Low Fuel Pressure Indication

UPDATE 

I received a new Rotax fine fuel filter and a new fuel pump assembly from USA. Found a U.S. FAA certified mechanic to work on it. But still no solution. Checked the fuel pressure on the Garmin display with engine off pumps on. It was down to 19 psi. 
He replaced the fuel filter and we ran the pumps again. Now it was 18 psi. Started the engine and ran it for a few minutes. No change. 
Since there is no port on the fuel system to install a mechanical pressure gauge, we disconnected the AN fitting from the fine filter to the fuel rail. We attached a mechanical pressure gauge to the line from the filter. The engine side was not connected. Turned on the pumps and fuel pressure read on the Garmin display as high as 51 psi. On the mechanical gage it read about 90 psi. So the pumps were working. Called Super Petrel and they said replace pump assembly. The old pump assembly was contaminated with water but it seemed to be pumping fine. We had a bad odor of fuel for over a year but no one could pinpoint where it was coming from. Might have been the pump. 
He replaced the pump assembly and we ran it again. Garmin fuel pressure was now reading even lower than before. 16 psi. 
So we have exhausted our parts that were shipped in and still have no resolution. 
We are thinking it must be either the fuel pressure sensor or fuel pressure regulator. We took off the snap ring and inspected the fuel pressure regulator. All clean and no debris in the valve body. Since the engine was running fine, it’s probably a bad pressure sending unit. 
Any thoughts?

Scott,

Observations:

You replaced the fine filter and the pressure went down about 1 PSI... That is as would be expected since the pressure sender is before the filter and replacing the filter would lower the resistance through it. Nothing unexpected there. 

If I understand correctly, you put a mechanical gauge on the end of the line that would normally go to the fuel rail, effectively blocking the fuel line.  This is not a good idea.  These pumps are positive displacement and require the return line in the loop or they can/will be damaged from high pressure. I have seen a pump self-destruct under that condition by literally pushing the inner workings out of the end of the pump housing.  I'm hoping that you put your new pump in after that test and did not repeat it with the new pump installed.  If the NEW pump WAS run with the line effectively blocked with the mechanical gauge, then it should be carefully inspected and tested for proper operation and any possible leaks before use.  Moving forward, if you want to use a mechanical gauge for testing, then just replace the sender unit with a mechanical gauge (temporarily) and leave everything else connected as normal. 

It's not clear from your photos what the pressure range of your UMA sender is, but I believe the standard UMA differential sender used on the 912iS is 0-70 PSI.  If this is correct, I would have expected the Garmin readout to have been 70 PSI when you were seeing 90+ PSI on your mechanical gauge.  Since you were only seeing 51 PSI, this would indicate that the UMA sender is not working correctly. Verify which sender you have and that it is 0-70 PSI.  

Next steps:

1. Confirm your new fuel pump was not run with the output line blocked during your testing. 

2. Remove the pressure sender and install the mechanical gauge in its place. All other fuel lines should remain connected as per normal operating configuration.

3. Run the pump(s) with the engine off.  You should see 41-45 PSI on the mechanical gauge.  If you do see this (correct) pressure, then your problem is either the UMA sender unit, or the Garmin GEA-24 engine information module that its connected to.  The sender is cheaper, so replace it first and see if the problem is resolved. 

However, if when using your mechanical gauge (in place of the UMA sender) the pressure still reads low, then the problem is likely the Rotax pressure regulator. Replace the regulator and retest with the mechanical gauge still installed.  If this now reads 41-45 PSI, then reinstall your UMA sensor and verify the problem is resolved. 

If I was a betting man, I would put my money on the pressure sender. 

Were the ~19 psi reading real, the engine wouldn't run, or only just barely run. I'll wager it's the sender. Perhaps contributing, perhaps not, I'm not a fan of how the sender wires have been routed. Zooming in on one of your pictures:

The u-turn bend in the white wire bundle is asking for eventual trouble - it's too tight and stressing the wires. Add vibration, and it will get a lot being essentially tied to the engine, and it will eventually fail, if it hasn't already. If the bundle is too long, either take up the slack a little in several places with more gentle bends or make a loop.

As an aside, the use of zip-ties pictured is not "aircraft grade" in my opinion. For example, look at the medium-sized tie around the two blue tygon tubes; it's already partially crushed the lower tube and the one touches the other. A much better way to do such things is to use 3 smaller ties. Simply make a circle around each hose with 2 of the ties. Then loop the third between the two "circle ties" to hold them together. That one you can adjust as tight or loose as you want without putting crush force on the tubes. The same applies only worse to the pair of hoses above that, tied to the engine mount. Mounts vibrate. You don't want that vibration to sand a hole in the hose or rub the paint off the mount. Wrap a medium tie around the mount by itself. "Circle tie" each hose individually with small ties. Then use another small tie through the "circles" to connect them to the one around the mount. Again, less hose-crushing plus a "stand-off" from the vibration of the steel mount. Lots of words, but do it once and you get the idea.Sorry I don't have a picture handy. Also note that these plastic zip-ties come with differing temperature ratings or none at all for the cheap ones. "Heat resistant" ties aren't hard to find. Cheap ones will not last long inside engine compartments.

Thank you Jeff for your detailed instructions. We ordered a replacement fuel pressure sending unit and the weatherpak pins to plug into the wiring harness. We did notice that the zip tie that held the sensor in place, put it tight against the air box. It likely experienced a lot of vibration. 
it will take about a week or so for the parts to arrive and clear customs. I will post the results once we install it. 
The old pump was used for the pressure test and was only on for a moment. Once we changed the pump, we did not reinstall the old pump. It is in a difficult spot to reach. 

Jeff,

If you can find a photo or two of your recommended zip tie placement that you described, I would love to see what you mean.

My aircraft came from the factory this way as have almost 400 Super Petrels before mine.

Scott