This is probably a dumb question but I wanted to be sure. It's a 4.9 300 I know the pressure is supposed to be 55 psi. I'm changing the injectors out and I wanted to make sure they hold pressure before I put everything back together so I checked the pressure. I jumped the 2 and 6 pin to make the pump run continuously and I was only getting 41 psi. I clamped off return line and it made no difference. I wanted to isolate the pump to check pressure and I clamped off the feed line (I had pressure guage hooked by the fuel filter not the valve on fuel rail, didn't have correct fitting for it). I ran the pump continuously and still only got 41 psi with it isolated. My question is, shouldn't the pump be pushing more than 55 with koeo pin 2 and 6 jumped? I'm pretty sure its just a constant 12v to the pump, I don't think there is a variable voltage regulator or something where it would limit the output of the pump since it's not running, am i correct? as far as i know the only thing that regulates the pressure is, well obviously, the fpr. I'm 90% sure I'm right I just wanted to double check before I swap the pump out

 

With the fuel return clamped fuel pressure should skyrocket, so something is up. Does pressure hold after the pump is shut off? If not then it's bleeding off somewhere and the only 2 possibilities are through one of the injectors or back into the tank... assuming the truck only has 1 tank. Since your pressure is low to begin with that suggest the pump is weak or one of the check valves in the fuel delivery module is leaking,,, there are seperate control valves on the send and return lines. There is a possibility your truck has the wrong regulator too, 41psi would be roughly correct for a V8 motor.

 

Right, I'm pretty sure it was leaking injectors which is why I changed them out but forget all that lol. It was dual tanks but the selector valve has been taken out. So here is the set up, fuel pump-fuel line-fuel pressure tester- fuel filter- fuel line-clamp. Only gets to 40 or so. Fuel is blocked off before it gets to injectors or for, so it is weak fuel pump correct? Nothing should be limiting the output of the fuel pump right? As far as I know there isn't anything that would drop the voltage to make it not run the same koeo vs koer

 

I also checked the voltage at battery and it's at 11.5, I'll charge it up a little bit and see if it makes a difference

 

Yes it should make 80-100psi very quickly with that setup and then the pump should audibly struggle.What year is the truck?

 

It's complicated lol, it's a 90 truck that was a 7.3 (blown motor) swapped to a 4.9 out of a 91. The fuel pump was a cheap Amazon one and very well could have been for the 5.0 motors is my guess or it's just a cheap pump that isn't any good. I just wanted to double check so I'm not throwing things at it

 

All the fuel pumps put out the same volume and pressure. The fuel pressure regulator on the fuel rail is responsible for bleeding excess fuel back into the fuel tank to maintain the proper pressure. If you clamp off the return line, you should see about 100 psi on the fuel rail. As pumps age and wear out, the maximum the fuel pressure the pump can provide drops to below the regulators set-pressure.

 

Did you swap over the tank and lines frm the '91?
Does the fuel pump look somehing like this? If not then yeah you have the wrong pump.

 

It's running on the rear tank so it's a different sending unit and I also used the diesel tanks, it fit in there just fine so I'm pretty sure the tanks are same between gas and diesel. It's probably just a cheap pump, got the whole sending unit with pump on Amazon for 70. I'll swap the pump out next weekend. I was just making sure there isn't any electrical limiter on the fuel pump causing it to not pump as hard but the more I think about it, the pcm just tells the relay to be on or off so there really isn't anywhere where the pcm could limit the fuel pump voltage to limit pressure which was what I was trying to figure out