odd dips in fuel pressure, pump problem?

" I do worry that it will bring back my old problem of  I do worry that it will bring back my old problem of high fuel pressure on descent and taxi.  on descent and taxi. "

Please quantify "high fuel pressure" ?. 

It is normal for the fuel pressure to vary according to engine power setting / demand ie high demand / low pressure, low demand / high pressure.

Assuming you are using  a Rotax, or Rotax like, diaphragm pump, you should never have a fuel pressure issue. The pump self regulates to maintain a high pressure between 5 & 7 psi -

Rotax Operators Manual - "Fuel pressure Max. 0.4 bar (5.8 psi) (0.5 bar (7.26 psi) Min. 0.15 bar (2.2 psi)"

To me there can only be two concerns relating to high fuel pressure-

1. Will the pressure overcome, the ability of the carburettor float bowl valve to cut off supply when bowl is full ie flood the carburettor, cause to leak into the engine compartment - fire! 

As long as the pump & float bowl valve are operating within specifications this should not be an issue.

 2. Fuel system burst /leak possibility ie fuel leakage leading to potential in flight fire, or engine failure.

As long as the pump is operating within specifications and all fuel connections/pipe work is in good order, this should not be an issue.

From my notes, I was seeing pressures as high as 9 psi on taxi, but I was also chasing a lot of bad senders.  I originally had the sender mounted on the engine, which kills the VDO units.  When I remotely mounted the oil and fuel pressure senders on the firewall, I also had read about installing the bypass, so I did both at the same time.  I feel like I have a fairly reliable sender now, so I'll be really interested to see what the fuel pressure is with the bypass closed.  If it's below the max, I'll happily ditch the fuel return until maybe some time in the future when I have a chance to add another return line to the tank. 

In looking at the latest manual, I also found this, which would seem to indicate that I can just ignore all this if I wanted to.

"Due to the technical design and installation conditions (construction of the return line, etc.) pressure fluctuations, at the fuel pump are possible. These pressure fluctuations within the specified operating limits are not considered a problem."

Hopefully I'll get to do some more testing this week. 

Rusty

All the best Rusty.

Speaking for my compulsive self - I am always looking for ways to improve or maintain my aircraft/engine performance (within its inherent limitations) so I empathise/understand your concerns in this area. I recently went through an intermittent  low fuel pressure problem - after much consultation (inc with this Forum) some physical investigation and various "fixes",  I now hope the latest will remove all symptoms (aircraft undergoing several changes so the proof is still about 4 weeks off)

I made a long (for me) flight yesterday, after removing the fuel bypass.  While it doesn't get as low in pressure, it still seems to be unsteady.   The mystery continues.  Maybe next time I have the cowl off, I'll replace the fuel pressure sensor, since I have a couple spares.  It's going to be embarrassing if it's that simple...

Rusty

Fuel pressure sensor failure is quite common when you have pressure fluctuations on the panel. It's  the #1 reason I keep spares in the shop.