Boost Pump, Fuel Pressure issue

[Sue Bon]

3 hours ago, PT20J said:

So, how did you know the pump was bad? Did it stop making a sound, or did you just not see fuel pressure?

It made a very tiny effort (I could barely hear it) before going quiet. The fuel pressure gauge didn't even twitch. After running the engine for 10-15 minutes, I taxied back and shut down. I tried the boost pump again and heard a tiny little whirl, then nothing. When I tried again a few minutes later, complete silence. The circuit breaker remained in the whole time. It was the sound of a motor dying IMHO.

[N201MKTurbo]

I didn’t take them apart, but I blew through one and there is a check valve of some sort in the pressure regulator. I could hear and feel it opening. 

 

If anybody needs a core, let me know…

 

[PT20J]

4 minutes ago, Sue Bon said:

It made a very tiny effort (I could barely hear it) before going soundless. The fuel pressure gauge didn't even twitch. After running the engine for 10-15 minutes, I taxied back and shut down. I tried the boost pump again and heard a tiny little whirl, then nothing. When I tried again a few minutes later, complete silence. The circuit breaker remained in the whole time. It was the sound of a motor dying IMHO.

Agree, that sounds like a bad motor, so I would not expect it to have anything to do with fuel pressure when the engine is running.

If it were mine, I would send the pump for overhaul and while I was waiting I would verify the gauge accuracy which could be done with a compression tester and some plumbing.

[Sue Bon]

2 minutes ago, PT20J said:

so I would not expect it to have anything to do with fuel pressure when the engine is running.

This is the part that confuses me. It seems like too much of a coincidence that the boost pump fails and the pressure is in the red at the same time. My shop opens again on Tuesday. I'll have them take a look and let you all know what they find. Maybe it is two separate issues.

[ArtVandelay]

Could be a poor electrical connection as well, less likely, but would explain lack of enthusiasm of the pump.

[hammdo]

Take clear plastic tube, a vacuum cap, and put it on the vent on the pump, run it and let it sit —

see if the tube has gas in it after sitting…

-Don

[PT20J]

42 minutes ago, hammdo said:

Take clear plastic tube, a vacuum cap, and put it on the vent on the pump, run it and let it sit —

see if the tube has gas in it after sitting…

-Don

That will let you determine if the internal seal is leaking a small amount, but if the motor is shot, it doesn't really matter 

[hammdo]

Right -just if it leaking…

[Shadrach]

3 hours ago, ArtVandelay said:

Could be a poor electrical connection as well, less likely, but would explain lack of enthusiasm of the pump.

My dukes pump was unenthusiastic for most of its life. It sounded so inconsistent it sometimes generated strange expressions from passengers during prime.

[A64Pilot]

A vane pump allows flow even if the motor is seized, the Weldon I believe had a bypass valve

I know because on the S2R-H80 I had two Weldon pumps in series, only one pump was run, the second was just a back-up in the event the first failed.  There was no failure mode that wouldn’t allow fuel to flow through the pump.

A diaphragm pump can provide constant pressure through a wide range of flow, until the pressure relief valve closes. At that point as liquids are incompressible then pressure will fall off of a cliff.

Often for a simple GM type of diaphragm fuel pump pressure is regulated by a spring on the diaphragm, when set pressure is reached instead of the diaphragm moving the spring is compressed so the stroke of the pump is variable to maintain a constant pressure.

[PT20J]

 

[A64Pilot]

Well I stand corrected then, I thought what the video called the lost motion linkage would maintain set pressure.

So does everyone else but me see significant pressure decrease in cruise compared to taxi RPM?

If so how much?

95% or better of my time is turbines and they have either vane or gearotor pumps not diaphragms, the one other aircraft I’ve had that was fuel injected didn’t have fuel pressure gauge, it had a fuel flow gauge that was really a pressure gauge calibrated in GPH.

[Shadrach]

1 hour ago, A64Pilot said:

Well I stand corrected then, I thought what the video called the lost motion linkage would maintain set pressure.

So does everyone else but me see significant pressure decrease in cruise compared to taxi RPM?

If so how much?

95% or better of my time is turbines and they have either vane or gearotor pumps not diaphragms, the one other aircraft I’ve had that was fuel injected didn’t have fuel pressure gauge, it had a fuel flow gauge that was really a pressure gauge calibrated in GPH.

I don’t recall any fall off in pressure. IIRC, it’s reliably constant from taxi to cruise. 

[A64Pilot]

19 minutes ago, Shadrach said:

I don’t recall any fall off in pressure. IIRC, it’s reliably constant from taxi to cruise. 

Thanks, but if you watch the video it makes the point that it’s a variable volume but also variable pressure pump so many times that it seems apparent that many like me thought it was a variable volume but relatively constant pressure pump, but it’s not. Pretty hard to argue with that video.

Diaphragm pumps that are constant volume and constant pressure pumps do exist, they work like vane and gearotor pumps do, that is they simply waste the excess volume through a pressure relief valve.

So either we operate in a range that likely doesn’t even come close to max flow of the pump so that pressure remains relatively constant or really I don’t have another explanation, just be guessing.

[Shadrach]

13 hours ago, A64Pilot said:

Thanks, but if you watch the video it makes the point that it’s a variable volume but also variable pressure pump so many times that it seems apparent that many like me thought it was a variable volume but relatively constant pressure pump, but it’s not. Pretty hard to argue with that video.

Diaphragm pumps that are constant volume and constant pressure pumps do exist, they work like vane and gearotor pumps do, that is they simply waste the excess volume through a pressure relief valve.

So either we operate in a range that likely doesn’t even come close to max flow of the pump so that pressure remains relatively constant or really I don’t have another explanation, just be guessing.

I believe we operate in a range that is considerably lower than the max flow of the pump. Do recall that the narrator said that a single high-pressure pump could feed both engines of an Aztec at max power.