how much fuel pressure?

[mystic04]

I've been chasing a intermitant miss so I started by changing my plugs which were white from a lean condition so I changed the fuel filter and ran some seafoam through the tank then checked fuel pressure I get 30 psi with key on engine off then 20 psi at idle and somthing like 30 when I hit the gas and if I hold say 2500 it will settle back down to about 22 psi so does any one know what the fuel pressure should be from the factory and has anyone had this problem before thanks

 

[CDW6212R]

That pressure is too low, it should be well above 30psi at all times. I think 35psi is the standard, with near 42psi at WOT(pull the FPR vacuum line to check it while idling). That pressure is for all pre 99 Fords.

You might try the regulator first because it's easier to get to than the pump. Look over the lines carefully for any odd appearances or leaks first.

My 99 truck had only 50-55psi last year, it threw a lean code often. Mine was likely the rubber line between the fuel pump and FPR(in tank with pump), or the regulator itself. ((+ Fords should have about 62psi at all times. Regards,

 

[MrShorty]

As mentioned, those pressures are definitely too low. Two things I might look at before replacing parts:

1) Is the gauge accurate?

2) Is fuel being returned to the tank? Probably easiest to do by just listening to the tank for the sound of fuel draining into the tank. Ground the fuel pump test lead so you don't have to be running the engine. If fuel is being returned to the tank during a low pressure condition, it is almost certainly a bad FPR. If fuel isn't being returned to the tank, then there is something preventing the pump from generating sufficient pressure to open the FPR.

 

[mystic04]

Thanks for the info fellas

 

[CDW6212R]

As mentioned, those pressures are definitely too low. Two things I might look at before replacing parts:

1) Is the gauge accurate?

2) Is fuel being returned to the tank? Probably easiest to do by just listening to the tank for the sound of fuel draining into the tank. Ground the fuel pump test lead so you don't have to be running the engine. If fuel is being returned to the tank during a low pressure condition, it is almost certainly a bad FPR. If fuel isn't being returned to the tank, then there is something preventing the pump from generating sufficient pressure to open the FPR.

I'm trying to understand that, help me with this? If the FPR was bad keeping pressure low, doesn't that mean that it would be allowing too much fuel back to the tank? I am trying to see the difference in return fuel, what that would sound like, and the difference.

 

[Maniak]

Thats how I would understand it.. bad fpr (low pressure) would allow too much fuel back to the tank.. Bad FPR (high pressure) would give you less fuel going back to the tank..

I've never been able to hear fuel going back into the tank.. I can only hear the fuel pump in the tank.

~Mark

 

[410Fortune]

ditto
I can hear the pump go WHEEEEEEEEEEEEEEEEEEEEEEE nothing else

 

[Joes4x4Ranger]

ditto
I can hear the pump go WHEEEEEEEEEEEEEEEEEEEEEEE nothing else

I hear that same WHEEEEEEEEEEEEEEEEEEEEEEE noise, and then at times, a "gurgle" like someone blowing air back into tank with a straw or a "cavitating" pump. The gurgle is not real load, but noticeable and intermittent.

 

[CDW6212R]

That gurgle would be from fuel spraying out of the return nipple into the surface of the tank gasoline. The nipple is a closed end so fuel likely sprays down out of it quickly.

 

[mystic04]

I rechecked the fuel pressure with a better gauge and got 28-29 at idle and 39 at idle with the fpr vacuum unpluged I guess this is still to low does any body know which pin to ground on the test plug and exactly what the fuel pressure should be thanks

 

[Opera House]

Those pressure readings are exactly what they should be. You still may have a bad FPR. Some were made with the spring offset and the seat wears only on one side causing it to stick open at times. You can check this by monitoring the pump current while you are driving. It should never drop below 3.5A, engine will stall at about 2.5A. Current easily monitored by inserting leads at fuse socket. Unless you can document some drivability problems, I really doubt fuel is your problem.

 

[mystic04]

Those pressure readings are exactly what they should be. You still may have a bad FPR. Some were made with the spring offset and the seat wears only on one side causing it to stick open at times. You can check this by monitoring the pump current while you are driving. It should never drop below 3.5A, engine will stall at about 2.5A. Current easily monitored by inserting leads at fuse socket. Unless you can document some drivability problems, I really doubt fuel is your problem.

Thanks for the info the problem with the truck is it idles really rough (shaking) and you can hear a miss through the exhaust but its not a hard miss it is intermittant there are no fouled plugs and no vac leaks it pulled 25 inches at idle so im stumped

 

[SPORTEXP]

In my books fuel pressure is 38-45 PSI. So what they are saying when you pull the vacuum line to the fuel pressure regulator it should jump to 45 PSI (simulating wide open throttle). If it doesn't do that I would agree with the rest of the guys here, but first start with the fuel filter then replace the fuel pressure regulator.

 

[CDW6212R]

I would feel uncomfortable if it doesn't have at least 30psi at idle, preferably 32-35psi. It sounds like the WOT pressure is not the issue, so concentrate on the normal idle readings.

 

[TJTrucker]

Just replaced a brand new Walbro pump with a Ford OEM pump, the OEM pump hooked up and the air bleed out of the fuel lines should show 40 -45 lbs KOEO. After starting it will settle back to 30-32 lbs. A good OEM Ford pump will deliver 60-70 lbs with when tested at the tank head with a pressure gauge on the high pressure outlet from the tank, with no return line. Only test this for the 2 initial on time pulse, DO NOT cylce the key more than 2 times. You can blow the check valve on the pump.
I have seen Airtech and Walbro pumps go as high as 85 lbs. A good pump will give you 40-45 lbs with in the 2 seconds of the start pulse, providing there is no air in the lines. If there is air in the lines it will show a lower pressure as the air will compress.