2003 f115 fuel pressure regulator

[rejesterd]

If it runs better with the sense line disconnected, then I think the fuel pressure isn't right to begin with. Have you measured fuel pressure at key-on and while idling after warmup (both with and without the sense line)?

[99yam40]

Is there a reason you have not started your own thread with all of the needed info on your motor and your testing?

[boscoe99]

Yes, vacuum test the regulator and fuel psi went down.

Down to what? At the same rate that the pressure went down?

[BoatmanNH]

Is there a reason you have not started your own thread with all of the needed info on your motor and your testing?

Yes, the thread was relevant to my situation. Like I stated I’did everything beachpoint did, only difference is I have a F150. My hopes were that beachpoint would chime in what resolved his issue. Maybe he would get an email notification that someone replied to this thread. I’m new here.

[rejesterd]

Boatman - Just consider what happens when you unhook the sense line: that causes the fuel pressure to rise slightly. If it runs better after the fuel pressure has risen, that means it's too low to begin with. If you can't measure the fuel pressure, you might consider just sending in your injectors to be cleaned/tested (many companies do that service). To me, one or more stuck/clogged injectors could cause the issues you described. If they're sticking open, that reduces fuel pressure. If the service report reveals that they were fine and didn't need to be cleaned, at least you'll know that injectors aren't the problem. But from what I've read on this and other forums, clogged/stuck injectors seem way more common than a bad pressure regulator. There's also a filter inside the VST that you might consider replacing.

[BoatmanNH]

Today I checked fuel pressure with key on 41 psi. Running 36 psi, remove sense line 41 psi. I give it throttle with the sense line on and it coughs then goes. Without the sense line it’s perfect.

i had the opportunity today to compare it to another F150. Key on 42 psi. Running 38 psi. If I remove sense line there’s no change.

Diagnostics readings we’re almost comparable. Nothing out of the norm.

Previously to this post the whole fuel system was cleaned, including injectors and it did seem to run better out of the water. Water test failed. I’m leaning towards that regulator.

[rejesterd]

The 2003 F150 service manual says it should be 45 psi within 3 seconds at key-on and 38 psi at idle after it's warmed up. So you are a little low. That could be from stuck open injectors, a bad pressure regulator (however, you already did the vacuum test, I thought), a damaged VST needle valve or float, clogged fuel filters, (seems unlikely if you cleaned the fuel system recently), or a problem with the mechanical fuel pump.

Strange that the other engine's pressure didn't increase when the sense line was removed, however that idle pressure of 38 psi is right.

[zenoahphobic]

I think rejesterd summed it up, there could be a number things all of which need to be methodically checked (cleaned).

But I am confused about the regulators role in deliberately reducing pressure at higher power output of the engine. Logic would dictate more pressure is needed rather than less to provide more fuel. Unless the reduced pressure function is to put a cap on overall pressure maximum, like in waste gate on turbocharged etc etc.

[boscoe99]

I think rejesterd summed it up, there could be a number things all of which need to be methodically checked (cleaned).

But I am confused about the regulators role in deliberately reducing pressure at higher power output of the engine. Logic would dictate more pressure is needed rather than less to provide more fuel. Unless the reduced pressure function is to put a cap on overall pressure maximum, like in waste gate on turbocharged etc etc.

The regulator reduces fuel pressure at low power and increases fuel pressure as the power increases.

When the key is turned on before the motor is started the manifold pressure is as high as it will ever be. The fuel pressure will be as high as it will ever be.

Motor is started and run at an idle. The manifold pressure drops to as low as it will ever be. The fuel pressure drops to as low as it will ever be.

Start advancing the throttle and the manifold pressure rises. At the same time the fuel pressure will start rising.

Retard the throttle and the manifold pressure decreases. At the same time the fuel pressure will decrease.

Fuel pressure is tied to the manifold pressure via the hose that connects the pressure regulator to the intake manifold.

[zenoahphobic]

Boscoe you write well, seems a reasonable explanation to me.

What I can't come to grips with: what's the point of con*****ing such a change in psi (41 down to 38 or 36?) surely this does not exactly equate to the amount of fuel used in that system? Seems to me more precise pressure should be maintained and the amount of fuel should be precisely metered by the length of time the injectors are switched on. Why would you do both?

[99yam40]

I have a feeling it has to do with what Rodbolt was saying about the need for extra fuel during acceleration.
as the throttle is opened up the pressure in manifold raises for a small time, but once motor comes up to a steady RPM the manifold pressure stables back out.
it does not stay at the same pressure as when accelerating

[boscoe99]

Boscoe you write well, seems a reasonable explanation to me.

What I can't come to grips with: what's the point of con*****ing such a change in psi (41 down to 38 or 36?) surely this does not exactly equate to the amount of fuel used in that system? Seems to me more precise pressure should be maintained and the amount of fuel should be precisely metered by the length of time the injectors are switched on. Why would you do both?

Fuel flow is a function of the pressure of the fuel on one side of the injector and the air pressure on the other side of the injector. Let's say the fuel pressure is 45 psi and the air pressure is 15 psi. The injector opens for x seconds and y amount of fuel will flow through.

If the fuel pressure is raised, say to 55 psi, while the air pressure stays at 15 psi, then more fuel will flow through. Can we agree on this?

By the same token, if the fuel pressure remains at 45 psi while the air pressure is lowered, say from 15 to 10 psi, more fuel will flow through. Hopefully all will agree.

Now what is happening is that the air pressure within the intake manifold changes as the throttle is opened and closed. Throttle closed, less air pressure. Throttle open, more air pressure. The pressure regulator is sensing the changing air pressure within the intake manifold and is adjusting itself so as to keep the pressure difference between the fuel pressure and the air pressure the same. If the difference is the same then the fuel flow will be the same.

Capiche?

[99yam40]

Fuel flow is a function of the pressure of the fuel on one side of the injector and the air pressure on the other side of the injector. Let's say the fuel pressure is 45 psi and the air pressure is 15 psi. The injector opens for x seconds and y amount of fuel will flow through.

If the fuel pressure is raised, say to 55 psi, while the air pressure stays at 15 psi, then more fuel will flow through. Can we agree on this?

By the same token, if the fuel pressure remains at 45 psi while the air pressure is lowered, say from 15 to 10 psi, more fuel will flow through. Hopefully all will agree.

Now what is happening is that the air pressure within the intake manifold changes as the throttle is opened and closed. Throttle closed, less air pressure. Throttle open, more air pressure. The pressure regulator is sensing the changing air pressure within the intake manifold and is adjusting itself so as to keep the pressure difference between the fuel pressure and the air pressure the same. If the difference is the same then the fuel flow will be the same.

Capiche?

well that does make sense to me.

they are just trying to keep the same amount of fuel flow thru injectors for a set open time with different intake pressures.
then the ECU can then open the injectors more times or a longer time to get what ever amount of fuel it wants

thanks Boscoe,

[99yam40]

I agreed with boscoe and the post went off to be reviewed for some reason

[zenoahphobic]

I agreed with boscoe and the post went off to be reviewed for some reason

I had problems too posting.

I was also going to suggest like you did, Rodbolts explanation 5 years ago indicates that this was done to temporarily enriched the mixture under acceleration (throttle opening) .

One would be less inclined to use manifold pressure for this purpose in an automobile because manifold pressure (vacuum to most) may vary too much and will play havoc with the mixture during gear change and deceleration, Outboards are single geared and boat speed only has a small effect on keeping the engine revving when the throttle is reduced.

The OPs problem seems to suggest the system has been designed within a tolerance of only a couple of PSI's drop to cause a problem, or somehow that tube to the manifold is blocked not allowing the pressure to rise (anywhere near atmospheric pressure). Is there a restricter or valve in this system?