Fuel pressure mystery

The vent tube should never be in the air flow. This is in the Installation manual. Actually none of the vent tubes off the engine are supposed to be in the air flow.

Sean,

Are you using 100 LL or avgas?  Currently auto fuel where I live (CA) has a very high RVP, I tested some yesterday and it was about 14 PSI.

Just thinking out loud about what changes occur during climb out….

1. More heat in the engine compartment from max power and reduced air flow… could warm the fuel lines.

2. Increased fuel demand…. but so minor that should not affect pressure.

3. Nose up attitude… on a low wing possibly enough to increase the net suction head at the mechanical pump a bit.

I’m wondering if these factors are enough to create some fuel vaporization on the suction side of the pump, which would be solved with the boost pump, and would also fade away after leveling off to cruise.  If you are using 100 LL or a 50/50 mix of 100 LL and mogas, this is not likely.  But if you are using 100% mogas, it might be worth trying some avgas.  

Jeff B, I like your reasoning and it may be true; however one thing about it puzzles me. If some fuel vaporization is occurring (essentially some vapor lock) You would think that someone would notice some engine hiccupping or rough running occasionally like you get from typical vapor lock. Yet I have not heard of anyone experiencing engine issues during this anomaly.

James, it’s a shot in the dark but here is what I’m thinking.

Let’s say the pressure on the inlet side of the pump is -1 PSI from ambient, and under certain conditions that lets some bubble form in the input fuel line and enter the pump. Since gasses are compressible, that would lower the efficiency of the pump, but may still allow the pump to make a 1–2 PSI on the output side. Then, as the (bubbly) fuel is compressed by the pump, the bubbles may be reabsorbed.  Any bubbles that remained would be vented out the float bowl vents, so as long as enough fuel reached the carbs, the engine would run as normal.  The only net effect would be a lower fuel pressure.

Roger Lee wrote:

The vent tube should never be in the air flow. This is in the Installation manual. Actually none of the vent tubes off the engine are supposed to be in the air flow.

Prior to yesterdays flight, I had two bottles/catch cans, located low down on the firewall. One bottle dedicated to engine oil breather and the other a combined air box & fuel pump drain bottle.

I have now separated the airbox & fuel pump drains into individual sample bottles, so that any fluids caught can be attributed to the correct source.

The air box drain/ bottle is still low on the firewall.

The fuel pump drain/bottle is now just below the oil pump (front of engine).

In truth, I have no idea what the pressures may be in each location - just taking a "punt" that the systems will work where placed.

NOTE: Bottles are all vented.