96 explorer 4.0 with low fuel pressure?

I am chasing a very slight consistant stumble at idle. I am getting
28psi off the fuel rail. When I unplug the vacuum hose off the fuel
pressure regulator I get 38psi. Turn truck off and it held 38psi fine
for 20 minutes. I have a new fuel filter....what can cause the low
pressure?? I believe it should be 35psi?? Thanks

wrote:
> I am chasing a very slight consistant stumble at idle. I am getting
> 28psi off the fuel rail. When I unplug the vacuum hose off the fuel
> pressure regulator I get 38psi. Turn truck off and it held 38psi fine
> for 20 minutes. I have a new fuel filter....what can cause the low
> pressure?? I believe it should be 35psi?? Thanks

Those symptoms sound more like the MAF sensor or IAC valve. Both can be
cleaned (carefully).

Here are instructions for cleaning the MAF:
http://www.4x4central.com/tips.htm#mas

Does it run well other than idle? Is the "check engine" light coming
on?
-Paul

Not sure what you are asking, because I don't see any problem with your fuel
pressure. Fuel pressure is regulated ABOVE the manifold. That's the purpose
of the vacuum hookup.
In any case, if the engine only stumbles at idle, the problem is not with
fuel delivery -- the demand for fuel at idle is very low. Did you check the
IAC?

" " > wrote in message
...
> I am chasing a very slight consistant stumble at idle. I am getting
> 28psi off the fuel rail. When I unplug the vacuum hose off the fuel
> pressure regulator I get 38psi. Turn truck off and it held 38psi fine
> for 20 minutes. I have a new fuel filter....what can cause the low
> pressure?? I believe it should be 35psi?? Thanks
>

Hello Paul Thanks for the response!! I have replaced the
IAC(probably didnt need to). Cleaned the Maf a few times. No check
engine light. Did all the 02 sensors. New fuel filter,temp sending
sensor(computer),coolant temp sensor(gauge),thermostat,catalytic
converter,egr valve,dpfe(egr pressure sensor),air filter,camshaft
position sensor,removed and cleaned throttle body,new platinum pulgs.
Yesterday I checked the fuel pressure and came up with the low pressure.
Truck runs perfect with exception of the idle issue. Thanks again
Paul!! Nel

Brand new Iac. I thought fuel pressure remains the same whether at idle
or at higher rpm?? ThanksNel

wrote:
> Brand new Iac. I thought fuel pressure remains the same whether at idle
> or at higher rpm?? ThanksNel

I checked the manual for my 94 4.0L OHV engine. It gives three specs
for fuel pressu

Key on, engine off: 35 to 45 psi.
Engine idling: 30 to 35 psi.
Engine idling, vacuum hose disconnected: 35 to 45 psi.

It also says that the pressure should drop by 3 to 10 psi when going
from key on, engine off, to normal idle pressure.

The vacuum hose mentioned is connected to the fuel pressure regulator-
I guess they want more fuel pressure at full throttle than at idle.
-Paul

wrote:
> Hello Paul Thanks for the response!! I have replaced the
> IAC(probably didnt need to). Cleaned the Maf a few times. No check
> engine light. Did all the 02 sensors. New fuel filter,temp sending
> sensor(computer),coolant temp sensor(gauge),thermostat,catalytic
> converter,egr valve,dpfe(egr pressure sensor),air filter,camshaft
> position sensor,removed and cleaned throttle body,new platinum pulgs.
> Yesterday I checked the fuel pressure and came up with the low pressure.
> Truck runs perfect with exception of the idle issue. Thanks again
> Paul!! Nel

Sounds like you've eliminated a lot of things. One other possibility- a
manifold vacuum leak. Those can be tough to find. Check the usual
suspects (hoses and their connections) and then there is the nasty one-
the lower manifold seals.

Explorers have had problems with bad lower intake manifold seals, so
much so that the factory paid to have them replaced. I haven't
experienced that myself, just read about it. Maybe someone who's
experienced this problem will describe the symptoms and any
troubleshooting tips they learned.
-Paul

Let's try again: Fuel is injected into the manifold. Hence what counts is
not absolute pressure, but the difference between the regulated pressure and
the pressure in the manifold. That's what stays (reasonably) constant. At
idle, the manifold pressure is low (high vacuum), so to compensate for that,
the absolute regulated pressure drops. At full throttle the manifold
pressure rises (low vacuum), so the absolute regulated pressure has to go
up. The regulator uses the vacuum hookup and a diaphragm to accomplish this
trick. Hope it's clear now.

" " > wrote in message
...
> Brand new Iac. I thought fuel pressure remains the same whether at idle
> or at higher rpm?? ThanksNel
>

Just came back from mechanic and he sprayed some carb cleaner around
upper intake..engine did funny things. Said to take off intake and
examine it as it is plastic. I am going to do that now. Just got a
gasket and am keeping my fingers crossed that the intake is not cracked.
Thanks and Ill keep you posted!! Thanks againNel

Yes that has cleared it up!! Thanks for making me understand! I am
used to my 71 460 big block mach1..its an alcoholic...just keeps
drinking and never complains or breaks down!!Nel