Low Fuel Pressure

You don't say what make/model of aircraft you are referring to -- High wing?  Low wing?  Fuel return line system? Electrical aux pump? 100LL and/or mogas (with or without ethanol)?  

Low fuel pressure on climb out is not unheard of in some make/models of aircraft.  For example, Van's Aircraft offers a higher pressure electric aux pump to address reported RV-12 "low fuel pressure" issues.

This is a 912ULS in a 2007 Sky Arrow 600 sport, High wing, engine mounted above wing in pusher configuration. Fuel goes from fuselage tank (under engine) to gas collator, to aux electric pump to engine pump to carbs and return to tank.

I've had numerous problems with both engine pump pressure and electric aux pressure all seemingly unrelated. Over the course of these many events I have replaced electric aux pump, engine pump, fuel pressure sending unit, and most hoses. (not all done at the same time).

The latest "event":  On ground, engine off, checked aux pump pressure. It was slightly low. (It always reads slightly below green even with new pump, and new fuel sending unit). After engine start, engine pressure in the green. Takeoff with electric pump on, pressure normal. After takeoff and takeoff power, aux pump turned off. Pressure slowly dropped towards zero. Turned aux pump on and pressure returned to normal. Leveled off and reduced power to cruise. Turned pump off and pressure dropped towards zero again. Turned pump on and pressure increased to normal. Gritted teeth and turned pump off again to see just how far the pressure would drop. At zero, nearly, turned pump back on and pressure increased to normal. Landed. After landing, turned pump off and at idle pressure remained normal. Ran up engine to nearly full power...brakes won't hold at full power and pressure remained normal. Shut down.

Next time on trouble shooting on ground checked aux pump on, engine off pressure, normal. Started engine and pressure bottom of green arc. Turned aux on and pressure increased. Sometimes when aux is turned off pressure drops and sometimes it doesn't.

I hope my synopsis is understandable.

Thanks for any help and suggestions.

These problems have occurred bot with mogas, with and without ethanol and 100LL. Currently operating with only 100LL.

Was the tank nearly FULL or was there some headspace available?

Have you confirmed that the tank vent is functioning properly?  Are you sure you're sure???

A single vent or multiple?

Is the vent pointed into the wind, downwind, crosswind or not in the wind?  Consider a venturi effect. ???

I also get this happen on a Bristell NG5 in summer months. Been doing it for 3years from new. Engine runs fine at all times.

In winter I have a steady solid reading of 5psi at all times, climb, cruise or whatever. In Summer it can drop to 2.0 but generally starts off at 4.5'ish but slowly drops to 3-3.5 psi and ocassionally as low as 2.0psi. It always recovers to 4.5 or 5psi when I put the backup electric pump on. I only usually use the electric pump for takeoff and landing.

I have put it down to warm weather fuel vaporisation. I use ethanol free Mogas not Avgas. I suppose I should try Avgas 100LL and see if it makes any difference.