Vapor Lock Problem

[jjet]

I have a RV6A with an O360 with an IO conversion. When it is hot out and after landing, my fuel pressure would drop to zero during roll out or while taxiing (engine faultering or quitting). When I turn on electric pump, pressure comes back up. All fuel lines are run clear of exhausts as much as possible. Put a shroud and blast tube on fuel pump. Replaced engine driven pump, problem persists. Up until this week it was just an annoyance, but it happened at Oshkosh in flight which is a bit more disconcerting. Anyone have any suggestions??

Thanks
Jeff

[airguy]

Solution is to run the boost pump during low-fuel-flow operations (low power setting) when the FWF area is heat soaked. You can route an air blast tube to a cooling shroud on the fuel pump that will help, but the ultimate "cure" for this problem is something that requires extensive modification of the fuel system which is something most folks shy away from.

[Fred.Stucklen]

Another suggestion: Be sure that you DO NOT have a 90* fitting on the input side of the engine driven fuel pump. A 90* fitting will cause the fuel to cavitate and generate bubbles when the fuel is hot....

Originally Posted by jjet

I have a RV6A with an O360 with an IO conversion. When it is hot out and after landing, my fuel pressure would drop to zero during roll out or while taxiing (engine faultering or quitting). When I turn on electric pump, pressure comes back up. All fuel lines are run clear of exhausts as much as possible. Put a shroud and blast tube on fuel pump. Replaced engine driven pump, problem persists. Up until this week it was just an annoyance, but it happened at Oshkosh in flight which is a bit more disconcerting. Anyone have any suggestions??

Thanks
Jeff

[SHIPCHIEF]

Remember that the mechanical fuel pump is being directly heated by the engine it is bolted onto, and the hot oil in the engine splashes into the top of the fuel pump casting to lubricate the arm. If you experience high oil temperature, your fuel will absorb some of that as well.
I run 2 electric fuel pumps and no mechanical fuel pump. I regulate fuel pressure with a 'turn-down regulator" which returns all fuel flow above the set pressure to the fuel tank.
I don't suffer rich hot starts or need a purge valve, but I do need to keep a close eye on bus voltage.
On the other hand, I also have a Thorp T-18 with a Marvel carb, and an electric fuel pump that feeds the fuel into the standard mechanical fuel pump, ala VAN's style. I have had heat problems with this when I flew with winter cut 87 octane mogas in the summer without removing the oil cooler air block off plate. Oil temp got up to about 210-220 F, high but not alarming, but after about a half hour the engine didn't run so well, no matter what mixture or throttle setting. After I drained out half of the mogas and topped of with 100 LL it was fine, the next day I put the facts together and removed the cooler plate.
The combination of high oil temp and high air temp behind the engine was vapor locking my fuel system even with the electric fuel pump on.