Fuel pressure warning on decent 912 ULS

[constantin_w]

Hi guys,

I am having some trouble with our RV-12, 912 ULS with Dynon D180. We keep experiencing wildly jumping fuel pressure gauges with frequent high fuel pressure warnings. The jumping gauge can also be seen on the ground but the warning comes mostly up on decent which points on some issues on the flow back to the tank.
We have already changed the fuel pump, the fuel pressure sensor and some fuel lines. We did check the valves as well. In general, the problem seems to disappear after some maintenance and comes back after a while. Today we had one warning on the ground which went away quickly. Not a single warning in the pattern (did 4 circles), but in the next flight (ca 4500ft MSL, WOT, 4900rpm), the warning kept coming up during decent again.
Did anyone of you experience a similar problem? Any idea where to look and how to solve the problem?

Thanks for your help in advance.
BR
Conny

 

[Piper J3]

Fuel pressure sender might be bad. Bad ground. Bad wiring connection. Did you replace mechanical or electric fuel pump? Electric fuel pump "on" all the time? Nobody added switch to shut off electric fuel pump?

 

[seagull]

Not a single warning in the pattern (did 4 circles), but in the next flight (ca 4500ft MSL, WOT, 4500rpm)s

I’m not sure about the fuel pressure fluctuating, mine does spike to 5.5-6 with a quick throttle retard.
More interesting to me was “WOT at 4500 rpm”. If that is all the rpm you are getting at WOT your prop needs some pitch removed.

 

[PaulvS]

How high in psi does the pressure gauge "jump"? Apart from the items that have already been checked and changed there is the restrictor orifice in the fuel return line. The orifice is normally 0.040" and should be clear of any debris that may cause blockage and higher fuel pressure readings.

Otherwise, it could be a fault in the D180 and a way to check that would be to install a temporary separate fuel pressure gauge to validate the reading on the D180.

 

[constantin_w]

Hi,

the electrical pump was exchanged. Sensors were exchanged twice already. The pump is always on with no switch. All pipes were checked and blown free.

I corrected WOT. It was 4900 not 4500. We pitched for speed. The engine runs very smoothly with the setting.

The restrictor orise has been checked too and is clear. PSI jumps between 1 and 7 (max scale).

I will try to get a separate fuel sensor to check for D180 faults.

Thanks
Conny

 

[bobg56]

sounds like a wiring issue

 

[constantin_w]

sounds like a wiring issue

Not sure about it. The problem seems to increase after some time of flight and especially during decent. I would expect a wiring issue to be continuously problematic

 

[seagull]

Fuel pressure is dependent on airbox pressure. A leak in the airbox may present as variable fuel pressure with altitude changes. Check fault logs for airbox pressure sensor errors, double check carb to airbox connections.

This is incorrect information.

 

[docjjrmd]

This is incorrect information.

yep you are correct I had my turbocharger brain on... (original post deleted)

 

[scottmillhouse]

Is your fuel tank properly vented? I had low fuel pressure problem in my 2015 build when full tank on take off with Vans external vent, fixed by drilling small hole in gas cap like original. Perhaps a clogged fuel tank vent could cause your fluctuations.

Am concerned about 4900 rpm’s since that means you are probably lower on takeoff. Rotax recommends minimum of 5200 at full manifold pressure (during take off) to prevent detonation.

 

[Tony_T]

I agree with Scott. A pitch that allowed 5200 at takeoff and 29 (in Hg) MP is a good compromise. I think this is where the Van's suggested pitch setting puts you.

 

[constantin_w]

Is your fuel tank properly vented? I had low fuel pressure problem in my 2015 build when full tank on take off with Vans external vent, fixed by drilling small hole in gas cap like original. Perhaps a clogged fuel tank vent could cause your fluctuations.

Am concerned about 4900 rpm’s since that means you are probably lower on takeoff. Rotax recommends minimum of 5200 at full manifold pressure (during take off) to prevent detonation.

The hole exists and is not blocked. I also had this in mind and checked it. Any other ideas?

 

[flightdoc101]

You have both a mechanical and an electric pump. Unlikely that both are at fault. I would suspect your pressure sender.