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Fuel pressure display on Dynon

PostPosted: Tue Dec 08, 2015 11:15 pm
by Chief
I'm using the Dynon FlightDek 180 in my Highlander. So far, all is working exactly as expected, except for the fuel pressure display. Occassionally in flight I'll get the red alarm bar accross the bottom of the screen indicating "Low Fuel Pressure". I immediately advance two pages to the fuel screen.The fuel pressure indicator is fluctuating very frequently displaying a range of numbers, e.g. 5.2, 4.4, 3.9, 2.3, 4.8, etc., two or three a second.

If this is just a glich in the Dynon, perphaps because of the rapid soundings, I can live with that. I replaced the original fuel pump after the AD with a new pump supplied by Lockwood. It has about 45 hours on it. Never has the engine (Rotax 912 ULS) burped or reacted in any way. If not for the Red Alert flash, I would never have known about this. I have a back-up electric fuel pump but have bever felt it necessary to activate it.

Its the fluttering of that fuel pressure that bothers me. The minimum fuel pressure, according to the manual is 2.2 pounds. Usually, the Dynon is in the 4.5 to 5.5 pounds.

Any thoughs ?

Re: Fuel pressure display on Dynon

PostPosted: Wed Dec 09, 2015 7:06 am
by Jack L

I have the Dynon D10 EMS and it did the same thing since day one. I spoke to Billy about it and he suggested to disregard the POS. As you would expect Billy to say he stated that if you don't have sufficient fuel pressure you will no about it as fast as the Dynon will tell you about it.

As a precaution I checked my gas collator screen and found nothing so I went into the fuel sensor menu and widened the monitoring parameters of the sensor to eliminate the warnings. Now 120 hours later it still reads erratically and all is good. You could change the sensor but risk that the same thing can happen soon after.

I checked the Dynon updates to see if there were any changes in software to address this problem and could not find anything. I also have an electric pump for back up but I only use it on the ground to prime before start.

I like my instruments and seeing erratic fuel pressure bothered me for a while but I am good with it now. Perhaps someday I will change the sensor.


Re: Fuel pressure display on Dynon

PostPosted: Wed Dec 09, 2015 11:09 am
by User GDS
sounds like a sensor or an intermittent electrical connection, but I would recommend trying the auxiliary fuel pump to see if it changes the reading, that would help rule out a problem with the mechanical fuel pump.

Fuel pressure is critical, so I would get the problem fixed. My FP sensor is hooked up to a Dynon Skyview and is stable and stays within a range from 3-6psi. When I was flying an RV12 it had the D180 and the fuel pressure read-out worked well.

Re: Fuel pressure display on Dynon

PostPosted: Wed Dec 09, 2015 7:03 pm
by Chief
Many thanks to both of you. I may to hook up a mechanical fuel guage to the pump to get a preassure readng to ensure the fuel pump is producing consistent pressure. If it is, then its the Dynon.

Re: Fuel pressure display on Dynon

PostPosted: Wed Dec 09, 2015 8:55 pm
by danerazz
Air in the system?

Re: Fuel pressure display on Dynon

PostPosted: Wed Dec 09, 2015 9:41 pm
by Jack L

GDS brings up some good points and believe me before I felt comfortable with the erratic fluctuations on my Dynon D10 I checked the entire fuel system thoroughly. I should clarify that when I refer to Billy I mean Billy Payne who is a certified Rotax technician and the guy that was doing all of the engine repair work for Just Aircraft and numerous other Rotax customers as well.

Before messing with fuel lines there are a few things that you can do. With the engine running and the indication erratic turn on the electric fuel pump, the pump will provide 4-6 PSI to the system, if the fluctuations are still there it is unlikely that either pump is defective and If it stabilizes the mechanical pump may be at fault. With the engine off, turn on the electric pump and check to see if the indication fluctuates, if it does not fluctuate tap the fuel sensor gently with the handle of a small screwdriver if it fluctuates like mine does than you know it is the sensor. If it doesn't you can check the wires as GDS suggested. It sounds like your fluctuations happen 2-3 times per second like mine do, if that is the case It almost has to be a sensor fault as it is hard to imagine the actual fuel pressure changing at that rate.

Following that talk to Billy.

Let us know what you come up with.

Re: Fuel pressure display on Dynon

PostPosted: Wed Dec 09, 2015 9:49 pm
by SuperFly
Back in my RV days, there was some discussion about pulsing in the fuel system due to the pump. Now I may be speaking out of turn here, but is that a possibility? Dont know what type of pump the Rotax pump is (I should though since I will be flying with one) but is it the type of pump that could create pulsing in the system? Could it be the return line orifice to large, or too small?
I actually have a fuel pressure dampener that I had from a project left over, it is a welded stainless steel ball about 2 1/2" in diameter with a 1/4" NPT nipple welded onto it. I was intended to damped the pulses in a fuel system to make fuel flow more consistent on a fuel injection system. Perhaps some brands of sensors are just more sensitive to the pulses than others. Anyone call Dynon to see if they can be snubbed in some way to get a more stable reading? I too have a D180, so if no one knows, I will likely call to inquire.

Re: Fuel pressure display on Dynon

PostPosted: Fri Dec 11, 2015 7:40 pm
by rmullins
I have had issues with the fuel pressure since I installed my Dynon D180 6 years ago. The original problem was the first sensor had a 0-60 PSI range but was operating in the 5 PSI range, not the best for accuracy to be operating that low in the range. I replaced the original sensor with a new one earlier this year and it hasn't been much of an improvement. A Dynon tech support guy once told me they were struggling to find a good sensor...

Re: Fuel pressure display on Dynon

PostPosted: Sat Dec 12, 2015 7:13 am
by Jack L
I have been investigating other possibilities and stumbled onto a few things:

The Dynon manual states not to install the sensor on the engine, they recommend a remote location like the firewall. A couple of years ago Just Aircraft started installing the oil pressure sensor on the firewall as it was also experiencing high failure rates. Mine like most is mounted directly to the fuel manifold on top of the engine;

Another possibility that can actually cause real fuel pressure fluctuations is if the fuel vent line coming off the fuel pump is in the airstream, this will cause positive and negative pressure fluctuations at the diaphragm in the pump. According to the Rotax Installation Manual the line should terminate within the cowl or by other means that will keep it away from the slipstream, the same is true for the oil vent line;

Finally, this Rotax Service Instruction addresses another possibility. If the banjo fitting is over torqued or if the crush washers are used repeatedly the orifice in the fuel manifold can become closed or blocked resulting in unreliable fuel pressure indications.

Re: Fuel pressure display on Dynon

PostPosted: Sat Dec 12, 2015 2:20 pm
by Chief
Many thanks for the input. I will go through all these and see if this resolves. The sensor was installed per directions from a Lockwood tech, but the suggestion there could be air in the line has come up twice. The first and easiest test will be to activate the electric fuel pump and see if the display is the same as with the mechanical pump.

I had another person off forum suggest that with the sensor between the mechanical pump and the carbs, the fluctuation could be a result of prssure build up in the carbs, and as it changes the sensor Display is affected.

I guess if necessary, I could bypass the Dynon and mount a mechanical fuel pressure gage on the panel

Re: Fuel pressure display on Dynon

PostPosted: Sat Dec 12, 2015 2:43 pm
by gkremers
I elected to not install any fuel pressure sender. The fuel flow (red cube) is rock solid at known power settings, I just monitor that. I do have a backup facet fuel pump but have only used it once or twice. The engine starts easily in all temps, just haven't had the need to use it.


Re: Fuel pressure display on Dynon

PostPosted: Mon Dec 14, 2015 4:14 pm
by Chief
Up-Date. After a good warm-up I toggle to the fuel pressure page and observed the usual randomness of the display. While watching the display I activaed the electric boost pump. The result was no change. The jumping of the numeric values for fuel pressure was the same with the boost pump on as it was with just the mechanical pump.

Re: Fuel pressure display on Dynon

PostPosted: Tue Dec 15, 2015 11:51 pm
by 957DK
Hi Chief,

I have the same setup as you, a 912ULS, Dynon D180 and an aux electric fuel pump. I've been watching this thread as I'm experiencing exactly the same issue you have. With the engine running I see fluctuating fuel pressures. Turning on the aux pump with the engine running has no effect on the fluctuating fuel pressure. While at the hangar earlier today I ran the aux pump with the engine shut down and the pressure is steady. I've been ignoring the reading as the engine runs great.

My plan is to cover the air intakes on the cowling and running the engine to see if it might be a carb vent that isn't routed correctly. I'll let you know how it turns out when I get a chance to get back to the airport.

Dan Kretchmer

Re: Fuel pressure display on Dynon

PostPosted: Tue Dec 15, 2015 11:54 pm
by 957DK
I should've mentioned that I've never had the pressure fluctuate to the point that I get the low pressure warning message...

Re: Fuel pressure display on Dynon

PostPosted: Fri May 20, 2016 12:31 am
by rkleim
Any update on this post from last year? I have the same problem.