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Low fuel pressure

Gagarin737

Gagarin737

Well Known Member
Before I open the fuel system once more I hope someone will come with a solution/things to check.
A few flights ago I had one drop of fuel pressure during cruise. Pressure dropped from 4,5 to 0,7, lasting 10 seconds. Pressure recovered in a few minutes. During the climb out after a low pass 10 minutes later the pressure dropped to 0,2psi. Rest of the flight normal values. All flights before I had a constant pressure of 4,5 in all flight phases.

I changed the fuel pump (still had the old one, not leaking) and replaced the sensor for a "Kavlico" sensor.
Test run thereafter didn't show any problems, steady pressure around 4,5 with both the electrical and mechanical pump, 4 with only the mechanical (engine off pressure is around 2,5).

During the flight thereafter I had a pressure drop during climb out to 0,2 , lasting 20 sec. Pressure climbed back slowly up to 5 psi after 45 minutes flight. The flight thereafter on the same day gave me a minimum pressure of 2,5 during climb out, normal pressures for the remainder of the flight.

Last flights the same problem; first flight very low pressure, flights thereafter on the same day no problems.

I checked the gascolator, totally clean, and did an other flow test. Flushed all fuel lines after the gascolator (bit redish fuel from the pressure line).

Fuel flow values are constant during the pressure drops, engine is running fine. Pressure drops occurs after giving power. In case of pressure drops due to supply problems I expect to see fuel flow will drop as well but I don't see that.
I forgot to download the last flight, will post that next week.

-Next step will be a mechanical gauge to check the digital values.
-Checking the finger screen in the tank
-checking the fuel valve

Any more suggestions?
 
I would purge/blow the line to the sensor (not just drain it) before opening the tank. Any restriction, debris, or possible intermittent kink might cause these symptoms.

I use all mogas. I have a 15 gal hangar container with a handcrank pump on it. I fill that container with jerry-canned gas. Whenever I pour into the container I use a Mr. Funnel to ward off debris. (Pretty awkward to lift a 5 gal container up to the fueling port.)
 
I had 2 Kavlico fuel pressure senders fail so far.The first one showed zero pressure after being installed for 5 hours,the other one became very erratic with having fuel pressure low and high alarms after a few hours in service.
Seems they are not as reliable as presumed.....or just my bad luck?

cheers
Karl VH-XKH 11Hours total
 
I would bet it is a poor wire connection or a bad sending unit. These two causes are 96% of the erratic low pressure issues.
The only real way to double check is put a mechanical gauge inline with the other.
 
Joeri,

Did you experience any loss of power? A lot will disagree with me, but I've flown plenty of airplanes that had no fuel pressure gage. If you lose pressure you react to the loss of power. The number of false alarms reported in this forum seem to indicate the fuel pressure reading is not a reliable troubleshooting indication. Having said that I should point out that I put a switch on my electric pump and only turn it on for start, takeoff and landing so I have a 'plan B' if the engine sputters.

Rich
 
Fuel Pump switch????

Rich, I'm thinking of putting a switch on the electric pump at the CI next week, but if you cit the red wire in the tunnel as described elsewhere, does that also shut off the avionics cooling fans? Don't want to do that around here in the summer!!

Wayne 120241/143WM
 
No, power flows back to the fuel pump through the red wire. The Avionics fans tap in at the control panel so cutting the red wire won't impact the fans. I'm happy with the mod powering the fuel pump through the unused Nav light switch.

Rich
 
Check/Rewire connections.

If you have tried two senders, it is probably a wiring issue. Check, or even re-do, the power, ground and signal connections on the sensor and monitor. Also look for any splices in any of the wires and check/re-do those. You can try wiggling the wires with the power and pump on a see if you can get it to fail.

If your pressure really drops to .2 PSI you will notice . . .
 
Thanks all.
The engine didn't sputter at any time.
Will try the mechanical gauge first and will connect the Kavlico direct to the EMS instead of the firewall forward connector.




Bill_H said:
I would purge/blow the line to the sensor (not just drain it)
Click to expand...

I did disconnect the sensor/fuel return line and used compressed air to blow the line.

RFSchaller said:
Joeri,

Did you experience any loss of power? A lot will disagree with me, but I've flown plenty of airplanes that had no fuel pressure gage. Having said that I should point out that I put a switch on my electric pump.

Rich
Click to expand...

No loss off power/no sputter, fuel flow indication was constant during all fuel pressure losses. I also have a switch on my electric pump.
The Katana flown for the transition training had also no fuel pressure indication (only a fuel pressure caution light).
 
A small piece of the graph from the flight I was talking about. First 0 psi indication at 600ft. Fuel flow stable. No drop in power. Third time in a row that I had these values during a first flight of the day. Still have to order a mechanical gauge.

fbn7.png
 
Prime the line?

Also, did you try priming the line? Crack the fitting with the fuel pump on and let some fuel flow out, taking with it the remaining air. I still can't figure out why this matters, both liquids and gasses should come up to the same pressure in the line, but it does seem to make a difference, especially on oil senders.
T
 
I think you have a point. After installing the new pump and Kavlico I primed the sender line. Results from the test run can be seen below. Pressure is ok. The flight thereafter was like the earlier graph. I also think its air in the pressure sensor line. But the first four flights pressures were solid (old sensor). The first low pressure indication was after about 10 hours. The sensor fuel line is a bit downstream and with that its strange air gets in (if thats the case).

w5s2.png
 
Last edited:
Gagarin737 said:
A small piece of the graph from the flight I was talking about. First 0 psi indication at 600ft. Fuel flow stable. No drop in power. Third time in a row that I had these values during a first flight of the day. Still have to order a mechanical gauge.
Click to expand...

I have seen the same. I have the old sensor and not the Kavlico. The low pressure condition alarm not not occur during runup, but, usually just after take-off. The condition persists for about 1 minute and then the indicated fuel pressure climbs back to normal. This condition does not occur on every take-off. Maybe 10-15% of the time. Never any engine hesitation. Fuel flow remains normal. I have yet to determine a cause. I have vented the fuel pressure sensor line.

Others that I have talked to have observed a similar condition. The alarm is observed only by those where the low fuel pressure alarm is set to 2.2 psig. If the alarm is set to 0.7 psig (as was the case with Van's earlier locked settings file), this condition is masked and the pilot never sees it.

Will be very interested to hear your results with the mechanical gauge. I've always wanted to try, but, have never taken the time to install the mechanical gauge.
 
Update

Forgot to give an update.
I flushed all lines from tank to gascolator and installed the fuel lines from aircraftspeciality.
Pressure is fine since then. No explanation found for the low fuel press warnings.

Schermafbeelding%25202014-02-24%2520om%252020.19.22.png
 
Quite likely that you have had some restriction in the old lines and with the engine hot during the climb and shortly in the cruise, the extra heat load causes vapour bubbles. You are sucking from a tank and up to the engine, all uphill in the climb.

Mogas has a high RVP, and in winter an extra dose of butane and the RVP is twice that of avgas. As you climb the outside air is at a lower pressure, the tank is lower, the pump is sucking harder and with any loops or restrictions vapour bubbles will form.
 
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