Fuel Pump issue

[BrianL99]

I was flying yesterday and my Fuel Pressure dropped to 0.

I turned on the electric fuel pump and the pressure immediately came back. I assumed it was an anomaly with the instruments, so I shut the electric pump off for a few seconds and the fuel pressure then went back to zero. Flew another 5 minutes (heading towards an airport), then tried shutting off the electric pump again ... all was back to normal with the mechanical fuel pump.

The engine ran fine for the entire 5-6 minutes I was playing with the 2.

Any thoughts or ideas?

[FastEddieB]

Remind me of the aircraft and engine type.

[roger lee]

The most common cause of this is usually a loose wire, bad ground or bad pressure sender. Since the pressure comes back with the electric pump I would highly suspect the pressure sender and because your engine didn't sputter on you with no fuel pressure. Normally I go after wires and especially grounds first, but your electric fuel pump leads me to believe it is the pump because the pump would affect a loose sender connection. Senders are notorious for going out especially if it is mounted on the engine where vibration gets to them.

[dstclair]

What Roger said

Brian -- I'm assuming there have been no changes to your Sting since delivered from the factory. If so, the fuel pressure sender is mounted on the engine and the leads have been bound very tightly such that there is very little play. The vibration from the engine can eventually break the wire underneath the insulation. I had this several years ago with symptoms lasting several months (pressure to zero and non-stable pressure) until the wire broke completely. Strip the insulation back around 1/4" to check as there is no way to see the break without doing so.

This doesn't explain why the pressure normalized with the backup pump although this may have been coincidence.

[BrianL99]

It's a Rotax 912S Eddie.

The reason I discounted the sending unit, was because the pressure came right back up when I turned on the electric pump ... 3 times.

I would have thought if the sending unit had an issue, it would be reflected in the reading with the electric pump on, also?

Given the fact that the engine didn't "sputter", would suggest that the mechanical pump was operating ok ... I think?

As for being "original"? The fuel pump has been changed out at least twice. Apparently Sting (Rotax) had a issue with fuel pumps and wanted everyone to replace there's with a different one. The original owner of my Sting replace it and didn't like the new one for some reason and put the old one back on. When I bought it, SportAir put the "new" one back on.

I'll check wires tomorrow, to see if I can see anything amiss.

Thanks all!

[3Dreaming]

I'm not sure what brand sending unit you have, but I have seen VDO units get bad spots in the middle of their range. I have tested oil pressure units on the bench to trouble shoot this before, and at certain pressure going in the reading goes to zero. Add just a little more pressure and it goes right back to reading where it should. Testing the lower pressure fuel unit will be a little more difficult without a low pressure regulator.

[FastEddieB]

It's a Rotax 912S Eddie.

But what kind of plane?

Given the fact that the engine didn't "sputter", would suggest that the mechanical pump was operating ok ... I think?

I asked again because in a high wing, a working fuel pump may not even be necessary.

Most motorcycles with these same carbs just use gravity feed, as do myriad Cessna carbureted high wings - it doesn't take much fuel flow at all to keep the float bowls full and the engine running.

[BrianL99]

It's a Rotax 912S Eddie.

But what kind of plane?

Given the fact that the engine didn't "sputter", would suggest that the mechanical pump was operating ok ... I think?

I asked again because in a high wing, a working fuel pump may not even be necessary.

Most motorcycles with these same carbs just use gravity feed, as do myriad Cessna carbureted high wings - it doesn't take much fuel flow at all to keep the float bowls full and the engine running.

A low wing Sting.

[FastEddieB]

A low wing Sting.

Thanks. I may have known that at one point and forgotten.

In that case you'd better have at least one pump working - or get to play glider!

Check the sender first. As a data point, my fuel pressure gauge sometime drops very low - almost to zero - when switching off my boost pump, before climbing back up into the green. Has always done that with never a sputter, so I tend not to sweat it - but I'm still in no hurry to turn the pump off, preferring to have some altitude first. And my first sender, mounted to an engine baffle, went bad. The relocated replacement seems to be longer lived.

[dstclair]

BTW -- the Rotax SB has a note near the end of the document allowing for low pressure:

Slight, short-term pressure undershoots are allowed. But the pressure must stabilize to the operating
limit within 5 seconds. If not, the cause should be determined and rectified.

Mine does this about 1/3 of the time on longer climbs. I used to never run my aux pump but now I've changed my procedure to turn on for take-offs/landings/climbs. Basically, what I used to do with my Continental and Lycoming powered low-wings. Don't really like to do this but an occasional 'yellow' and/or fuel pressure alert tends to make passengers nervous

I'm guessing high-wing folks rarely see this due to gravity feed + the fuel pump.

[BrianL99]

So I flew today.

I had zero fuel pressure all day and turning on the Electric Pump didn't make a difference.

I'm guessing it must definitely be the sending unit?

[dstclair]

99.9% chance that it is the VDO sender or the connections themselves. They cost around $25 so might as well swap it out.

Do you have Dynon displays? If so, I'd suggest going with their solid-state sender and relocating to the firewall at your next maintenance event. These are much more durable and relocating to the FW will protect it from vibration.

[MrMorden]

I'm guessing high-wing folks rarely see this due to gravity feed + the fuel pump.

My understanding in the CT is that the gravity feed is insufficient for full power if the pump totally fails. I think there's enough fuel to provide for about 4000-4500rpm, which is plenty to maintain altitude. But the additional gravity feed should help in intermittent low pressure situations as are being described here.

Other types of aircraft might have better flow under pure gravity.

[BrianL99]

99.9% chance that it is the VDO sender or the connections themselves. They cost around $25 so might as well swap it out.

Do you have Dynon displays? If so, I'd suggest going with their solid-state sender and relocating to the firewall at your next maintenance event. These are much more durable and relocating to the FW will protect it from vibration.

I have the TruTrak. I'll take a look and see what sending unit I have. For $25, might as well change it out.

Thanks

[FastEddieB]

As an interesting footnote:

I replaced my wonky fuel pressure sender with the same part # VDO sender.

When it arrived, it had a red tag attached saying: "Not for use with fuel".

Thought that odd, but its the same part that came on my plane in the first place.

Go figure!