Fuel Pump Issue

[9GT]

I also like to use these Del Fitting Seals when I final assemble aluminum fuel and brake lines. They are a bit pricey but help make a leak free sealing joint.
https://www.aircraftspruce.com/catal...37fittings.php

[lr172]

Originally Posted by Bill Boyd

Updates: flew to town and grabbed some 100LL - 2/3 ratio in each tank with balance 93 mogas. Left tank will now keep pressure with boost pump on, and loses prime on the engine pump much more slowly than before. Right side still runs fine, as before.

Definite improvement related to vapor pressure, I assume...

One thing to consider, is that fuel tank fill height will change that amount of suction required to pull fuel. The fuel pump draws a good bit less suction when pulling fuel from a full tank than a near empty tank. Something to consider when reviewing symptoms. I would expect a small air leak to show more significant symptoms on an empty tank compared to a full one.

Larry

[Bill Boyd]

Originally Posted by myv65

Have you noticed anything with regards to the rate of pressure loss versus fuel consumption? You'll always see increasing vacuum as measured at the pump suction with increasing flow. If you've got a moderate obstruction on one side, you should clearly see a difference in the vacuum between the two sides, but also should find that the severity your problem varies with fuel flow rate. Clearly not a solution to the problem, but measuring that vacuum and how it responds to different conditions should confirm whether this is a restriction issue or not. If on the other hand the vacuum is lower (closer to ambient pressure) on the problematic side, you have a leak and you're drawing air.

When it comes to measuring vacuum, you also may want to consider using a differential gauge/sensor (other side open to atmosphere) rather than a single port. With single port you'll need to consider your current density altitude when comparing values.

Best luck getting to the bottom of this.

I have not done reduced flow rate pressure testing - purposefully avoiding prolonged ground runs with CHT 300* upper limit and running >=75% power all the time for engine break-in, except in the pattern which is not the best place for inducing an expected loss of fuel pressure. As things stand I will probably have the left side plumbing swapped out before the break-in is completed.

I don't anticipate that ground-based fuel flow testing using only the boost pump would tell me what I need to know, but it might...

I am hatching a scheme in my head today at work - to use my garden sprayer brake bleeder rig to pressurize the fuel lines and leak test with soap solution before I dismantle the plumbing and send it to Tom.

[1001001]

Originally Posted by lr172

One thing to consider, is that fuel tank fill height will change that amount of suction required to pull fuel. The fuel pump draws a good bit less suction when pulling fuel from a full tank than a near empty tank. Something to consider when reviewing symptoms. I would expect a small air leak to show more significant symptoms on an empty tank compared to a full one.

Larry

YES. The suction head available is related to the head of liquid in the tank. The problem should be worse when the tank level is low and it might appear to be non-existent with a full tank.

A combination of a filter introducing an excessive dynamic head loss (pressure drop when flowing) and a low tank level will result in too low a suction head at the pump.

This will be particularly evident for a positive displacement pump, like the engine driven fuel pump. If the suction head is too low, a volatile fluid such as gasoline will tend to vaporize on the pump's intake stroke.

A pump with an internal recirculation may not show this, as some fluid on the discharge side will return to the suction side and increase the average suction pressure. Think of a jet style well pump here. I'm not certain of the configurations of the various electric fuel pumps, but it's likely they have some sort of internal recirculation setup to allow flow past the pump if it fails/is turned off.

[Electrogunner]

Originally Posted by 9GT

I also like to use these Del Fitting Seals when I final assemble aluminum fuel and brake lines. They are a bit pricey but help make a leak free sealing joint.
https://www.aircraftspruce.com/catal...37fittings.php

I used those to fix a pesky fitting leak. Worked great so far 2yrs leak free.