Fuel Pressure Gauge Fluctuations

Question: I am starting to get fluctuations on my fuel pressure gauge under 1,000 rpm. After checking the entire fuel system I cannot find anything. Could a small leak in the diaphragm cause the fluctuations, as there is a red stain under the fuel pump where it bolts to the engine block? There is a small oil seep at this same location.

Answer: First, the fuel pressure gauge fluctuates because there is a vacuum leak before the engine pump. It is sucking air, causing the fluctuation; it is not due to air in the gauge line. This line DOES NOT need bleeding. Use the hand wobble pump to pressurize the fuel system, then look under the cowl on the right side (this should be part of the preflight.) If it leaks anywhere from the wobble pump to the engine pump, you will be able to see it. Leaks have been found in the diaphragm of the wobble pump, the top gasket of the engine pump, and the rubber hose connection between sections of the metal tubing. You are sure to find it just by running the wobble pump.

Check the fuel valves if you can't find any other leaks. These can be disassembled and lubricated with Parker 'fuel lube' to prevent leaks there for a long time. Ask your FBO for a small amount of the Parker product. If you disassemble, make sure you get the handle back in the right place as it will have to be removed during disassembly; there is an AD to pin the handle.