Fuel pressure and restriction test

[DDlighttruck]

What’s the difference between suction and pressure? That’s two different words for the same thing...?

[Mattman]

Suction is a vacuum reading. Pressure is a positive reading.

[DDlighttruck]

That makes sense

So why does the pressure gauge needle bounce so bad? Do you average the middle of the bounce to guess at the pressure?

[Lonestar10]

gear pumps atleast the ones ive worked with work with 2 gears spinning together when the teeth on the gears come together they create a seal that forces whatever its pumping to move so your constantly getting pulses of fuel from the gear pump not a consistant flow like you would see from a cars fuel pump. that is why the needle is bouncing.

[DDlighttruck]

Alright, thanks.

So other than slightly high restriction on that top suction side port, everything looks normal?

[Lonestar10]

that isnt showing restriction the gear pump is sucking the fuel from the fuel tanks to itself there is going to be negative pressure on that port as its pulling the fuel through the fuel filter. if you wanted to see restriction you would have a gauge at that top port and then another gauge before the fuel filters between filters and tank that would tell you your true fuel restriction upto the pump by measure the true resistance of the fuel filters.

lets just say with 3/4inch fuel lines going from tanks to gear pump and for this lets say there are no fuel filters. lets say there is 3 gallons of fuel from the pump suction to the suction in the fuel tank the gear pump is going to create close to the weight of the fuel in suction vacuum to get that fuel from the tank to the pump add in filters and now you would probably add another 10-15% additional vacuum with clean filters.

[DDlighttruck]

If it’s measuring a vacuum then how come the resistance gauge is measuring pressure, not vacuum?

[Lonestar10]

how long after turning the key on did you take the reading? if the electric pump was still running that would explain a positive reading there.

[DDlighttruck]

Ooohhh so turn the key on, when the lift pump stops, fire the motor and retest...? I went back and re-read the steps and it is saying 6 inches vacuum with a clean filter, and 12 with. Aa dirty one.

I honestly can’t remember how long it was running before I started testing, but I know I did the bottom one first