FUEL PRESSURE - where to test, how many PSI?

[mkino]

I'm troubleshooting (again!) the fuel system on my 1991 454 mercruiser (carbureted). Two questions:

1) Can I put the gauge before the fuel pump (convenient), or does it have to go between the fuel pump and the carburetor?

2) How many PSI should it be?

Thanks!!

 

[Bt Doctur]

Re: FUEL PRESSURE - where to test, how many PSI?

can put gauges in both areas, one will show suction and one will show pressure. Should be 4-8 psi

 

[UncleWillie]

Re: FUEL PRESSURE - where to test, how many PSI?

The fuel pump sucks in fuel and pumps it out.
You will see a slight vacuum when measuring before the pump depending on how far uphill it is pulling fuel.
After the pump you will see some pressure.
The actual pressure is determined by a spring behind the fuel pump diaphragm.
The pressure does not change with rpm.

Without knowing the specs, expect to see something in the 5 psi range.
More than 2 but less than 12, would not be out of line.

Pull the fuel line off the carb and stick it in a container.
Crank the engine over a few turns.
A good squirt of fuel should appear on each revolution.

Stick your gauge on the end of this same hose. No fuel flow is needed to measure the pressure.

 

[achris]

Re: FUEL PRESSURE - where to test, how many PSI?

The spec you're looking for varies according to which fuel pump you have, Airtex or Carter.

Carter: (@1800 rpm) 5.25 to 6.5psi
Airtex: (@1800rpm) 5.5 to 7.0psi

And as already said, pressure can only be measured AFTER the pump, otherwise it's suction. If you do measure the suction, it should not be more than 2" Hg... If it's more, check for restrictions in the delivery system. Things like clogged screen on the tank pick-up, clogged anti-siphon valve, fuel lines too small, blocked fuel filter.

What problem are you chasing to want to check pump pressure?

Chris.....

 

[mkino]

Re: FUEL PRESSURE - where to test, how many PSI?

The spec you're looking for varies according to which fuel pump you have, Airtex or Carter.

Carter: (@1800 rpm) 5.25 to 6.5psi
Airtex: (@1800rpm) 5.5 to 7.0psi

And as already said, pressure can only be measured AFTER the pump, otherwise it's suction. If you do measure the suction, it should not be more than 2" Hg... If it's more, check for restrictions in the delivery system. Things like clogged screen on the tank pick-up, clogged anti-siphon valve, fuel lines too small, blocked fuel filter.

What problem are you chasing to want to check pump pressure?

Chris.....

Boat is surging under load, won't stay on plane. I also had to crank up the idle just to get it to run. I checked all fuel lines and pickups and anti siphon valve and replaced the fuel filter. So I think either the two month old fuel pump is bad or there is some junk in the carburetor. I want to be sure the fuel pump is good before I get the carb rebuilt.

Thanks for your input!!