I have dual 780 Demon carbs, dual Holley blue pumps. I used to hear the fuel pumps shut off but never knew why. I found the oil pressure sender was bad last year and replaced it. Since then I noticed if I leave the key on with motors off the pumps keep running. I am scared just to "see what happens". I can take about 90 seconds of the pumps running with the motor off before my gut says turn it off stupid. Now I am afraid to leave the key in the on position because I do not know if the pumps will shut off. So as the title reads, what shuts off the fuel pumps?

depends, if yours are triggered by the ignition wire through a relay that could explain why they run in the on position. The other more plausable explanation is your pump was grounded through a oil pressure switch that may has gone bad. Or, when you replaced the sender, you maybe hard grounded the relay on accident and that would explain why it runs with the engine killed now. Most fuel pumps are triggered through oil pressure, pressure drops and it kills the pump saving what is left of the engine

i don't think it is to save the engine. i think it is to save the boat and passengers. fuel line breaks, engine quits, key stays on, bilge fills with gas...next step? BAD THINGS!!! something to think about for those who run a holley red instead of mechanical pump on a carb set-up...

Should be a Hobbs switch that is activated by oil pressure as said above.

Ok I found the part number to the switch and looked it up. I do not understand how the open pressure range is between the closed ranged. So i put the wrong switch on. I should be looking for a 3 prong switch that opens under 7 psi and is normally closed? Right?

oil Pressure Switch Pressure Rating : No. 2 Opens At 2-6 psi, No. 1,3 Closes At 1-9.5 psi
http://www.napaonline.com/Catalog/Ca...617_0222440261

you wamt a open one that closes,,when you crank the engine it closes the switch (on) and then the fuel pump will come on..each engine should have its own seperate switch..