I am trying to determine if my small block mechanical fuel pump needs replacing. At idle, it puts out 8psi and while driving easy, anything over half gas it drops to about 6 and sometimes 4psi. Should it always be consistant at all rpm? Motor is a zz4 350 with a 750 holly vac secondary, all stock, nothing modified.

 

As you press the gas pedal the fuel pressure will have some swing, mine runs about 6 psi and goes to about 3 and back to 6 as the pump catches up... It needs replacing when you start the car and it only goes to 1 or 2 and then to 0 that's a problem..

 

Be certain you've got a decent fuel pressure gauge too! I've seen a number of inexpensive gauges that give all sorts of erroneous pressure readings. Some are susceptible to temperature changes, and will vary if near engine heat.

 

Fuel pump soul purpose is keeping the fuel bowl filled with fuel at any RPM.
The higher the RPM, the less the pump will output because it is relying on suction for filling the pump chamber and it has a natural enemy - forward motion - pump has to suck fuel forward as Camaro moves forward, but only in acceleration.

 

no pressure gauge for me .
I have seen this twice from the gauge rupturing