Fuel pump

Would be helpful to have some pressure readings: 

# Boost pump alone.

# Boost pump with engine pump.

# Engine pump alone.

# Why did you replace the boost pumps?

# Aircraft type

Speculation:

# Hot fuel 

# Too large a return line restrictor valve

# No return line

# Faulty boost pump

Any one or combination of the above.

Hi, yes, sorry here are more info.

The plane is a Zenair Ch601 XLB with a Rotax 912ULS. we have a facet pump that failed but till that point we had not any problem.

We replace it with the same model "facet fuel pump 574a" with a fuel pressure between 4 to 7 psi.
with this pump we got: boost pump alone: 5.2psi; boost pump with engine one: 5.2psi.
when engine cold, the boost pump works fine, but after landing with the engine off when we turn on the boostpump the pressure cant go over 1 or  2 psi. if i leave the plane stationary for half an hour or more, the boost pump gives pressure again.

We thought was a foulty pump and too high pressure so we replace it with the same model with lower pressure bought from Aircraft spruce.
this time the boost pump pressure performance (on the package) should be 2.5 to 4 psi.
whit this new pump we got: boost pump alone: 4 psi; boost pump with engine one: 5 psi and excaclty the same situation as the previous one when engine just shut off.

your speculation:

Hot fuel: the pump doesn't get hot and during flight everything works fine. moreover we didn't had any issue with the previous pump. (which was the same model)

Too large a return line restrictor valve: there is no return line, and again, no issue since now. it's a 2008 build.

 Faulty boost pump: 2 in a row from different suppliers? Possible but that would be very unlucky.

"Hot fuel: the pump doesn't get hot and during flight everything works fine. moreover we didn't had any issue with the previous pump. (which was the same model)"

It's not about the pump getting hot - its the fuel being heated by the engine. The fuel boils, creating vapour - often referred to as vapour lock.

This is a common problem with Rotax 9 carburettor engines and the main reason for recommending that the fuel system have a fuel return/bleed line back to the tank.

Fuel vaporisation is mainly/not exclusively, a problem for the engine once the aircraft has landed. This is when air flow through the cowling is much reduced/even nil, on shut down. Heat (radiant & convective) rises in the cowling . The temperature of all fuel hoses rises, especially those that are routed over the engine and the fuel within, may vapourise.

If there is vapour, in the boost pump, it can not pump fuel - nil pressure.

If there is vapour in the fuel lines, the boost pump may have difficulty - minimal/nil pressure.

Dino

My guess is the fuel you have now, buy today, is not the same quality as in 2008.  No return line is contrary to the installation manual.  I believe you have vapor locking.  The fuel gets hot, it is not a matter of how hot the pump is.  Because you can not clear the air at the engine side (you have no return line) the fuel must cool before it will work again.

Just my guess

Cheers

LOL Sean...guess we posted at the same time.  I agree with you. 

cheers