the engine ran at idle speed with it both plugged and unplugged. I think I will run it back into the top of the tank, via the overflow valve, pictured below.

Now, if I can just figure out why the engine runs so erratically since the dual tank conversion.
 

Attachments

·
Premium Member
U1000Ag
Joined
·
2,254 Posts
See any bubbles in the glass fuel bowl ? If so, air is getting in.
If you can, plumb a low pressure gauge into the fuel supply line leading to the fuel filters. Should see about 4psi - the injector pump needs that much in order to function properly.
I am guessing air is getting in somewhere.
And Montez is right on the money regarding those hose-clamps - get rid of them.
 

·
Premium Member
Unimog U1250
Joined
·
1,650 Posts
Get a facet pump and use it to pressurise the fuel lines. I have one and it works , makes it easy to find leaks and purge the air out of the fuel lines. One of the weaknesses of the Pollack valve is that even a tiny amount of dirt stops the seal. I have CAV filters upstream of my Pollack valve to protect it
 

·
Registered
1980 U1300L
Joined
·
13 Posts
Discussion Starter #7
Thanks for the suggestions Iain. It's definitely something I will investigate.

When I received your message, I immediately looked at a few facet pumps, and there seems to be a wide variety. Krietpiel has suggested in a previous reply there should be 4 psi in the supply line to the engine. Is that your understanding? I have seen several facet pumps that have a range of pressures, e.g. 4-7 psi. Acceptable?

The other question I have about the facet pump is placement. Are you suggesting the pump should be between the Pollack switch and engine, or between each tank and the Pollack switch? So, in other words, there are two facet pumps, each inline between the tank and switch, pushing the fuel through the switch?

CAV filters sound great. And since this suggestion is to keep the switch clean and clear, I assume there would be two mounted inline between each tank and the switch. True?
 

Attachments

·
Registered
1980 U1300L
Joined
·
13 Posts
Discussion Starter #9
continuing saga of dual tank conversion fuel problems

I have completed all the suggestions of those on this forum for solving my fuel flow problems since the dual tank conversion. To backtrack/update for those who may be reading this thread for the first time, I removed the original single tank from my U1300L Ambulance, and added two 50 gallon aluminum Freightliner tanks with a Pollack 6-port electronic fuel selector switch, along with new supply (3/8") and return (5/16") fuel hose and fittings(see photos from previous posts). Once I completed the conversion, I bled the lines, and though the engine will idle fine for 20-30 minutes, once I begin driving, it stalls every 100-200 meters. I then have to bleed the air out of the lines to get it restarted.

As mentioned, I completed the suggestions of those on this forum, i.e.; new fittings and hose on the engine "vent" return line (blue hose in photos) routed into the main return line, installed a facet fuel pump downstream of the switch to pressurize the supply line to the engine, installed filters upstream of the switch to protect it, and replaced all worm-drive clamps with injector clamps to prevent accidental air induction into the lines (see photos attached to this post).

Still, I have the same problem. Idles fine for 20-30 minutes, begin to drive, and it dies within 100 meters. Open the bleed valve, and the line is full of air. I am frustrated enough with the problem I may not be seeing it with a clear mind. Since the truck ran well for over a year with the original tank and lines, I feel the problem must be related to the new fuel setup. Since everything from the tanks, to the lines, to the switch are new, I am not sure which link is the most likely culprit.

HELP!
 

Attachments

·
Premium Member
U1000Ag
Joined
·
2,254 Posts
Very frustrating for you I am sure. However, you have done good work so far, eliminating a few things. I would suggest you plumb in a short piece of transparent pipe so you can see what is going on - bubbles should be visible. Start at the tank end and work your way up the line. The tank might be new, but still, it remains possible the pick-up has a manufacturing fault and sucking in air. You will get there.
 

·
Premium Member
Unimog U1250
Joined
·
1,650 Posts
I guess it is a process of elimination. I had a similar problem, would get an intermittent air leak. If you run the Facet pumps whilst driving and still have a problem, then it must be downstream of the pumps as they will pressurise everything up to the lift pump.
 

·
Premium Member
U1000Ag
Joined
·
2,254 Posts
Like Iain said - process of ellimination.
Looking at your photos, I would start with the elbow connection. Cannot see what it is - but, if it is a Swagelok hydraulic style - it has that look about it, it will need serious tightening to make airtight. Unlike the softer brass or nylon ferrules, steel ferrules can be more difficult to seal.
 

Attachments

·
Registered
1980 U1300L
Joined
·
13 Posts
Discussion Starter #13
Thanks guys. I'll keep trying, but I want to start by eliminating the most probable cause first, rather than just shotgunning it and randomly attacking each potential problem.

I want to be clear, Krietpiel, to mention that the tanks are "new" to the truck, but were taken off used trucks at a junkyard. The thought has already occurred to me there could be a problem inside one, or both, of the tanks. I cleaned and drained them before I mounted them. But I did not test them. I'm not sure how I could test them for fuel flow.

I considered splicing the supply line into one tank at a time, thus bypassing the switch, and running it temporarily as a one tank setup. In this situation I test each tank, and line from the tank to where it meets the switch, independently. I also take the switch out of the equation in this scenario.

What appears to be happening is that I clear the line (at least from the pump to the engine) of air, the engine runs fine until it burns all the fuel in the line, at which time the newer, air contaminated fuel has reached the engine. What is baffling to me is where the air is being introduced. I have not found any visible leak - reasoning that where air can enter the system, fuel would escape. I have had some thoughts similar to that of a broken straw in a drink - when suction is applied to the end of a straw, air is introduced into the fluid traveling through the straw where the break in the straw is above the fluid level in the glass. In this manner, there may be a small hole in the fuel tank siphon tube, above the fuel level. I have no idea how to test for this possibility. Krietpiel suggested plumbing in a transparent section of tube, but where? I know air is eventually entering the system because the lines are full of air every time it dies. I bleed it, and it runs fine, until that "clean" section of fuel is burned.

So, in short, any suggestions for systematically eliminating the most probable areas of concern?
 

·
Registered
1976 416.141
Joined
·
350 Posts
Here's a couple of questions to ask yourself.....

How much fuel is in each tank? Could the fuel level be so low that when the truck is moving the dip tubes may be momentarily uncovered?
Does the problem occur with both tanks?
Can you temporarily bypass the switch and plumb directly to the Facet pump?
Can you temporarily run a single piece of 3/8" line (no splices) from one of the tanks directly to the lift pump?
Do you have the old style or new style priming pump? How old is it?

The suggestions from Krietpiel and Iain are a great place to start and hopefully the questions can assist with developing a methodical approach to isolating the problem.

Rick Montez
416.141
 

·
Premium Member
Unimog U1250
Joined
·
1,650 Posts
When I first fitted my dual tanks, I had a lot of problem with one of the tanks, turned out to be an air leak on the threaded nipple on the CAV filter, a leak on the Facet Pump and a crack on the inject return line T piece. I would suspect that hydraulic compression fitting as well - they are great for massive pressure, but hopeless for vacuum.

I would do the following.

1. Try drive the truck with the Facet pumps on - if it runs then the problem must be between the pumps and the tank or in the tank itself.
2. Since the problem is with both tanks, then it has to be downstream of the Pollack valve unless you have the same compression fitting Pieter mentioned on both tanks or exactly the same problem on both tanks. Replace the two copper washers on the lift pump end fitting first, you only get one or two uses out of them at the most before they harden.
3. There is one line between the Pollack valve and the inlet to the lift pump and if it is old, replace it, it might have a pinhole leak. The T piece for the little injector return line on that line is another source of problems, the OEM one is plastic and can crack, get a brass one if you can.
4. I found the little filter/non return valve on the end of the facet pump leaked on mine, I had to replace it with a straight connector - no need for a non return valve anyway if you don't have any leaks.
5. I used the air line that I have for the air system, just heater it up with an hot air gun and pushed it over the fittings, added a crimp clamp or similar for extra insurance, being rigid they need long radius bends so the don't collapse but vacuum won't suck them closed like rubber hoses if there is a restriction somewhere so it doesn't look nice and I have made sure there is lots of line to flex normally a full spiral between the camper where the filters are attached to and the tanks / rest of the truck. Using air lines means I don't have to carry too many different sizes of hose - just the air line sizes.

My plumbing is a lot more complex as I have some Ts and valves on the feed and return lines to allow me to transfer fuel between the two tanks through the filters using the Facet pumps. Helps clean up dodgy diesel at the cost of a $6 filter or two, which is not bad considering 150Lt of diesel can cost over $400 in remote places. If I forget to close the transfer valves the air gets sucked in but it is easy to prime with the Facet pumps on I just pump the hand pump slowly and all the air goes out in a minute or so. I also have lines for the Webasto diesel stove and 90ST Hydronic heater, hence the mass of lines everywhere.

2017-12-05 21.38.23.jpg

2017-12-05 21.38.02.jpg

2017-12-05 21.39.19.jpg
 

·
Registered
2001 W163 ML270CDI / 2006 W164 ML500 / 1986 U1300L Unimog
Joined
·
185 Posts
Obviously Iain and Pieter are all over this but I'd just be taking it back to basics. Take a 3/8 length of temp hose with an inline filter and start excluding components from the circuit. Better yet do it using clear line. It doesn't have to be used for long or stand up to large pressure so you could probably get away with cheap clear vinyl tubing, or if you're worried use Tygon. Run it from each tank separately to the primer pump. If she works and there are no bubbles you know that it it's somewhere in the middle. If it doesn't work but there are no bubbles you know its from the primer pump up. If it doesn't work and there are bubbles you know its the tank pickup. You get the picture. If it really is air ingress then its quite a simple (yet painful) process of elimination like the others have said. Just as a side note I changed my fuel lines to 3/8 too but instead of changing the fittings I just kept the original banjos. It may be worth trying if you find it is on the fitting ends.

Last question I'm opening up to the masses. Could it be something to do with the small injector return hose being run into the main pump return line? Could the main return line be backing fuel up into the injector return line and stalling it? Alternatively could it be causing a venturi? I'm guessing there's a reason the original system didn't just join the two returns together. Did you ever try driving it with that return just disconnected? The little bit of fuel spilled while driving to test it wouldn't hurt anyone.
 
1 - 16 of 16 Posts
Top