The Espar D4 in my rig never worked well. Usually these heaters carbon up from being run too cool or from high altitude use, so I bought a new atomizing screen and glow plug to repair this issue. I also bought one of the very cool solid state altitude compensating devices Espar sells.
I used these in my TDI Syncro builds many moons ago and know they work well providing the proper fuel ratio at nearly any altitude. My rig had the dual pump set-up assuring the mixture was never right.
When I pulled the glow plug and screen, I noticed they were spotlessly clean--ie. no carbon. This indicated a fuel starvation issue. We all know the crap about not being able to run the furnace with less than 1/4 tank--not sure what to do about it--but my tank was nearly full, yet there was no fuel at the Espar pump. My thesis is that when the fuel tank is run down, it creates a siphon effect that pulls the fuel from the Espar circuit leaving the pump dry. To cure this issue, I installed a small plastic fuel filter in the pump inlet circuit.
This will create a reservoir of fuel for the furnace, but will break the siphon effect if that is the issue--we'll see.
Once I supplied some fuel to the pump, it fired right up. So when this tank runs down, I expect to see a supply of fuel in the filter that will feed the furnace until it can pull in more from the tank.