I burned out the Mallory pump I put in 2 years ago for VO. If I replace it with the FASS do I now need a separate return line? or can I purge back thru the FASS like I did with the Mallory? any other suggestions?
Yes, I believe you will need a dedicated return, as well as a air return. I had to draw out a schematic once for a Pureflow technologies FASS system. You will need at least 3 valves maybe 4; one before your FASS assembly to control the fuel going into it, one after to send it to the appropriate filter, one to control the return coming out of the engine, and one to control the air mixture coming out of the pump.
Since the FASS assembly needs to be frame mounted, you may decide to relocate your veg filter under the truck now too, otherwise the fuel will have to go up to the engine, then back down to the frame and pump, then back up to the filter, and into the engine. This would be extremely inefficient, as the fuel will probably be running in 20+ feet of hose before actually making it to the engine.
The other option would be to just use the FASS system for one fuel, and by a rebuild kit for the Mallory. All the Mallory pumps will inevitably burn out, just to the nature of there design. The good news is that brush kits are readily available (GC even has some if the pump came from them). The kits shouldn't be too much, maybe $20, and are very easy to install.
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