Fuel Pressure loss on Loop upline
 

Airplane - F1 Rocket, IO-540 D4A5, recent install of Red Cube between fuel servo and spider. Engine driven fuel pump has ~350 hours.

Pulling 3 g's at the start of a loop I am experiencing a fuel pressure loss from the normal 20+ psi to ~8 psi (engine loses power) after which my boost pump automatically kicks in (power picks back up) and brings the pressure back to normal.

I can fly around all day, pulling 2 g's in 60* banks, and no issues, fuel pressure stays 20+ psi.

Anybody experience the same? Am I correct to assume a new engine driven fuel pump is in my near future?

Consider the changes you made, and start there.

First, it worked fine prior to relocating the red cube, yes?

I assume you're measuring pressure at the engine pump outlet. If so, nothing you plumbed between the throttle body and the divider would make a difference.

I further assume the red cube was previously firewall mounted between the bulkhead fitting and the engine pump. If so, you changed a hose and/or hard line as part of the relocation.

Thus three possibilities; the new lines pick up more heat than the previous install, or one of the fittings has a small air leak, or you installed a right angle fitting and (combined with heat) it is forming vapor bubbles in the flow.

Mark----did you have previous flights in this configuration and 3g's with no issues?

Tom

20 PSI "normal" is too low (18 minimum), typical pump pressure should be around 24-26, I'd start there.

I have had 10 flights since the change where I performed some aerobatics, only the last 3 - 4 have shown these symptoms. BTW, the other change I made at the same time was replacing my Andair gascolator with and Aerolab.

Correct, just to clarify this is not a measurement issue, there is a corresponding loss of engine power till the boost pump cuts in.

Close, it was a Floscan that was replaced, and it was located in the tunnel between the stick well and the firewall. Replacement hose is a straight run.

To add the Red Cube I put a bracket on the front of the fuel servo and mounted the cube there. Not a likely source for additional heat.

Will check the fittings for a leak, although why "G" forces would exacerbate a leak I am not sure, but possible.

No right angle fittings were introduced.

EDIT: This is not fuel flow related, I have confirmed via flight testing.

I should have been more clear, the normal fuel pressure fluctuates between ~22 and ~25. If I look at this on a graph it has a very constant oscillation - is that oscillation normal?

Approximately what is the frequency? I've noticed this at about 2400 RPM as monitored by the G3x Touch. I believe it may have to do with the sampling frequency of the engine monitor vs pressure pulses in the mechanical pump. Unverified, but the oscillation goes away when I dial the rpm down from 2400 to about 2350. The period of the osc. I have observed is about 5 seconds, and it will vary between maybe 21 and 25.

So check it for an air leak. Seal the inlet and pull suction on the outlet, see if it will hold a vacuum.

If fuel weighs 3x more, it requires 3x the pressure drop for the same lift. More suck means a small air leak becomes a big air leak. Entrained air doesn't pump very well.

Hey, it's the long distance diagnosis game, with the usual lack of information. Maybe right, maybe not, but I usually have a logical reason for a suggestion.

location, location, location
 

Where did you mount the aerolab relative to the exhaust tips?

Or how many inches aft of the firewall? I am guessing it is somewhere in the center tunnel belly skin.

I am also guessing on take off, below 500ft, you have your boost pump on?

The exhaust pulse is a vibrational shake and bake to anything on the floor skin near the center tunnel. Might just be bubbles manifesting from the increased flow demand when you pull the nose vertical. A 2 G level turn will need less power and fuel flow than a 3 or 3.5 G pull up.

This I will do

Of that I have no doubt.