Air Bubbles in Fuel Bowl

swat · Post by **swat** » Mon Aug 29, 2011 9:52 am

My M38 runs great, but I can see air bubbles in the glass bowl in the fuel filter when it is running. Does this mean I have a leak in the vacuum side of the fuel pump?

Post by **wesk** » Mon Aug 29, 2011 10:47 pm

Usually bubbles in an add on filter between the pump and the tank mean you have an air leak in your plumbing between the pump and the tank. Often you can such a small leak in your plumbing that it evaporates from the fitting or tube before you ever see it. Then when you are running the engine the fuel pump suction will keep the fuel inside the plumbing and actually start sucking small amounts of air in through that leak.

Bubbles in an add on filter between the pump and the carb will usually mean the same thing but could also be a diaphragm leak in the pump.