Car fuel pumps and Water pumps

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On Moderation
16 Nov 2009

This is driving me crazy!

Does anyone know why these websites appear to contradict one another or is it different for car fuel pumps as opposed to water pumps? When the diaphragm is raised does the inlet valve open or close?

The lowering of the diaphragm makes the inlet valve of the fuel pump open. With this valve open, fuel is drawn from the gas tank and into the pump. At the same time, the lowering of the diaphragm closes an outlet valve.

Read more: How Car Fuel Pumps Work |


A vacuum is created inside the pump casing each time the diaphragm is raised (see figure below). This opens the inlet valve and seals the discharge valve allowing water and air to enter the pump. When the diaphragm is lowered the resulting pressure seals the inlet and opens the outlet valve purging the pump housing of water and air..

I presume discharge is another name for outlet
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John Webb

Established Member
5 Jun 2010
St Albans
They work identically - just that the fuel pump is worked from below and the water pump illustrated on the linked site is worked from above. So the up-stroke on the fuel pump forces fuel out of the outlet while in the water pump the down-stroke forces the water out.
Hope this helps you.


11 Dec 2005
East Durham
The strict answer is both, depending upon how the pump is designed.

The most likely answer however is the link you posted from the pump manufacturer ie pull the diaphragm= suck, push =eject

It is possible to design a pump with the Inlet(suction) and outlet (delivery or discharge) on the same side of the diaphragm as the actuating rod- this would result in the pump drawing in fluid as the rod is pushed and ejecting it as it springs or is pulled back. However this design would put the fluid on the same side as the actuating rod meaning you would need an additional seal around the rod itself, introducing a potential leak path in a position prone to wear.

There are of course many other design of pump, the most common being the centrifugal pump (often used as car water pumps), lobe and gear pumps (often used as car oil pumps), screw pumps, sliding vane pumps (usually vacuum pumps), peristaltic pumps and various other designs of positive displacement pumps as well as other exotic and bespoke designs!


PS the fuel pumps you describe are normally the engine driven type used on older carburettor fitted cars, fuel injected engines tend to have various designs of pump fitted at the tank outlet proving fuel to the injector system on a ring main type set up.

PPS beat me to it John!
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