Fuel and return lines

Hello all,

I am putting a VW diesel engine in my sled driving a trimmable surface drive. My next issue is the fuel system. I picked up the motor a couple weeks ago. It has a supply and a return line.


This brings up a couple questions. First off, I want to put the tank in the front of the 20' aluminum sled for better balance of the boat.

How much pressure is in the return? Can it push all the way to the front?

What is the best material for those long runs?

Thanks!

Chris

 

It should not build up any great pressure if it is open to your tank. It will flow back unless your tank is higher which I don't think it is in your case.
If you decide on stainless or copper tubing make sure you get the type that is coated with PVC and make your supports from aluminum, using insulated clamps. You have to keep other metals away from your aluminum or it will corrode. Seal wrap any connectors, couplings, as well. You could run it in a fuel rated, hydraulic type hose as well, some of these are quite tough.

As a matter of interest I was running diesel lines to a pair of 160 KVA generators last month. It was a gravity run from a storage tank. The mechanic had decided that he was not going to use the belly tanks as the fuel we get is of poor quality and was always causing problems in the other generators. I said I would get a spill return line installed back to the storage tank but he said not to bother as he would be happy to tee it back into the feed line upstream of the engine lift pump. I was a bit concerned about this as there was a lot of work involved, trenching etc, and I wanted it all done in the one contract. I had also believed that there were temperature issues.

Anyway we went with his method and the system was still working fine when I left. I go back next week and if I remember I will let you know if it is still trouble free.

 

With a 150kw MTU, I return diesel 9 meters to the tank with no problems. You may check with the engine manufacture for max permissible back pressure in the return system

 

The return line should have a pipe that lets the fuel into close to the bottom of the tank. If you just use a fitting at the top of the tank it will aireate the fuel and may cause problems.

 

The existing lines ore on top of the motor/pump. So the lines drop down to the bottom and then through through the frames to the front of the boat. I was just worried there might be too much head pressure to get back up and into the tank. Sounds like "not an issue".

Gonzo, thanks for the tip....... That may well save an unseen problem.

There was a guy selling USCG rated rubber fuel line for $2 a foot. So about $90 would get me running. That route back into the supply sounds great. But does it work?

Thanks! Chris

 

Return line fuel is hot. Not recomended to feed this hot return fuel back into the engine. Be careful. The fuel tank acts as a cooler.

 

Actually, the return fuel flow is just a trickle; there is a restriction in the banjo bolt to provide back pressure because that determines the injection timing.
Make sure that you do not exchange the two banjo bolts on the pump. They look the same, but the input has a much wider opening, for the return it is just a pinhole.

Length of the return line is irrelevant, so is the point where the return connects to the tank. Also the material doesn't matter: VW uses plastic fuel lines. I would use copper because that won't damage easily.

My advice is to go back to the guy who sold the engine and ask for the VW fuel filter that belongs to the engine. It has a thermal valve which normally returns fuel to the filter. Only when temperatures are above approx. 130 F. the fuel is sent back to the tank.

 

Damn, good question and something I hadn't even considered

I have set my day tanks up to gravity feed 3ft 6 down with a 14 ft run to the motors but never even considered if return pressure would be enough to get it back.

 

You will have about 1.5 psi at the inlet to the engine lift pump (I assume you are using the lift pump), and you will only require the same pressure to run the spill back up your return to the tank. There will be a lot more pressure than that coming back even from a low pressure injection system.

Here is some more about not using a return line, although I would recommend that you install one as some engines recirculate the fuel to purge and bleed it. It's purely for interest.

http://www.ybw.com/forums/showthread.php?p=3378844

 

Oh! That brings up amother question. Does the vw need a pump in the fuel tank? I have a 15g tank from a donor boat nearly new, shaped to fit the bow. Havent removed it yet.

 

From what you described earlier your engine pump should be able to pull the fuel through. Have you got somewhere low to place your course filter as it will be easier for you if it syphon fills, at least with a half full tank?

 

No, the Bosch VE pump fitted on the engine has a low pressure part for getting the fuel and a high pressure part for the injection.

 

IF the boat is large enough to have 2 fuel tanks , marine sources will have a fuel valve with connections to switch the supply and return at the same time.

FF

 

Thanks! Im glad to have the information. Not sure how much diesel I need to carry. My sons insist on 100gal. or stay home on their boats. But the go out on the ocean for salmon, halibut and tuna. Sometimes 75 miles out for tuna. Not me! Id take this 20' sled across the bar on a nice day, but within range to zip back across if it picxs up.

This 15g is almost 3 outboard tanks worth. Will see how thirsty this little motor is. My buddy had on in a rabbit and got 51mpg and only had an 8 g tank.

 

So is that a yes, I will be ok?
or No, I need to make some modifications?

This is on Cummins B3.3 65hp
http://www.tadiesels.com/cu-B3.3M.html