Fuel Pressure on a '95

[deiaggie]

Symptoms: lack of power at highway speeds, accelerates faster at half throttle than at full at highway speeds and in top gear.

I tested my fuel pressure today at the rail and with the key off I got 10 psi. Turned the truck on and it read about 29 psi. Revved it up and it went up to about 30 and upon slowing down it droped down to 26 psi and then went back to 29 psi.

All the posts I have found refer to the later 4.0s regarding fuel pressure on what it should be, like 20-30 with the key off and 35-40 with it running/revving

what should I be at with a 3.9?

 

[ptschram]

Tha's a little bit low, but not yet cause for concern. But, the 10 psi, key off is a little low, you likely have a leaking injector.

 

[deiaggie]

Tha's a little bit low, but not yet cause for concern. But, the 10 psi, key off is a little low, you likely have a leaking injector.

after i shut it down it held 28 psi for at least an hour before I took off the gauge...

 

[deiaggie]

my manual states it should be 2.39-2.672 kg/cm^2 right after you bleed down the pressure and first turn on the pump without running the engine. That would be 34-38 psi with a max pressure drop of 0.7 kg/cm^2 (10 psi) after 1 min. I am getting 10 psi after it is bled down and I first turn the key before it starts...and this is with a relatively new fuel filter (replaced within 2000 miles) and it lacked power before i changed it.

how else would I test for a leaking injector if its not really dropping the pressure after the pump is off?

 

[robertf]

check the vacuum line from the regulator to the engine. mine dried out and cracked. It was running very rich because of it and I think its the reason I had o2 error codes.

the part about the engine being 25 - 30 with the key off and 35-40 running is kind of misleading. it might rise a little since the fuel pump is running, but the engine running and producing vacuum will only lower the presure reading on the fuel rail. Its purpose is to keep the fuel pressure 30psi over engine pressure, and at atmospheric pressure that is 0 + 30 (gauge pressure, not absolute)

the part throttle acceleration might be normal, my 95 does it too. Could be a worn out tps. I need to check mine.

i checked leaking injectors by removing the fuel rail but leaving the injectors in the rail. put some paper towels down and turn the key on so the fuel rail primed. Didn't get any leaks. Didn't check spray pattern but the plugs all looked fine.

 

[deiaggie]

check the vacuum line from the regulator to the engine. mine dried out and cracked. It was running very rich because of it and I think its the reason I had o2 error codes.

the part about the engine being 25 - 30 with the key off and 35-40 running is kind of misleading. it might rise a little since the fuel pump is running, but the engine running and producing vacuum will only lower the presure reading on the fuel rail. Its purpose is to keep the fuel pressure 30psi over engine pressure, and at atmospheric pressure that is 0 + 30 (gauge pressure, not absolute)

the part throttle acceleration might be normal, my 95 does it too. Could be a worn out tps. I need to check mine.

i checked leaking injectors by removing the fuel rail but leaving the injectors in the rail. put some paper towels down and turn the key on so the fuel rail primed. Didn't get any leaks. Didn't check spray pattern but the plugs all looked fine.

vacuum line is new the regulator...it cant believe that it would be normal for a disco with stock tires not be able to go over 85 mph without going downhill in 3rd gear. Before the lift/tires it would pretty much stop accelerating after 80 mph and would slowly gain speed...any kind of hill it would slow down. I cant imagine towing anything...with 4 people it felt even worse

dont know how a actual test proceedure can be misleading...if it is supposed to have a certain pressure at the rail without the engine running and then a different pressure when the engine is running depending on speed (vacuum controlled). if the measurements were misleading then what would be the point?