Narrative:

En route to ZZZ I saw the float in the ercoupe header tank sight gauge descending. This typically means the fuel in the main wing tanks (fed to the header tank by the engine fuel pump) is exhausted. But with known fuel quantity in the mains; yet the bobber is going down in the sight gauge; this implies either failure of the engine fuel pump or an obstruction in the fuel lines from the mains to the header (or the main fuel shut off valve under the instrument panel on the pilot's side is turned off). I was getting flight following from [approach] and immediately advised them of a potential fuel system anomaly. First; I planned diverting to ZZZ1 where I knew maintenance was available. But as the bobber continued to descend; I feared imminent fuel exhaustion and diverted instead to ZZZ2. On the ground; I called the number given to me by [approach] to confirm I was safely on the ground. There was fuel self-service available. Initially; I planned on topping the header tank to its 6 gallon capacity to provide an hour plus range which would permit the legal VFR flight to ZZZ3. To my surprise; the header tank accepted barely a gallon of fuel. Examination of the main tanks revealed an appropriate reduction in the fuel level for the flight time [our departure airport] to where I first noticed the sight gauge anomaly. I started up; and the sight gauge was normal. I took off and the indicator rod stayed in then normal position for the remainder of the flight. So; I concluded there was no issue with the fuel transfer from the mains to the header tank. Rather; there was an unexplained (for now) anomaly in the float elevation of the sight gauge rod). When I get to my final destination of ZZZ4; I will consult with the a&P there; and consider replacing the float assembly; possibly with a non-cork impervious float. Beyond that; I will explore the feasibility of adding a fuel pressure sensor to register fuel pressure at the fuel lines from the main to the header tank. With an instrument panel gauge it would be possible to verify fuel flow to the header tank. If you have to rely solely on the position of the crude bobber and sight gauge; it becomes impossible to verify proper fuel flow; including the actual fuel level in the header tank; without landing and performing a physical inspection of all tanks; and a top off of fuel in the header tank as I did at ZZZ2.

Google
 

Original NASA ASRS Text

Title: An Ercoupe 415 pilot reported that the fuel indicator for the header tank indicated he was losing fuel.

Narrative: En route to ZZZ I saw the float in the Ercoupe header tank sight gauge descending. This typically means the fuel in the main wing tanks (fed to the header tank by the engine fuel pump) is exhausted. But with known fuel quantity in the mains; yet the bobber is going down in the sight gauge; this implies either failure of the engine fuel pump or an obstruction in the fuel lines from the mains to the header (or the main fuel shut off valve under the instrument panel on the pilot's side is turned off). I was getting flight following from [Approach] and immediately advised them of a potential fuel system anomaly. First; I planned diverting to ZZZ1 where I knew maintenance was available. But as the bobber continued to descend; I feared imminent fuel exhaustion and diverted instead to ZZZ2. On the ground; I called the number given to me by [Approach] to confirm I was safely on the ground. There was fuel self-service available. Initially; I planned on topping the header tank to its 6 gallon capacity to provide an hour plus range which would permit the legal VFR flight to ZZZ3. To my surprise; the header tank accepted barely a gallon of fuel. Examination of the main tanks revealed an appropriate reduction in the fuel level for the flight time [our departure airport] to where I first noticed the sight gauge anomaly. I started up; and the sight gauge was normal. I took off and the indicator rod stayed in then normal position for the remainder of the flight. So; I concluded there was no issue with the fuel transfer from the mains to the header tank. Rather; there was an unexplained (for now) anomaly in the float elevation of the sight gauge rod). When I get to my final destination of ZZZ4; I will consult with the A&P there; and consider replacing the float assembly; possibly with a non-cork impervious float. Beyond that; I will explore the feasibility of adding a fuel pressure sensor to register fuel pressure at the fuel lines from the main to the header tank. With an instrument panel gauge it would be possible to verify fuel flow to the header tank. If you have to rely solely on the position of the crude bobber and sight gauge; it becomes impossible to verify proper fuel flow; including the actual fuel level in the header tank; without landing and performing a physical inspection of all tanks; and a top off of fuel in the header tank as I did at ZZZ2.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.