Boost pump ?Airlock?

AviatorJ · #1 05-09-2019, 12:53 AM

When I first did my fuel testing I struggled to get fuel to actually move through my pump. I ended up putting a vacuum on the line right before the mechanical pump which I assume broke an airlock or primed it.

Fast forward a few months and this week I did my 50ish hour oil change. With that checked my fuel filter. All was well and the plane started right up.

Today my son and I put the cowling on and I took him for a few laps around the pattern. Once I landed I noticed a small leak on the AN fitting to the boost pump. This actually may have been there a bit because it was so small that by the time I would remove the inspection panel to the tunnel I couldn?t find it. I actually invented a game called ?fuel or leather?? I would ask hangar neighbors what they thought they smelled. Anyhow left the panel off for this flight and put a piece of white paper under the assembly to locate it.

I replaced the fitting, connected everything back up and tried to start it.... no joy. Worked it for a bit, thought maybe I somehow introduced a containiment so went through some measures before pulling the top cowl and fixing it once again with the ole shop vac trick.

So my question is there a trick to either prevent this? If it occurs a method to correct it that doesn?t involve a shop vac?

Talking to a friend who owns a lot of large diesel equipment said his method if they swap a pump is to run it up 10 cycles before engaging the ignition (he defined a cycle as running it until the noise pitch changes then turning it off). Said failing to do that will cause it to airlock and you either have to let it sit overnight or manually dump fuel in the pump.

dlloyd3 · #2 05-09-2019, 07:34 AM

Some electric fuel pumps are not self priming. Airflow Performance warns of this in their instructions. Mechanical engine driven pumps do a pretty good job of self priming, hence you friends recommendation of a number of turns of the engine. Didn't think of that, used a little pressure on the tank vent to force fuel to the pump.

BillL · #3 05-09-2019, 07:47 AM

Quote:

Originally Posted by AviatorJ

When I first did my fuel testing I struggled to get fuel to actually move through my pump. I ended up putting a vacuum on the line right before the mechanical pump which I assume broke an airlock or primed it.

Fast forward a few months and this week I did my 50ish hour oil change. With that checked my fuel filter. All was well and the plane started right up.

Today my son and I put the cowling on and I took him for a few laps around the pattern. Once I landed I noticed a small leak on the AN fitting to the boost pump. This actually may have been there a bit because it was so small that by the time I would remove the inspection panel to the tunnel I couldn’t find it. I actually invented a game called ‘fuel or leather?’ I would ask hangar neighbors what they thought they smelled. Anyhow left the panel off for this flight and put a piece of white paper under the assembly to locate it.

I replaced the fitting, connected everything back up and tried to start it.... no joy. Worked it for a bit, thought maybe I somehow introduced a containiment so went through some measures before pulling the top cowl and fixing it once again with the ole shop vac trick.

So my question is there a trick to either prevent this? If it occurs a method to correct it that doesn’t involve a shop vac?

Talking to a friend who owns a lot of large diesel equipment said his method if they swap a pump is to run it up 10 cycles before engaging the ignition (he defined a cycle as running it until the noise pitch changes then turning it off). Said failing to do that will cause it to airlock and you either have to let it sit overnight or manually dump fuel in the pump.

Diesel fuel systems are a way different animal, so don't try to relate that experience to your 10.

Did you do an air pressure test (30psi) of the fuel system between the wing root and the mechanical pump (or servo) ? You might have an air leak on the suction side that is contributing to the problem. I have tested only two (7 & 10) but there was no hesitation of the boost pump to pull in fuel and prime. My friend had his 10 pump out-and-in and I did my 7 with all drained, we had no issues. We both have stock (Vans) Airflow Performance pumps.

AviatorJ · #4 05-09-2019, 09:11 AM

I didn't do any air pressure test. All I did prior to connecting everything during build was blow out the lines and made sure the fuel selector and everything was accurate.

Next time this happens I'll try to push some air through the vent to see if that works. I guess that's similar to what I did with the vacuum only easier to get to.

bsvantho · #5 05-09-2019, 09:24 AM

I agree with the previous comments about possible air leak. I have stock AFP pump and stock fuel valve. I have had the system apart for fuel flow tests and servicing filters and have not had any trouble with priming using the pump after draining. I put fuel in the tanks for the first time only a few weeks ago. I added 5 gals. and the pump drew fuel and pressurized a with no trouble.

RV Jerry · #6 05-09-2019, 10:16 AM

Just curious how you hooked up shop vac to draw fuel through pump

skylor · #7 05-09-2019, 03:51 PM

Quote:

Originally Posted by AviatorJ

...SNIP

I ended up putting a vacuum on the line right before the mechanical pump which I assume broke an airlock or primed it.

SNIP...

Using a shop vac to draw a vacuum on the end of a gasoline line is a REALLY DANGEROUS practice! Shop vacs generally do not have explosion proof motors and you're potentially exposing the motor directly to any fuel vapors being drawn from the tubing by the shop vac.

Skylor

DennisRhodes · #8 05-09-2019, 06:39 PM

Quote:

Originally Posted by skylor

Using a shop vac to draw a vacuum on the end of a gasoline line is a REALLY DANGEROUS practice! Shop vacs generally do not have explosion proof motors and you're potentially exposing the motor directly to any fuel vapors being drawn from the tubing by the shop vac.

Skylor

What Skylor said!!! All those fuel vapors go thru the brushes in that motor!!! Also that same DANGER exist when one uses a shop vac for cleaning out a fuel tank( thats had fuel in it) Please don't do either of these with a shop vac. There is a vac device that uses compressed air as the motive and that would be acceptable. But please no Craftman shop vacs.

rv7charlie · #9 05-09-2019, 06:45 PM

Listen to Skylor.

To prime the pump, did you try cracking a fitting downstream of the pump? With no flow to the engine, it's probably just pumping the air back to the inlet.
Edit: that's not the same as failure to prime. Gerotor pumps will pump air.. for a short time, anyway.

rjcthree · #10 05-09-2019, 07:27 PM

I had a a similar experience, and while it was a PITA to find, it was a leak at the the flare to the electric pump, no fuel would leak, but that persistent smell, very light. I used a hand vac pump (brake bleeder) to confirm a leak somewhere, and reset each flare, and retested. It only took four, but you know how the joy of boost pump access.....

The fitting was tight. Todd did the hoses, it just didn?t seat with that first torque.