Low fuel pressure alarm

[Al Downs]

Went to fly today and on climb out I had a low fuel pressure alarm. Went around and landed. Alarm still showing on taxi. Saw pressure of 1.2 to 1.7. Any thoughts

[FastEddieB]

I think its probably the sending unit.

 

Mine failed gradually and I relocated the new one to a position where there would be less vibration (on my Sky Arrow).

 

Interestingly, the new VDO sender with the same part number as the original had a red tag saying "Not for use with fuel".

 

BTW, it takes very little fuel pressure to fill a Bing float bowl and keep a Rotax flying.

 

If you don't have one you can pick up an inexpensive pressure gauge at any auto parts store and see what the pressure really is - photo to follow...

 

Here you go:

 

 

IMG_1682 by fasteddieb, on Flickr

[chanik]

Try scrubbing and crimping tighter the spade lugs that connect to the sender. Mine were loose and giving these alarms as well since high resistance shows as low pressure.

[FlyingMonkey]

If the fuel pressure *were* to actually be low, how could you manage that before landing? Maybe lower the RPM to prevent fuel starvation at the higher RPM fuel flow rate?

[FlyingMonkey]

You won't have to alter your landing because the engine will run just fine on that pressure. It will run even if the pump were to totally fail. It will run even if the pump is not on the engine.

 

That's great to hear...I'm assuming it will not run all the way up to 5500+ rpm, so where will it start to stumble with a failed pump and just gravity feed?

[FlyingMonkey]

If you pull the hoses off the fuel pump and just join them together on the CT it will run full throttle without a stumble. If you leave a totally dead fuel pump in line it will run over 5000+ rpm without a stumble, but at max rpm it will stumble. Tested in actual run test.

It may not be the same in every type aircraft, but it is for the CT.

 

That's awesome! So, um...why do they use a pump at all? To ensure good pressure at high AoA?

[Dan Kent]

Went to fly today and on climb out I had a low fuel pressure alarm. Went around and landed. Alarm still showing on taxi. Saw pressure of 1.2 to 1.7. Any thoughts

 

Al,

I was having flaky fuel pressure reading for nearly a year. They would be both low and high. I have an auxilliary fuel pump on my low wing so when they were low I would turn that on for a few minutes and then they would normalize.

 

I decided to get rid of the VDO sender unit as I was convinced it was at least part of the problem. Dynon sell a Kavlico sender (they have one for oil pressure also) and I had that installed on the firewall and my fuel pressures have been very steady (after a new fuel pump though as its pressure was over 7 psi at 4000 rpm).

 

The only issue with the Kavlico is that it requires a 5 volt source so there are 3 wires to it. If you have a Skyview system in your plane you can get the 5 v from it.

 

[FastEddieB]

That's awesome! So, um...why do they use a pump at all?

 

Not sure.

 

In my plane the fuel tank is below the engine, so either the engine driven pump or the boost pump must be working to supply the carbs with fuel, as would be the case in any low wing plane.

 

High wing, not so much. The majority of recent BMW "airhead" motorcycles use simple gravity feed to nearly identical Bings putting out close to the same power, so I'd think gravity would do the trick.

 

[Jim]

They probably use the pump because it comes with the engine,

[FlyingMonkey]

They probably use the pump because it comes with the engine,

 

Ha, that's probably true! Though my high-AoA idea makes some sense too, some planes have problems with gravity feed at high AoA...I know the Luscombes do, with the factory tank behind and above the seats, though when converted to wing tanks the problem disappears.

[Al Downs]

Thanks for all the information. Cleaned up and tightened all the connections and ran the engine. No alarms now. Seems to have taken care of it. Crummy weather right now but will take it out as soon as I can to verify it is fixed.