fuel pumps

It seems that you need to first confirm if the problem is on the suction side or pressure side. If it’s the suction side, then the expected problem would be fuel vaporization from the low pressure created by the suction of the Rotax pumps as we have been discussing.  I don’t understand the erratic amp draw of your boost pump. I would think the demand would be quite constant. I don’t know of a pump that would give you a pulsed amp draw like you are describing….but I’m ready to be corrected on that.  This needs more investigation.  

If the problem is on the high pressure side, the places to examine would be the fuel pressure regulator and the fine filter.  I don’t know the placement of your fuel pressure sender, but if it’s downstream of the fine filter and there is no bypass around the fine filter, then a partially clogged filter would give you low fuel pressure and it may be improved with a second pump running. In this case, you would expect to have very little return flow back to the fuel tanks since the pressure regulator would be closing off the return flow trying to raise the pressure.  If you have a healthy return flow, but low pressure, suspect the fuel pressure regulator.   

Since this situation seems easily repeatable, then you or your mechanic should be able to use an analog fuel pressure gauge temporarily placed in various positions in the fuel system to isolate the problem.  

Not sure which plane you have but if it's a Sling tsi (or plumbed like one) then I think I know the reason. there are 3 pumps in the system, 2 Rotax pumps in one box on the firewall, and one boost pump under the dash. You can test this by turning on the master and listen to each pump run separately. You will probably hear a different sound when you run pump #2 because the boost pump is also running. The boost pump only runs with pump #2. I think the 2 Rotax pumps like to have their suction side fed with pressure. They evidently don't like to have low suction pressure, and the boost pump adds this to pump #2.  That's why the pressure drops more when you turn pump #2 off, you lose both pump #2 and the boost pump. I just use pump #1 as the backup and leave pump #2 on all of the time. Otherwise, if you run both pumps on all of the time it would be very difficult to know if one pump stopped working. All you would see in that case is a slight drop in pressure and no light would necessarily come on. If you only have one pump on and it fails then the engine doesn't quit instantly, your pressure would start to drop, your warning light would come on, and you would have some time to turn the backup on. Someone else said earlier they just switched the wires which essentially just means that when you turn the pump #2 switch off you are now actually turning off just the Rotax pump and not the boost pump. 

Just an idea...would using two Andair PX375-TC pumps at the wing roots and power them both on with either Rotax pumps is enabled be a good solution to the feed side?  The Andair pumps have a pressure relief for when one is pumping against a closed valve when the other tank is selected.

They are just spendy...

Within the body of the PX375-TC Boost Pump is contained the Bypass.This feature allows for fuel to flow around the positive displacement pump, whilst the pump is not running.The porting within the body has been computer optimised to ensure the best possible flow conditions.

The pump also incorporates a pressure relief system. Here, if the pump is pumping against a closed line, the pressure relief port will open and fuel will re-circulate within the pump. With theAndair Brushless Fuel Pump the motor is immersed within the fuel.This has 3 distinct advantages; the fuel can be used to cool the internals and the design ensures there will never be any leaks between the drive to the pump, furthermore, using advanced materials and machining techniques we can use the fuel to the internals giving us better performance

GENERALPX375-TC SPECIFICATION :

11-16.6 VDC 6 AMPS MAX / 4.5 AMPS NORM.

16 GPH @ 27-30PSI / OPEN FLOW 55-60 GPH @ 5PSI

NO FLOW MAX PRESS. 30-33 PSI

PRESS. DROP IN BYPASS MODE @ 36 GPH <0.5 PSI

SELF PRIMING TO 10ft (3m) THROUGH 3/8” TUBE

FITTINGS: 1/4 NPT INPUT AND MALE AN-6 OUTPUT 1/4 NPT TO AN-6 ADAPTER INCLUDED.

SOFT START / CONTINUOUS DUTY

WEIGHT 383g /

AEROSPACE GRADE ALUMINIUM BODY