First Engine Run Low Fuel Pressure

A pressure drop indicates that the pumps are not keeping up with the needs.

Before the engine run, you should have performed the fuel flow test.

Static fuel pressure into a blocked hose should be 70-110psi on each pump.

Return flow from the regulator will typically be 20gal/hr with one pump and 30gal/hr with both.

The orifice in the Bypass hose will typically return 3-5 gal/hr.

If you are bypassing excessive fuel the pumps will not be able to keep up.

What were your results?

- - -

Power ON the fuel pumps with the engine OFF. Note the fuel pressure. ~43psi???

Now turn OFF the pumps and monitor the fuel pressure dropping to Zero.

It will typically take about 4-6 second to fade completely away.

If it Zeros out in under 2 seconds, there is too much fuel being bypassed.

Hey Bill, 

Thanks for the reply. I have not yet done a fuel flow test as my wings are not installed I intend to test the whole system later but do not have room to install wings now. 

Another thing to note. On the kitfox a bypass is not typically installed and there are many (most) builds out there that run without. So mine also does not have a bypass. 

With engine off and one pump on I see 43 psi. Without the bypass, if I turn them off I see much higher pressure for quite a while. 

thanks for the help! 

Running a 912ULS without a Bypass is one thing.

If the fuel pump ingests a little Vapor and loses pressure, once the Carb floats drop, the pressure in the fuel lines will drop to Zero and the Fuel pump will pump the air and restore the prime to the fuel pump and you may never even notice anything happened.

Running a 912iS without a Bypass is another thing.

If the Fuel pump ingests a little Vapor and loses pressure, the fuel pressure regulator will stop bypassing fuel in an attempt to keep the pressure up.

The Fuel injectors will become fuel-starved due to the loss of pressure. the engine Stops!

While the fuel pumps have the capability of pumping air at 2-3 psi it will take a long time for that air to make it to the only exit remaining in the system, the fuel injectors.  And once the engine stops, that exit also goes away.

Your only option is to start cranking the engine continuously in an attempt to clear the vapor.  This may take a while.  All the time you are sweating bullets hoping the battery holds on long enough to get the engine going again.

If the fuel bypass line was installed there would be someplace for the low-pressure fuel and air to exit the system and get a prime back to the Fuel pumps.

Sure, the odds are low that this will ever happen.  But Murphy likes long Odds!!!

The bypass line may be an arguable option in a 912ULS equipped Aircraft, but it is NOT optional in the 912is fuel-injected version.

Hey Bill, 

I really appreciate the input and explanation and I just went through this back and forth with Rotax, and kitfox owners at Oshkosh. Most of the guys I was taking with were running the 915is and all did not have the bypass. I assume the 915 is the same from a by pass standpoint. I will re-consider adding a bypass. 

correct me if Im wrong but not having a bypass wouldn't be a reason for the lower pressure Im seeing? Any ideas about that issue? Id guess I would see higher if all else is working correctly. OR much lower pressure due to vapor as you describe above. 

Thanks,

As in many things, there is what you know you should do, and then there is what you can usually get away with.

If you have a high wing aircraft and you never run out of fuel and never have a vapor lock condition or any other reason that there is other than liquid fuel always present at the Input of the fuel pumps, then you will never have a need for the bypass hose.

The bypass hose is a lot like seat belts.

Most people go a lifetime without ever needing them until the moment comes that they do.

At that point, they need them badly, and NOW!!!

If you are of the opinion that seat belts should be a personal choice and do not request the passengers in your vehicle to buckle-up, then the lack of a bypass hose should not be of any concern either.

- - -

The lack of a bypass will not lower the fuel pressure.

Five Seconds after you saw 43psi, the entire fuel system was purged of Air.

I would pull the Fuel hose Off the fuel rail near Cyl#3 and put a plug in it.

Each pump should be able to produce more than 70psi of static pressure into the blocked hose.

The pressure you are seeing is a function of the Fuel pump's capacity exceeding the pump's internal pressure relief valve.

- - -

You did NOT talk to "ROTAX" at Oshkosh.

You talked to the Dealer's Representatives from California, Florida, and Wisconsin.

Rotax supplies the funding and the dealers staff the exhibit.

Notice that no one you talked to had an Austrian Accent.

You did not get an official Company opinion, just the personal opinions of some local folks.