Low low fuel pressure and flow at high altitude

HI Walt

I do not believe that 2 gallons will give you enough.  To see if it is really RVP issues and vapor drain it so you don't have a mixed tank.  Just my opinion.

Cheers

OK Wizard, you made me put straight 100LL in this girl, I just lost her respect.😂

The OAT on the ground today was 102f, I put 6 gallons auto premium E10 in the tank, I flew to 5000', same as yesterday. I shut off the electric pump and after about 30 seconds the engine lost rpm and kept trying to stop but would catch itself, I use this test as my baseline. I landed and emptied the tank and lines then put 6 gallons 100LL, no mix in the tank. Back in the air I flew to 5000', turned off the electric pump and the pressure dropped to 1.4 but the engine didn't stumble, I continued climbing to 6500 before it started missing. Clearly 100% 100LL made a difference. Compared to the mogas I gained 1500' in DA before fuel vaporization.

I have my Dynon set to give audio warnings on low pressure and low flow so I am very ahead of a potential vapor issue. Reducing power is a quick resolution. Add to this when using summer blend the vapor only occurs if the electric pump is off. Vans recommends running the pump 100% of the time. I only performed these tests to verify the condition of the mechanical pump.

This engine has 1500 hrs of mogas E10, it runs great, excellent economy and power. I like having no lead inside the engine so I won't be changing fuel at least for my local flying. Cross country is different when I don't know the fuel source for mogas using some avgas will now be an option.

Hi Walt

So just to be sure everyone knows I think that Vans has done a great job addressing this issue.  For all to see I attach the Vans Service Letter on the issue.  

Perhaps it is time other OEM have a serious look at the problem as it will only get worse as fuel producers are not giving us a fuel we can depend on for our aviation currently, short of using expensive avgas.  

Cheers

Hi Walt

"I shut off the electric pump and after about 30 seconds the engine lost rpm and kept trying to stop but would catch itself, I use this test as my baseline. I landed and emptied the tank and lines then put 6 gallons 100LL, no mix in the tank. Back in the air I flew to 5000', turned off the electric pump and the pressure dropped to 1.4 but the engine didn't stumble, I continued climbing to 6500 before it started missing. Clearly 100% 100LL made a difference. Compared to the mogas I gained 1500' in DA before fuel vaporization."

There must be a more to  your problem than appears at first glance - In my previous 912 ULS aircraft, I regularly flew between 6,500-10,000 ft . I never experienced fuel vaporisation in flight. With two exceptions, in over 900 hrs, never used Avgas (& then as a shandy/blend unknown ratio).

My new 912 ULS aircraft (92 hrs) has rarely gone above 7,500ft (yet to do any long trips) and so far no vapour lock.

There is no doubt in my mind that you should be able to take a, ULP/MoGas fueled, 912 ULS to well over 10,000 ft without any problems ie your vaporisation problem (if thats actually what it is?????) has something to do with the way your fuel is being delivered to the engine.

Sean,

Either I did not explain my situation properly or ??

For the record flying my RV-12 with the electric pump ON (as I always do except for these tests), (only once was a problem) and 12500' is not foreign air to this plane.

Once in 1300 hours / 4 years I had a vaporlock issue and that was winter gas bought in Texas, on a flight into New Mexico on a very hot day. I regularly fly at 10500' over the Arizona Desert into CA mid-day as recently as last month departing A20 at 1350 with 108f on the ground, NO vapor issues, always using mogas.

This whole discussion started because for a test of the mechanical pump I turned off the electric pump at 7000' on a very hot day. After much discussion and testing, I think what I experienced, as noted in this thread, is typical for any RV-12. 

An airplane with wing tanks has the benefit of fuel with head pressure, the engine does not have to "suck" fuel, also a looser fitting cowl will not build as much heat on the fuel lines.

The RV-12 fuel system is designed very well with all considerations for a mogas setup. Fuel lines are run away from heat sources, fuel return line to the tank, boost pump at the tank, fuel is pushed not sucked to the engine, its all done right. The POH says to use electric pump all the time. What I was doing for my tests was against the POH. My results do ask the question "what if the electric pump quits in flight".