Low low fuel pressure and flow at high altitude

That particular thread is 10 years old.  The Pierburg pump was dropped back in 2007 or 2010.  If you have the current pump, Corona, it is all aluminium and has a multi-layer silicone diaphragm system.  This is highly unlikely to occur with the new pump.

Cheers

Kevin Stewart wrote:

Could it be a problem with the vent hole on the spring side of the diaphragm? The Rotax expert Conrad Beale talks about it here: https://www.rotax-owner.com/en/912-914-technical-questions/4952-fuel-pump-pressure-variation

As stated in the original post it is not an instrumentation issue, ie sending unit problem. A sending unit error will not cause the engine to starve. 

EDITED;

Please accept apologies, I had not read the post and assumed it was another post discussing the "atmosphere" hole on the Kavlico pressure sensor being plugged and throwing off readings at higher altitude.

That article does not suggest that it is anything to do with instrumentation, it is suggesting that if the pressure on the spring side of the diaphragm is not what it should be, either by blocking or pressurising the vent hole, then this will affect the resultant pressure on the fuel side of the diaphragm. I don't see why the same problem could not afflict any of the pumps unless they all operate on a different principle.

I wanted to try "Rotax Wizard" vapor lock 100LL suggestion, so I flew today to do some testing. The OAT on the ground was 105f, I put 6 gallons auto premium E10 in the tank, I flew to 5000', 8000' DA. I shut off the electric pump and after about 30 seconds the engine lost rpm and kept trying to stop but would catch itself. By dropping the rpm to 4500 it would run smooth but the fuel pressure was still low and fluctuating 2.5 psi and below. I repeated this several times with the same result. I landed with 4 gallons E10 in the tank, I added 2 gallons 100LL. and went back up to 5000', fuel pump off and the same result, exactly. 

I flew at 3500' DA 6800' and the had 3.9 psi engine pump only at 5400 rpm.

After landing I ran the engine to 5000rpm and shut off the electric pump and it maintained 3.9psi.

As I flew higher, eventually up to 8000' the fuel volume / pressure from the mechanical pump kept reducing. It never stopped but as I increased altitude I had to reduce rpm to keep the engine from starving.

My conclusion, if this is a vapor lock issue the addition of 100LL should have increased the vapor temp enough that a difference would have been noticed.

It may not be vapor lock. It’s possible it’s a fuel starvation.