Mercury nightmare.

[Turdle]

30 PSI is not "way too low" ---however it is a tad low IMO
(ok, it is 25% low) which could be termed "way too low" but it is not down by 50% as one guy insists.



The spec is indeed 30-45 PSI for a 98 v8. I would want to see more like 35-40 though at idle, with 45 psi shown during a throttle blip

Question to the OP
What is the fuel pressure while cranking the engine over?

What is the fuel pressure during a short full throttle "blip"

 

[Joe Dirt]

Alright I'm not sure why everyone thinks I'm so uncertain but I think I've said it enough times to be clear. I'm going to stop chiming in. Good luck OP, be sure to let us know what you find as you trace the fact that your fuel pressure is way too low.

Dude, don't get all pissy. The point was, that the spec numbers don't jive completely with your comments. You're very certain- but the manuals, and performance numbers don't necessarily jive with your position. If we knew why you were so certain rather than just saying that's the way it is, and it's half under spec when it isn't, it might be easier to take this path as a fix- especially since your first comment was that it needed to be at 100psi.

Maybe more information would help us recognize your position better.

 

[SoNic67]

I am puzzled... Why would the pressure be higher when you press the throttle? Is the PCM speed-controling the pump? I always assumed that the pump gets constant voltage... in that case, the pressure would drop when the fuel flow increases.

 

[cm8809]

I had the same symptoms in my truck and my fuel pressure was ~30 psi. Replaced fuel pump, assembly and filter and now it runs like new with twice the pressure. I'm just trying to help.

 

[Turdle]

I am puzzled... Why would the pressure be higher when you press the throttle? Is the PCM speed-controling the pump? I always assumed that the pump gets constant voltage... in that case, the pressure would drop when the fuel flow increases.

The FPR is vacuum controlled. Less vacuum=more fuel pressure. More throttle=less vacuum.

 

[cm8809]

I am puzzled... Why would the pressure be higher when you press the throttle? Is the PCM speed-controling the pump? I always assumed that the pump gets constant voltage... in that case, the pressure would drop when the fuel flow increases.

Fuel pressure regulator. More throttle = more fuel (= higher pressure)

 

[Turdle]

I had the same symptoms in my truck and my fuel pressure was ~30 psi. Replaced fuel pump, assembly and filter and now it runs like new with twice the pressure. I'm just trying to help.

60psi is way too high for a return fuel system. It is spot on for a returnless system though.

 

[cm8809]

60psi is way too high for a return fuel system. It is spot on for a returnless system though.

Well, my truck runs awesome and that was the problem. Just my experience, It is what it is.

 

[Turdle]

Well, my truck runs awesome and that was the problem. Just my experience, It is what it is.

Could be your fuel pressure gauge is off? Just saying

 

[cm8809]

Could be your fuel pressure gauge is off? Just saying

Nope.

 

[SoNic67]

The FPR is vacuum controlled. Less vacuum=more fuel pressure. More throttle=less vacuum.

But then the pressure measured in that point won't have nothing to do (or verry little) with the pump itself, correct?
The discution was about the fuel pump pressure. I was thinking the pressure was measured ahead of FPR (to be relevant for the pump)...

 

[cm8809]

But then the pressure measured in that point won't have nothing to do (or verry little) with the pump itself, correct?
The discution was about the fuel pump pressure. I was thinking the pressure was measured ahead of FPR (to be relevant for the pump)...

That's what I've been referring to, yes.

 

[Turdle]

I'd like to know how you measured fuel pressure upstream of the regulator-there is no test port for this.

 

[cm8809]

I'd like to know how you measured fuel pressure upstream of the regulator-there is no test port for this.

Um take the pump out?

 

[gavin]

Um take the pump out?

yes, the pump itself should be able to put out around 60-65psi.

but when testing at the test port on the fuel rail, the reading should be 30-45 to be within spec. Since the FPR regulates the fuel pressure at the rail, and the test port is after the FPR.

All the talk about fuel pressure, in this thread, is based on the testing being done at the schrader valve/test port on the fuel rail; not the pump itself.

 

[cm8809]

Nowonder everyone seems to think I'm mistaken.

 

[ranger7ltr]

Taking the pump out of the tank will give...

Lots and lots of pressure...Yet the return line system[pre 1999] has a regulator that keeps the fuel system pressure between 30-45 psi...As the vacuum is higher on the regulator at closed throttle, the pressure is lower in the fuel rail... 30 psi is within Ford spec...Oh and Ford did not use the PCM to control the fuel pump so the pump just runs as long as it has power...

The Ford returnless fuel system runs the same pump yet they run a higher pressure on the fuel system...65-72 psi...There is no regulator on the fuel rail and the regulator just prevents overpressure getting to the fuel rail...Thus the regulator is in the fuel tank without a vacuum line...

As lots of other posters have stated, the 30-40 psi your 1998 Explorer sees is good pressure...Most people would have a problem with the pressure regulator NOT working and letting the pressure going higher than spec...And 30-40 psi is the Ford spec for a return type fuel system...65-72 psi is spec for a returnless Ford fuel system...Anything higher than that is too high and the engine will run pig rich if it runs at all...

 

[gavin]

Nowonder everyone seems to think I'm mistaken.

because fuel pressure is always tested at the rail first, since it requires no dismantling.

 

[Turdle]

I'd like to know how the pump pressure was tested without a bunch of special fittings or special tool. Fill us in please.

 

[ranger7ltr]

I have used a stock pump for draining and bypassing the tank and pump setup to eliminate them from the truck to provide fuel to the engines' fuel rail...I have done this on Fords and other vehicles as well...

The stock pump output when it has power will hit 65-75 psi...And I don't worry about the pumps' output pressure since either the vehicle has a fuel pressure regulator upstream of my fuel inlet[usually at the filter] or the pressure output from the pump is close enough to run within the manufacturers specs...