Low Fuel Pressure

Problems have occurred with tank nearly empty to full. Only one vent for the tank and IS clear. Vent has not been modified and is configured as installed by the manufacturer.

Alan,

I've had this problem in hot and cool weather and with mogas with and without ethanol and with 100LL.

Where is your Fuel pump drain line routed? 

If your pump drain line is in a negative pressure zone it can cause the fuel pump to lower its pressure due to vacuum on the backside of the diagram. The best thing for you to do is upload some photo's of your installation and fuel system so people can have a look at what your system looks like and identify any issues.

Had the same problem with a high wing ICP Savannah aircraft. Changed mechanical pump, aux pump, filter/gascolator, checked all the lines, checked tank vents, checked pump diaphragm vent all were ok but the problem remained. On full power , pressure would drop to 0,11 bar and it would stay there until I was reducing throttle. Engine was running fine though. During cruise , occasionally the pressure would drop from 0,26 bar to 0,15. The aux pump would raise the pressure to about 0,29 bar. It would happen on a cold day, a very hot summer day or even a very cold winter day. No Avgas was used, only mogas without ethanol. Finally I believed that it was actually a sensor fault (sensor was checked for accuracy) due to air being trapped inside the hose that is connected to the sensor from the fuel manifold ( not Rotax standard but custom made by ICP). I changed the installation and moved the sensor to a lower position than the manifold in order for any trapped air to be vented away by the return line and now I get solid readings (0,25-0,26). So I guess that air was being trapped between the sensor and the manifold , giving fake pressure readings to the sensor. Although it is normal to have air inside the fuel manifold, it appears that somehow air does go inside there and if not vented from the return line, it can cause problems.

Mr RTX, Thank you for your comments about the drain line. I had these problems with the old style pump as well but what you describe with the new pump may be a problem. I plan to reroute the drain line to insure that is not part of the problem.

Spiros, thank you for your comments. I have tried to make sure there is no air in the line leading to the fuel pressure sending unit. My mechanic is of the opinion a little air in that line would not make any difference as he believes the air would be compressed to the same pressure as the fuel  pressure. Anybody else have thoughts about that?

Thanks to all who have contributed their thoughts!