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200tdi Mechanical or Electric Fuel pump - Veg


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Hey,

Very recently I installed a twin tank veg oil system, about 600 miles later I have blown one of the seals on the lift pump. The lift pump was purely for the veg so was dead heading when running on diesel, but I don't see this being a issue as it is a diagram type.

I would like your opinions here, which is better electric or mechanical lift pumps? both are about the same cost so don't mind converting

Adam

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You can't use a diaphragm pump without it being allowed to let its fuel out - by having the pump pressurise the SVO fuel but running on diesel, the pump pressure will exceed its limits with the resultant damage. Electric rotary pumps would be fine to have running as they will only be able to produce their nominal pressure and no more, though switching the power off when not required would be preferable for pump longevity.

If you're sticking with the engine driven diaphragm pump (I'd recommend you keep it), then you need to have it drawing the fuel, not pushing, by re-rigging the entire fuel system so that you have parallel diesel and SVO systems, from tanks, feed lines and filters, only then joining at the selector valve, to then continue to the lift pump and directly to the injection pump. That way, the lift pump will serve both fuels, never operating without allowing fuel flow and not requiring an additional pump for the other fuel.

The only down side with this arrangement is that the fuel filters will be under negative pressure, so you need to be careful about their seals because leaks in will be harder to spot, being air inward rather than fuel outward.

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Ah right I see, that is contrary to what I knew previously, as I was told that diagram pumps could dead head.

Mine is currently set-up without a pump on the diesel circuit and the lift pump is pushing (initially cold veg), so I think I need a slight re-think. Although other than that I do like the system, both fuels are filtered before the valves (veg heated) then go into the closed loop with the main heater in the loop

So would it be best to just put the mechanical lift pump in the closed loop with a in-line filter just in case and have the whole system in suction instead? Just like to confirm before I order a new pump and assosiated pipework to change it, My car is intended for overlanding so need the best reliability I can get. Would paddocks one be ok as they only list one at £21?

Thank you

Adam

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Closed loop systems greatly increase the purge time when switching back to diesel, so you could end up with the coking problems I had when I ran dual fuel. I'd recommend a switchable return line system that changes to the SVO tank as the same time as the feed (a little diesel in the SVO tank does no harm), but is switched back to diesel at least 30 seconds after the feed.

The biggest vulnerability in the system is the coolant circuit through the heat exchanger - plastic T-pieces are a bad idea, as I found out personally half way up a mountain. Use brass T-pieces.

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Mine is currently set-up without a pump on the diesel circuit

Is the injection pump sucking fuel all the way from the main tank then?

Whilst it'll run like this (as you've found out), I don't think it's good at all for the injection pump to have to draw fuel itself. After all, the lift pump was fitted for a reason.

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Is the injection pump sucking fuel all the way from the main tank then?

Whilst it'll run like this (as you've found out), I don't think it's good at all for the injection pump to have to draw fuel itself. After all, the lift pump was fitted for a reason.

I have a second small diesel tank fitted. This goes through a filter to a six port valve. The main tank (containing veggie) runs through a heater, then a filter, then to the six port valve, from the six port valve the selected feed then goes to the lift pump and then the injector pump. When it is very cold, that first slug of oil from the heater to the injector can slow the engine down quite a bit! I have also found that on cold days, after a while the spill going back into the tank warms it quite nicely!

My six port switches feed and return simultaneously, the advantage is that it purges very quickly a disadvantage is that veggie will end up back in the diesel tank. A neighbour who has a similar conversion has independent valves, so he switches hi s feed first and a time later, his return.

As has been said, you end up using the lift pump to suck, so air tightness becomes very important!

Cheers

Peter

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Sorry I should have been more clear

I use Solenoid valves, one to switch the fuels, another that by default forms a closed loop and while active opens a return to the main tank for quick purging. The diesel tank has no return.

New pump is on order which will go just before the injection pump with a small inline filter just before the injection pump in case of lift pump failure. So hopefully that will sort all the problems out. Air getting in on suction hopefully shouldn't be a problem as I have been quite careful on what fittings I have used.

I will look for some brass t-pieces, as, as you might expect, mine are plastic and I have reducers made of plastic too!

Have to admit, I'm surprised we don't have a FAQ style thing on here, I could write one up with things I have learnt?

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  • 6 years later...

I'm trying to get to grips with designing a veg oil twin tank conversion for my 300tdi Defender and am trying to study up. I wanted some feedback on whether the following would solve the issues regarding fuel pump types and configuration?

From reading other forum posts I:

Would rather not use original mech lift pump for both fuels because the veg oil heater / filter need to go before the pump (so diesel isn't heated and to enable the use of separate filters for each fuel) and there would be negative pressure on the seals which isn't ideal.

Would rather not use electric pump for both because decent ones are quite expensive and mech pump seems tried and tested for veg oil.

Would like to pre heat the veg oil for initial flow at switch over from diesel to veg.

Within that in mind....

In diesel start up mode: 12v fuel pump pushes diesel through sedimentor, filter, solenoid, injection pump and returns to diesel tank. Veg oil is simultaneously pumped via original mech pump through filter, heater and returned to veg oil tank in it's own loop created by solenoid 1.

In veg mode: Veg oil is pumped as above but allowed through solenoid 1 and 2 to injection pump and returns to veg oil tank.

Purge mode: as per diesel start up mode but diesel is returned to veg oil tank.

This means that the mech pump never dead heads and is able to push through filters (and therefore no neg pressure on seals) with the added benefit that the veg oil is heated via the heat exchanger whilst engine starts and warms on diesel.

I want to know if i'm missing something which is perhaps why this hasn't been suggested before.

 

Screen Shot 2017-02-06 at 12.59.46.png

Edited by yourekiddingright
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I had a 1990 110 (initially 19J then 200Tdi) that I converted to a veggie. It took a while to tweak the system, but this is what I ended up with.

Twin tanks. A small tank of diesel was installed behind the centre middle seat. I also installed a heater unit on the oil line (fed from the cooling water system) a changeover valve, did the feed AND return (stops you feeding oil back to the diesel!) and a temperature switch (70degC) on the heater.

The engine was always started on diesel.  When the engine was sufficiently warm, the temperature switches automatically swapped from diesel to veggie (I had an indicator on the dash that showed the state of the valve (red - diesel, green - veggie!) about 1 mile before I was about to get to my destination, I would flip a switch that out the engine back onto diesel to purge the veggie.

That would leave it in a state ready to start again the next time.  I also had a hidden switch that would allow me to force a switch either way.

The only thing I would do differently is to have a slightly larger tank of diesel and either a fuel gauge on that tank or a warning when it was getting low.

If you want more details, send me a message.  

 

Cheers

 

Peter

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