What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Low fuel pressure when climbing

von_flyer

Well Known Member
Advertiser
I have had low fuel pressure during climb in my RV-7 with Lyc 0-360 carbureted. Now I have reading varying all the time. I am changing the VDO sensor to correct that.
Question I have, are there factors in plumbing orientation that would cause low pressure reading only during climbing? It is mounted high on the firewall.
 
While the VDO sensors aren't the best, your problem is likely not the sensor. Do some searches here on that problem, it's well reported.
 
In my 10, I get moderately low fuel pressures in the climb in hotter ambients; Never on cooler days and always in the later part of the climb where temps are rising due to the thinner air. In the 6, I get this very ocassionally. In both cases, the boost pumps makes it normal. Boost pump is in the fuse, so not heat affected.

I believe that the reduced cooling air flow is allowing the fuel to get too warm and creates some vapor and the resulting disturbance in the suction side of the mechanical pump causes reduced flow and therefore the lower pressure. Have NEVER seen it other than the climb in warmer air.

In my case, I don't believe it is worth remediation efforts, as the pressure never drops below what the servo can live with (around 12-13 PSI). I have heard that 90* fittings (excluding the AN fittings with long sweep tubular structures) or other sharp changes in direction can increase vapor production when fuel is near the boiling point, but cannot comment on how true or valid that is.

Larry
 
Last edited:
Lycoming Operators Manual states:
for O360 engines with Carburetors
Max 8psi, Desired 3psi, Minimum 0.5psi.

I have seen planes with warnings set to flash at 2psi because their VDO senders were not articulate enough to show anything less than that... makes for very nervous pilots, for no reason.
 
That was how a mechanical fuel pump failure manifested itself on my airplane. A drop in fuel pressure during climb, while operating with the boost pump off. The fuel pressure appeared normal in level flight and descent.
 
Back
Top