Fuel Pressure Issue Related To Altitude

Hi Brian

First, I would run both pumps all the time.  The only time you use one is on the fuel pump check while on the  ground.  Remember that these pumps run in series, one inline with the other.  When one is turned off this design of pump allows no flow, it must bypass via a check valve beside the pump as shown in the drawings in the installation manuals.  In a fuel injected engine you will be changing both pumps at the end of their useful life.  The running with only one pump should it fail the engine stops (rather abruptly I might point out, and you have to carry out a restart on the engine inflight.  

Just my opinion

Cheers

That makes sense. I was going to reply that the main reason I turn it off is because the POH says to do that but to my surprise when I went looking it does not say that. I must have taken that part in my checklist from my old carbureted plane I did training in. I guess my thought process had been that by only running one fuel pump you are kind of building a staggered failure of the pumps vs if you have them both always running at the same time theoretically they would have more chance to fail at the same time. Technically I can still get some staggered usage by the fact that on the ground fuel pump 1 is the main one that is on so it will get more use there. What you say makes sense and think I might change to keeping them both on during all phases of flight just want to make sure I'm not ignoring something I should be paying attention to. Originally I was thinking maybe it could be something more to do with the pressure regulator at the end of the fuel rail but the check valve taking a little time is a good point as well.

Brian,

Are you using automotive fuel?  If you still have a winter blend gas in your region this can happen as ambient temperatures warm up.  At higher altitudes fuel vaporizes easier (lower ambient pressure) and the fuel on the suction side of the fuel pumps used on the injected engines can vaporize and create a bit of cavitation in the pump. Running two pumps may mask the condition, and then it is revealed when one pump is turned off.  I suspect if you switch to Avgas as a test the problem goes away. 

That was actually one of the next tests I was thinking of possibly doing as I do run mogas. 

Brian,

Assuming your plane has two fuel tanks….

Here is an interesting test. Put Mogas in one tank and Avgas in the other.  Leave the plane in the afternoon sun for an hour or so to get the fuel warm. Then with the engine off, run a fuel pump and switch between the two tanks allowing 15 seconds or so for the fuel to fill the lines. Often the Avgas tank will provide quieter pump operation than the Mogas (winter fuel) tank.  This is because of the cavitation with Mogas as bubbles are formed in the suction line.