Low fuel pressure... again

[Rick]

I would be suspecting the fuel pressure sensor. I have seen sensor issues in the past with these units. It is located behind the alternator under #3 cylinder. These sensors are the ones recommended by Garmin for the G1000 installation. It sounds like the sensor could be giving fluctuating readings. If it were a wiring issue the fuel pressure indication would X out on the Garmin, So I would not suspect that.

I would also check your fuel filters. There are two screens, the main being the gascolator on the belly, the other is a small finger screen inside the fuel feed line to the servo on the LH side. Remove the feed line from the servo and the screen is right there. If those are both clear then I would try a new pressure sensor. They do use the same manufacture of sensor as the oil pressure by they are different so make sure you get the right one.

Welcome to the forum, Dave! And thank you for the very informative first post - it is exactly that kind of information that makes this forum such a valuable resource for all of us Diamond owners.

[Brichards]

We switched the oil and fuel pressure switches per Diamond's request but there is no change in the indications. At this point, I do not think there is a problem with the sending unit.

[Brichards]

The nearest Diamond service center is 45 miles from where my hangar is so imagine my surprise when while incidentally at the service center's airport, I had the electric fuel pump fail. Well I know the sending units are not causing the low pressure indication so now we will find out if the electric pump is the culprit.

[Brichards]

After replacing the aux fuel pump, I took a test flight, #1307 in the reports section), and sent it to Mr Lemaire at Diamond. I received the following response and decided to post it as I think it may be good information that all DA40 owners should be aware of.


The second flight looks really good and so far I see nothing abnormal. I discussed this with Glenn Lawler as well and the one thing mentioned is that you may want to try leaning above 3000 ft when climbing, to bring the cyl head temps up to around 400 degrees. This will ensure it gets hot enough to burn off any water in the oil system.[/color]

The pump should be used only to clear the low pressure fluctuation then turn it off. If you want, please continue to send data logs anytime for review.

What you may see at times with the DA40 is a fluctuating or low fuel pressure warning sometimes with associated erratic or higher fuel flow indication. There is no abnormal engine operation associated with these indications. Turning on the electric fuel pump extinguishes the indication and returns the flow/pressure to normal.

The IO 360 with the Precision Fuel Injection System does not have a vapor separation and return line to the tank(s). Vapor in the fuel system will travel through the fuel system including flow and pressure transducers, into the injection system. Vapor can form under higher ambient temperatures and low ambient pressures. Fuel gravity fed through the system (electric pump off) from the tanks is “lifted” by the mechanical pump. The suction of the mechanical pump can cause some cavitations (vapor formation), especially as the fuel flows through resistances such as elbow fittings. Vapor bubbles will cause momentary low pressure indications and may also cause momentary erratic or high flow readings as the flow transducer turbine spools up with the loss of fluid resistance. These momentary occurrences can be suppressed with the electric pump. The electric pump activation increases system pressure to compress or return to liquid, the formed fuel vapor bubbles.

The AFM normal procedures for climb, cruise and descent, include a Caution that vapor bubbles could form with the electric pump off. The steps above each of these cautions indicate the electric pump should be switched on, but the intent is not to leave the pump on. The electric pump is an auxiliary and emergency pump, not an emergency pump only. Switching the pump on in relation to these Cautions does not constitute an emergency response but rather an acceptable method of suppressing fuel system vapor formation.

The G1000 engine indication and annunciation systems are highly sensitive, where as a slower conventional panel may mask the vapor formation. From Diamond’s point of view there is no safety of flight issue with these momentary fuel pressure and flow fluctuations. The use of the electric pump to suppress vapor is considered normal operating procedures. Once the fuel system stabilizes, the electric pump should be switched off.

The intention of the steps for electrical pump selection, above each of the Cautions in the AFM is not for continuous operation of the pump, as this could potentially mask a defective mechanical pump and eliminate the redundancy of the fuel delivery system. If the abnormal fuel pressure or flow indications do not stabilize with the electric pump on, emergency procedures and maintenance action may be required.

I am still not sure why this problem did not exist until after 18 months of ownership, but I will pay more attention from this point forward and hopefully, the new aux pump will survive longer than 500 tach hours.

[Lance Murray]

Diamond sent me the same explanation as above.

[Brichards]

If this is indeed a vapor issue, I wonder if insulating the fuel lines forward of the firewall would help?

[Lance Murray]

I asked the same question. Diamond will not support that but your mechanic can do the install under 337. Many of the lines already have fire-sleeve. Some experimental aircraft install a cooling shroud around the fuel pump. I have thought about doing this.

http://www.vansaircraft.com/cgi-bin/cat ... uct=shroud

[Harry Range]

After my last flight on Jan 28. with the mutiple alarms my bird went in for its annual and the warnings for low fuel pressure was on the "check it out" list. My mechanic could not find anything wrong and decided to change the sensor unit as a precautionary and that our diagnosis was leading us in this direction. $950 for the sensor ... ouch ! Flew yesterday with no warnings nor any pressure readings below about 23 psi on the data card. So far so good. But then I did have 600 hours prior to that with no issue either. We will continue to monitor it.

[Brichards]

950 for the sensor ... ouch !

When we talked to Diamond, we were advised that since the fuel pressure and oil pressure switches were identical, switching them around was the recommended procedure to check for pressure switch malfunctions.

[Lance Murray]

I am now having fuel pressure indication problems. More than once my indicated fuel pressure has dropped to zero. It will sit there for a few seconds to minutes and then return to normal. The timing that it happens is not consistent and turning on the fuel pump makes no difference.

I am thinking that this is a failing fuel pressure transducer.

Does anyone have an alternate part number for this part so I don't have to wait for one from Diamond or pay the factory price?