Fuel pressure question
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Joined: Oct 2006
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Fuel pressure question
I'm running some tests to determine what is causing a lean code 172 on my '94 F-250 5.8l. Key on pressure not running is only 20 psi. When checking running fuel pressure I get 32 psi from the front tank and 34 psi at the rear tank. When disconnecting the regulator vacuum line the pressure jumps to 42 psi and 44 psi.
I'm having problems finding the pressure info I need but so far I find one post saying 45 psi is normal running pressure. Could this be a regulator issue? The fuel filter can is pretty rusty so may be original. Can a dirty filter cause this?
Thanks for any info!
I'm having problems finding the pressure info I need but so far I find one post saying 45 psi is normal running pressure. Could this be a regulator issue? The fuel filter can is pretty rusty so may be original. Can a dirty filter cause this?
Thanks for any info!
Fleet Owner
Joined: Aug 2005
Posts: 23,668
Likes: 301
From: Easton,Ks
Fuel pressure at idle is 30-32psi.
Fuel pressure at snap WOT is 40psi.
44psi is too high as it is set up for 39.6psi across the injectors at all times.
45psi is wrong.
Sounds like your gauge may be a little off.
There is no such thing as just a key on pressure as it could be anywhere from zero to the spring pressure of 40psi. 40psi if the key is on with pin #6 of the self-test plug grounded to pin #2 or grounded to the NEG post of the battery.
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Fuel pressure at snap WOT is 40psi.
44psi is too high as it is set up for 39.6psi across the injectors at all times.
45psi is wrong.
Sounds like your gauge may be a little off.
There is no such thing as just a key on pressure as it could be anywhere from zero to the spring pressure of 40psi. 40psi if the key is on with pin #6 of the self-test plug grounded to pin #2 or grounded to the NEG post of the battery.
/
Logistics Pro
Joined: Mar 2005
Posts: 4,698
Likes: 47
From: Mexico, Missouri
45 psi isn't normal on the V8 versions.
If in doubt about the last fuel filter change, do it 1st. Scratch that problem off the list.
You should see 30-32 psi running, 38-40 with vacuum line disconnected from the FPR. Also, snapping the throttle should show an increase in fuel psi (with vacuum hose connected to FPR).
When you say that you have 20psi, engine off, what will it do if you cycle the key on and off several times? How long does your fuel psi stay at (X) amount when you shut the truck off? How long does it hold (X) amount after it primes? The fuel pump is designed to only prime for 1-2 seconds after the key is rotated to the run position.
If in doubt about the last fuel filter change, do it 1st. Scratch that problem off the list.
You should see 30-32 psi running, 38-40 with vacuum line disconnected from the FPR. Also, snapping the throttle should show an increase in fuel psi (with vacuum hose connected to FPR).
When you say that you have 20psi, engine off, what will it do if you cycle the key on and off several times? How long does your fuel psi stay at (X) amount when you shut the truck off? How long does it hold (X) amount after it primes? The fuel pump is designed to only prime for 1-2 seconds after the key is rotated to the run position.
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