Thursday, 6 July 2006

Fuel system diagram

A few people have asked me to publish a diagram of the fuel system used. Well, here it is. My apologies for the rather rudimentary sketch. I'll try to improve it one of these days.

There are a number of possibilities available for a twin tank conversion such as this, and there is no such thing a the "perfect system". There is always a compromise of some sort. I’ve found that my system works very well. Initially, I was concerned that this particular system would be prone to air intolerance, but as it turned out, it is very good in this respect. In fact there are NO air leaks at all, and even after the car has been left standing for a few days, the system remains perfectly primed.

Initially, this was not the case, and I was going crazy trying to eliminate the air when bleeding the lines. I refitted all the fuel lines and retightened the hose clips, so I was satisfied that the air leak was not from poor attention to the plumbing work. The source of the air leak turned out to be the design of the 3-way return line banjo fitting on top of the stock Mercedes fuel filter. The fitting has a hole through the middle of the screw so that air was entering from the canister. It was easily fixed using some metal epoxy to block the hole. Once that was done, priming using the stock priming pump on the side of the IP was a cinch!

Note that the spill line for the diesel circuit goes back to the diesel tank, whereas the the spill line for the UCO/WVO circuit re-enters the UVO/WVO line just before the heat exchanger. This helps to conserve a small amount of heat.

A separate filter for each fuel system ensures that changeover times are minimised. A one-filter system would require that the entire contents of the filter would need to be emptied before effective changeover occurred.

No electric heaters are employed at this stage. I wanted to avoid placing additional load on the alternator, as a decent electric heater would draw something like 200-300watt.

You will also notice that I have not used a separate fuel pump in the WVO system. I’ve found that the stock lift pump is quite capable of the task.

One disadvantage of my system design is that the vehicle must be started and shut down on diesel fuel. There is always the possibility that the driver could forget to switch over before stopping for extended periods, thus leaving UCO/WVO in the IP and injectors. While I've found that this is not really a problem and re-starting from cold is still possible on WVO, albeit a little more cranking is involved, this is not really ideal because it can cause coking of the combustion chambers.

Overall, I’m quite happy with it. I’ve found that the 300D seems to reach operating temperature quite quickly, and that I am able to switch over to WVO fuel after about 3 minutes. Change overs are not noticeable at all…


Anonymous said...

the system seems good but i would install a non return valve between the UCO w.v.o tank and the "T" to eliminate the fact that you would eventually warm the whole tank.

please e-mail me on

OZTayls said...

Hi JP. Actually, the system is designed to warm the tank :) This is desirable to aid fuel flow, especially in winter.

During spring, summer and autumn I remove the glow plug heater before the IP as its not necessary.

The car has done over 50,000 km now without a problem and gives me no problems.

Cami said...

I noticed in your diagram that you keep the return line from going back through the banjo bolt into the filter. How did you circumvent it?

OZTayls said...

Hi sorry for the delay in my reply. All that is needed is to block off the return with a metal epoxy. You could also braze it closed, but the metal epoxy is fine and is reversable if ever you need to "undo it".