Hah! There’s two big debates in routing your fuel lines for a SVO-WVO-Vegoil system. First, to loop or not to loop. Second, to add a vegoil lift lift pump or not. We loop and in general don’t add a lift pump.
Not looping your veg return to the veg tank mirrors what most factory diesel system do, which is to return the diesel to the diesel tank. The benefit of a full-return system is that is minimizes diesel use, and purges any air you have in the system, avoiding the most common cause of poor performance and stalling (the other issue being a clogged filter).
Looping, however is what we do. Looping allows a gradual change from cold thin diesel to hot, almost as thin vegoil. It avoids the slug of cold vegoil that is inevitably sitting in the valve and the last few inches of unheated lines that are found in full-return system.
The looped system keeps circulating the heat up front, and allows us to backflush the veg filter with diesel. This backflush primes the supply valve and the few inches of unheated lines with diesel, preventing cold-weather clogging, and minimizing shock to the injection pump. There is no slug of cold vegoil, instead the heated diesel mixes with the vegoil/diesel blend in the purged supply lines, which then loops back again and mixes with the fully heated vegoil. The disadvantage of this system is that it uses a little extra diesel to purge the supply lines and uses the diesel left in the injection pump to blend into the loop.
To summarize: Looping is gentler on your injection pump and has better cold-weather properties. But, it doesn’t purge air and it uses a bit more diesel in order to purge and thin the initial slug of vegoil.
Of course, we use a 12v Vegtherm inside the loop, so we can maximize heat gain, and offer a full backflush. The Vegtherm is shut off for diesel driving, so we’re not lowering the lubricity of the diesel by unnecessarily heating it. Note that we use a Flat Plate Heat Exchanger (FPHE) on the veg side. Some folks create a longer loop into the FPHE, but that system won’t allow a full backflush. Only a 12v heater, of which the Vegtherm is the SVO standard, allows in-loop heating and a full backflush.
Okay second debate: to lift pump or not!
Older VWs do not have separate lift pumps. So no. Older Mercedes do have lift pumps, but the stock diesel filter can be rerouted before the lift pump, so the lift pump can pull from either the veg or the diesel tank, through the respective filter, and then into pump and on to the engine. So no. Fords and Dodges can generally be re-routed as well.
Newer high-pressure systems need a highly-regulated fuel pressure. So in general, yes. The pump that works well is a FASS pump. It’s guaranteed for 2 years on vegoil and has a 23 amp motor. This is not your standard pump, it’s not even the standard FASS pump. If you’re going to be converting a high-pressure (Pumpe-Düse or common-rail/CRD) system you will probably need one of these. An exception is the early Ford Powerstroke HEUI injection systems. In these, the stock filter cannot be rerouted, so Plantdrive.com provides a billet aluminum replacement filter “bypass” plug, and adds additional filters for both the veg and diesel sides. The good news is that you don’t need the extra pump.
I’ve only had one conversion where the owner wanted a cheap lift pump. They were having starting problems, and their mechanic couldn’t deal with the fuel line rerouting. We put on the cheapo $50 Solid-State Facet Pump (note: These have largely been replaced with their posi-flo pump). These should be considered disposable, but work. It should be noted that it ended being their glow plug relay, not fuel related at all.
Rerouting the lift pump allows the pump to work as it should, avoiding any chance of it pumping into a blocked valve (deadheading), and is cheaper since you don’t need an extra pump. Furthermore, the veg is always flushed from the lift pump, as a separate lift pump needs to be heated, along with the rest of the veg fuel system. The benefit of adding an extra pump is that it keeps your stock system stock, making it easier for mechanic to work on, and provides a fully redundant fuel system.
We believe in robust yet economical vegoil systems. In most cases a looped return and rerouting of the stock lift pump provides the best option for Alaskan winters.