[Lynn]

Ian,

To clear something up that my writing style evidently caused: I was not proposing the use of a pressure relief valve and float valve together. What I was trying to say was that one OR the other could cause an on demand fuel pump to cycle on and off.

You are spot on with the issue of venting that a pressure regulating valve would raise. Not only is this true on the positive pressure side of the equation, which you described, but it would also be true from a negative pressure point of view. If the HP pump was a positive displacement pump (which would make it self priming as well) and you ever got down to using that last gallon of gas in the pot, the tank would be in real danger of being sucked down to ball of cumpled aluminum because of a lack of venting. With most real world HP pumps for EFI, they would simply stop pumping in relatively short order.

In the closed header tank system in the drawing, the return line, via the main tank vents, is effectively the vent. Through the use of a float valve ONLY, this would continue to be true.

As to wiring the low pressure pump, your scenarios assume that the same power (ECU controlled) is used for both the HP and the LP pumps. While the same signal could be used to engergize two separate relays for each pump, I don't think it would be a good idea to run both through a single relay. Indeed, I plan to use a separate switch to control the LP pump relay so that I can prefill the pot prior to starting the engine. This could be done procedurally if the LP pump activation is energized with the ignition switch; but, I have seriously considered mounting the LP pump switch in a hidden location for anti-theft reasons. (Any would be thief would only get as far as the existing fuel in the header would take him.) For safety reasons, the diode protected grounds for the relay activation circuits will both pass through the inertia switch that is already in use. (The '93 Ford EEC-IV ECU does not have an oil pressure cutout for the fuel pump relay control; but, I may add this to the inertia cut out switch for added protection.) I will very likely also mount an indicator light near the fuel pressure gauge (HP) which will show when the LP pump is activated, especially if I put float valve conrol on its output.

So having an intermittent low pressure pump is technically feasible and I am sure there are going to be plenty of times when the LP pump can stay way ahead of the HP pump. But, just because it can be done, I am still not sure if it should be done. First and foremost, it violates the KISS principle! And, I am not convinced that the heat dilution gained from constantly returning the overflow to the main tanks is overshadowed by less noise, less heat generation and less power consumption of non full-time operation.

Lynn