Fuel Drainback from Lift Pump

[BDCarrillo]

Solved: Fuel Drainback

Alright so I've gone crazy chasing a loss of prime issue... I've fixed all of the issues on the filter head (pipe plug in heater hole, replaced Schrader core) and return lines (tygon tubing, new Viton O-rings). The filter head and return lines/caps hold pressure and vacuum without any leakdown, no bubbles from soapy water, etc. The lift pump is a new Delphi unit, and also holds vacuum and pressure just fine, so it's not leaking. Lift pump hose from the hardline is new 3/8 Aeroquip hose. Tank pressure relief valves are clean and operate properly, tank pickups are new 3/8" Aeroquip tubing, tank O-rings and caps seal properly.

Now the real head-scratcher... the metal line from the FSV up to the lift pump goes bone dry within a few hours of sitting. No signs of leaks. Any insight would be appreciated.

 

[laserjock]

I've not had this specific problem but I do a lot of interesting plumbing for my day job so here are some things to think about. Do you have a section of rubber line between the metal line and the lift pump? I would look there if you do. Sometimes the leak will be such that air gets in but liquid won't leak out. Remember that side is suction so there is no positive pressure to force fuel out of tiny leak. What does teh aeroquip hose look like? Ive not heard of it. What type of clamps are you using on it? Did you just push the hose onto the tubing? If there is no bubble flare or anything at the end (like a hose barb). You could have problems.

Just a thought.

 

[BDCarrillo]

Aeroquip AQP hose is used for the hardline to the lift pump, and lift pump to head. All have barbed fittings, but do not use clamps. The AQP hose instructions say to NOT use a clamp, and I've used them in a turbocharged gasser pushing over 65 psi of fuel pressure without clamps.

I did pressurize from the lift pump up to the filter head, and tried to pull vacuum from the lift pump but it held.

I hadn't thought about the suction not 'pushing' liquid out but allowing air in. I'll try pressurizing from the fuel tank forward and pulling vacuum from the hose that feeds into the lift pump.

**Update: Pressurized from the rear tank to the lift pump. I checked every inch of the line from pickup to pump inlet, nothing found. Only a slight bubbling near the air vent on the tank, which likely needs a new grommet. Since that's at the tail end of the system I doubt it would cause the hard line to drain (air would have to travel downhill, through the line, and up to the pump).

 

[BDCarrillo]

Found the issue... original metal fuel filter bowl had the drain hole plugged with silicone. It was painted over and I hadn't ever seen a proper fuel bowl before.

At operating temperatures it would soften and fuel would sneak out. Today it "sneezed" on shutdown and blew the contents of the filter out. It was quite a shock to see! Miraculously it held up until I got home. Sealing it up properly now.

PO bubba fix: 1 Me: 0

Also put in a check valve between the pump and filter, for peace of mind.

 

[laserjock]

That will do it. Glad you figured it out.