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Fuel Pump OUTPUT Must Be Checked Too!

Mar 9, 2009
Few folks check their fuel pump's pressure and even fewer folks check their fuel pump's output. Just because a pressure gauge shows say 5-6 psi at idle that doesn't tell you how much pressure or volume the pump is delivering under actual running conditions. About 5 years ago I had a power problem even though my fuel pressure was showing 5 psi at idle. I ran a 300# push-on hydraulic hose into my cabin with a 0-15 psi pressure gauge on it and found the pressure was showing 5 psi under a light throttle at 60 mph but only 3 psi at 70 mph under a full throttle and 0 psi at 100 mph under a full throttle. So the fuel pump (a new Pierce) wasn't any good. I installed a 100 gph Carter electric fuel pump with a 1/2" hose feeding it and a 1/2" hose going to my engine compartment along with a 5/16" return hose. With the higher volume electric pump it'll show 6 psi all the way to 130 mph at which time the pressure begins dropping just a little bit as I'm actually outrunning the pump. And electric fuel pumps are rated for their volume output AT the pump and by the time the fuel gets to the engine almost half of the volume gets eaten up by frictional losses in the hose/lines running along the frame. In my case I ended up with 54 gph at my #3310 Holley and under a full throttle @ 130 mph my engine is actually consuming ALL of that 54 gph.

A stock mechanical fuel pump should deliver 1-1/2 pints of fuel in 30 seconds of cranking (or running) so if yours isn't producing that much volume you either have a plugging sock in the tank or a failing fuel pump or a plugging fuel filter at/in the carburetor inlet. For checking your fuel pressure and volume I recommend sweat-soldering a brass 1/8" "T" into the pressure line so you can screw a gauge in whenever you want.

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