Fuel Pressure Problem

[rfazio1951]

Here’s the problem, sorry for the long post. I’m flying an RV-6 with a Dynon Skyview installed. I have the EMS module and all the engine probes hooked up. I’ve been flying with it for 3 years or so. Recently I’ve been getting a low fuel pressure warning from my dynon girl. I look down and see fuel pressure at zero or close to zero. It used to occur when doing aerobatics and in a high angles of attack. But I’ve been getting it increasingly and in level flight. No big deal back up pump on and pressure comes up. Increasingly it’s doing it more and more? Through this entire time my engine has never missed a beat. So I pull the cowl one day and my friend Skip is looking on and says, hey what’s that plug in your fuel pump? I have a Lycoming O-360 with mechanical fuel pump and electric facet backup. In the mechanical fuel pump I had a 1/8” plug in where the diaphragm is supposed to be vented to the atmosphere. Skip say’s you’re supposed to have that vented with a tube to below the cowl! Duh, so I pull the plug and vent it properly. My fuel pressure looks good and I go flying. A flight or two later and “FUEL PRESSURE” from my girl. So now I’m convinced that I’ve ruined my mechanical pump by having the plug in there for 500 hours since I’ve been flying the plane. I order a new fuel pump and install it. $400.00 later all is well for a few flights and “FUEL PRESSURE” again. What! So now I start going crazy. I did the following:

Disconnected all lines and check fuel flow with the electric pump on.
Blew through vent lines to make sure they were clear.
Checked gascolator – clean.
Checked fuel strainer in carb – clean though it would not affect pressure
Changed the fuel pressure sender – dynon old style

So I’m still having problems, and decide to install another fuel pressure gauge to make sure I’m actually having a low pressure problem or an incorrect indication. Remember the engine has never missed a beat. I flew around one day for an hour with zero pressure. My friend Adam says install a mechanical pressure gauge and eliminate all electric possibilities. I install an old mechanical fuel pressure gauge and line with a tee at the dynon sender. I take off and fly for a few minutes and “FUEL PRESSURE”. I look down and have between 2 & 3 pounds pressure on the mechanical gauge and 0 on the dynon screen. Ok, so now I know it’s lying to me. I’m looking at the dynon instructions for the skyview and it talks about how important the grounds are and senders should be grounded to the same ground as the EMS, etc. etc. I have the wire from the EMS going to the fuel pressure sender and the ground side of the sender grounded at the ground bar on the engine side of the firewall. The EMS is grounded on the cabin side of the firewall on the forest of grounds a-la a Bob Nuckolls type ground bar. I run a new wire from one of the EMS ground pins to the sender and disconnect the ground to the ground bar. Go flying and in a few minutes “FUEL PRESSURE”.

It seems to indicate correctly and the pressure is around 3.5 on the ground and as high as 4.5 flying. Then it will go down 2 or 3 pounds. If I see .9 or 1.5 on the ground I know it’s going to indicate low when I’m flying. Then I could put the backup pump on and it comes up a little to 1.5 or 2, then all of a sudden it’s back up to 3.5 or 4. I’m beginning to think it’s the EMS. I need some ideas.

 

[amolny]

Rich -

Two ideas come to mind:
Have you tried downloading you flight logs and uploading them to SavvyAnalysis.com? It's a free service you can sign up for. Then you can plot fuel pressure against engine RPM, fuel level and any other parameter you can think of. Does the fuel pressure trend up and down smoothly or does it look like a sawtooth pattern?

Also, there is an EMS settings file that lets you set the parameters for how that sensor gets read.
http://wiki.dynonavionics.com/Making_your_own_sensor_definitions

Lycomings have a diaphragm pump which causes pressure pulses in the fuel system. If the EMS sample rate is too low you can get 'aliasing' where the EMS sees a peak pressure on one sampling and a low pressure on the next sample. You can configure the EMS settings file to take samples over a longer period to average out the fluctuations.

Good luck,
Adam Molny

 

[rfazio1951]

when the pressure changes to - too low and I get hte warning, the pressure stays low for a while. Sometimes 20 minutes or so. low, even though I put on the backup pump it's still low like 1.5 or so. then all of a sudden it's up again, 3.5 and I'l shut the backup pump off and it remains high for an hour. It doesn't seem like the sample rate to me.

 

[swatson999]

What kind of pressure transducer do you have?

 

[rfazio1951]

The Dynon supplied sensor. It's not the new Kavlico units, it's the "legacy" pressure sensors.

 

[swatson999]

Those things suck. I had both a fuel pressure and an oil pressure transducer fail, replaced both, but I can see at least one of them starting to act up again. Switching to the new Kavlico senders at annual in January if I can make these last that long (running more wires is a pain, but doable).

Mine both failed *high*, though...I could see the pressures sloooowly starting to creep up, both over several flights and finally, within a flight. After changing them out, they were solid, at least for a few dozen hours or so.

Ditch the VDOs...they're either garbage or completely unsuited to an aviation environment. See if that fixes your issue, since it clearly appears to be instrumentation rather than actual pressure issues.

 

[rfazio1951]

Thing is it did the same thing with two units. It was giving me low readings, so I got a new one installed it and the exact same thing. Makes me think it's not the sensor.

 

[Dynon]

It's almost definitely not the EMS if a whole bunch of other readings aren't also reading incorrectly. In case you'd been changing any of your configurations, though, you might visit SETUP MENU > EMS SETUP > SENSOR INPUT MAPPING and make sure that the correct sensor type is still selected.

The pressure sensors aren't super-sensitive to grounding issues unless it's atrociously bad(the oil temperature sensor was the one that is most often the issue, and then not with SkyView).

Beyond that, we definitely have a strong bias towards the new sensors. The old VDO style were definitely not as reliable as we wanted to see in the fleet, hence why we replaced them.

 

[rfazio1951]

Ok so I guess I'm ordering a couple of the kavlico sensors. I can't see changing the fuel pressure and leaving the oil pressure. Is there a kavlico for the oil temp?

Here is the setup for my fuel pressure sensor in the .dfg file.
c37_p8={
valid=1
name=FUEL
id=0-30 PSI FLUID PRESSURE (100411-000)
alarm_type=SELF-CLEAR
range={
max_display=8
min_display=0
range1_valid=1
range1_top=0.3
range1_bot=0
range1_name=R1
range1_color=RED
range2_valid=1
range2_top=.7
range2_bot=0.3
range2_name=R2
range2_color=YELLOW
range3_valid=1
range3_top=5
range3_bot=.7
range3_name=R3
range3_color=GREEN
range4_valid=1
range4_top=7
range4_bot=5
range4_name=R4
range4_color=YELLOW
range5_valid=1
range5_top=8
range5_bot=7
range5_name=R5
range5_color=RED

 

[jakej]

'Old' fuel pressure is the most problematic but you're better off with the oil pressure Kavlico type as well IMO.  No need to change out the Oil temp sensor, I don't think that there is a Kavlico type for this either

Jake J

 

[dynonsupport]

Another reason we switched to Kavlico for fuel pressure is that we could get a better fit for various engines, especially those that have relatively low fuel pressure. We have these options for fuel pressure, depending on your engine:
P/N 101715-000 - 5 PSI Kavlico
P/N 101690-000 - 15 PSI Kavlico
P/N 101716-000 - 50 PSI Kavlico

For oil pressure:
P/N 101693-000 - 150 PSI Kavlico

All of them are $85 each, in stock at Dynon. Call Dynon Avionics Sales at 425-402-0433 for fast, friendly service.

As explained in the SkyView System Installation Guide and the one-pager that comes with them, for easier installation, if you have a (Dynon) Manifold Pressure Sensor, you already have the needed 5V and "EMS Ground" in the engine compartment, so you can tap those wires for the power and ground for the Kavlico sensors. Or if you want dedicated wires back to the SV-EMS-220
- Power is Pin 18 - White/Red wire
- Ground is any of Pins 3, 13, 16, 17, or 30 - Black wire

Ok so I guess I'm ordering a couple of the kavlico sensors. I can't see changing the fuel pressure and leaving the oil pressure. Is there a kavlico for the oil temp?

<snip>

 

[rfazio1951]

I installed the new kavlico fuel pressure sender today and it looks like my problem is fixed. Fuel pressure was higher than I have ever seen and it never varied more than a little. I hope I'm never going to hear "fuel Pressure" from my girl again.

 

[GalinHdz]

Which fuel pressure sender should be used on a Lycoming O360 A4M Carbureted engine? The P/N 101715-000 - 5 PSI or the P/N 101690-000 - 15 PSI Kavlico?

 

[rfazio1951]

I used the 0-15 unit. My pressure sometimes is at 5.1 or 5.2. So I went with the 15 psi unit. It seems to work way better than what I had. The pressure holds very constant.