How long is my 95 F150 w/ 5.0 302 supposed to hold pressure after shut off? It builds up about 40 psi maybe a little less while running or when the pump first primes with the key turned on. After that it immediately drops to about 20 and then slowly continues dropping.

Thanx

 

The fuel regulator sets pressure at 13-17 psi so anything higher and you may have something wrong.
After you shut the engine off, the fuel pressure should gradually drop off.

 

I thought it was supposed to be between 25-40 psi. I'll check the book when I get home.

 

The fuel Psi for a 95 302 should be KOEO - 35 to 45 Psi . KOER - 30 to 40 Psi . The fuel pressure regulator works off engine vacuum so therefore the fuel pump pressurizes the system to its max Psi when the key is turned to run , Once the engine starts vacuum is applied to fuel pressure regulator & it reduces the pressure . If you lose fuel psi soon after , a sec or two after the system pressurizes, then its likely the fuel pump is on the way out . Is the engine taking awhile to start in the mornings ? A simple test to determine if the fuel psi regulator is functioning properly is to start the engine & remove its vacuum line , if the psi goes to 45 or there abouts & returns to 30 to 40 psi when the vacuum is applied & no fuel leaks are present in and around the regulator then its likely not the cause of the pressure problems . On a properly operating fuel system the psi should drop off approximately 1 psi per min , The lower the psi drops the slower the drop rate , Ideally , it should take several hrs to return to 0 psi .

 

Sorry for the mix up about the fuel pressure.
It was late, and I was tired. It was about 4:20 am when I posted it.

 

[updated:LAST EDITED ON 21-Dec-01 AT 01:39 AM (EST)]Just a clarification on fuel pressure regulator. On the V8's, the center value is 39 PSI across the injectors. When doing the Key On, Engine Off, the intake manifold is at atmospheric pressure, so the vac line on the FPR see's atmospheric, so it's 39 PSI.
When the engine starts and runs, the intake manifold is now approx. 20" of vac, or -10 PSI compared to atmospheric. The FPR's vac line is sensing that vac. But when we measure a running engine via the Fuel Rail Test Port, our gauge compares the Rail pressure to atmospheric pressure, and that fools us into thinking that the fuel pressure dropped on the running engine, which is wrong. Need to take the running value, and add on to it how many PSI below atmospheric the intake manifold is. 30" of Mercury = 15 PSI, so 20" vacuum = 10 PSI to add on to the gauge reading.
You have to pass the Science Final Exam before you can go home for Christmas

 

What?.. did you say atmospheric pressure is 39psi? I didn't understand a thing you said. LOL

 

Torky , What elevation do you live ? For most of us we all live Id say within a 1000 or so feet of sea level . I understand what you said , but I cant see it applying to or making a whole lot of difference to anyone who lives below 5 or 6000 ft Asl . Even on a hot humid day the density altitude wouldnt affect the fuel psi as it relates to the reading enough one way the other .

 

Paul,
I was just trying to say that the fuel pressure doesn't really "drop" when the engine is started, rather, the reference point shifts down. And it's more difficult to measure properly.
For the KOEO test, the injectors spray end (in the intake manifold) is at outside pressure, because the engine isn't running. And it's supply end is at 39 PSI give or take some tolerance, above outside pressure. And the fuel pressure gauge, like pressure gauges do, measures the connected pressure relative to the outside pressure that the gauge head is at.

When the engine is started, the intake manifold drops below outside pressure (partial vacuum), so the spray end of the injector is in a area less than outside pressure. The Fuel Pressure Regulator, with its intake manifold vacuum pickup, still keeps the pressure drop, that is, the Fuel Rail pressure minus the pressure at the injector tip, at 39 PSI. But since that 39 PSI is relative to the negative pressure (vacuum) at the injector tip and intake manifold, the external gauge attached to the fuel rail will see a drop, because it measures relative to outside pressure, NOT intake manifold pressure.

What I outlined is a way to correct for that apparent, but bogus, "drop" in pressure when the engine is running. The fuel pressure regulator keeps a constant pressure drop across the injectors so the computer can meter out fuel by percentage duty cyle ON vs. OFF, and not get screwed up by the pressure changing the flow rate through an injector.

I hope this is more clear

 

[updated:LAST EDITED ON 22-Dec-01 AT 11:22 PM (EST)][font color="blue"]I was just trying to say that the fuel pressure doesn't really "drop" when the engine is started, rather, the reference point shifts down. And it's more difficult to measure properly.
For the KOEO test, the injectors spray end (in the intake manifold) is at outside pressure, because the engine isn't running. And it's supply end is at 39 PSI give or take some tolerance, above outside pressure. And the fuel pressure gauge, like pressure gauges do, measures the connected pressure relative to the outside pressure that the gauge head is at.
When the engine is started, the intake manifold drops below outside pressure (partial vacuum), so the spray end of the injector is in a area less than outside pressure. The Fuel Pressure Regulator, with its intake manifold vacuum pickup, still keeps the pressure drop, that is, the Fuel Rail pressure minus the pressure at the injector tip, at 39 PSI. But since that 39 PSI is relative to the negative pressure (vacuum) at the injector tip and intake manifold, the external gauge attached to the fuel rail will see a drop, because it measures relative to outside pressure, NOT intake manifold pressure.

What I outlined is a way to correct for that apparent, but bogus, "drop" in pressure when the engine is running. The fuel pressure regulator keeps a constant pressure drop across the injectors so the computer can meter out fuel by percentage duty cyle ON vs. OFF, and not get screwed up by the pressure changing the flow rate through an injector.

I hope this is more clear

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Torky, are you by chance an engineer by profession?

I ask because you've taken a fairly simple concept and made it seem far more difficult than necessary. You've also managed to be both correct and incorrect in the same post.

This is the correct part (your words in italics, mine in bold):
The fuel pressure regulator keeps a constant pressure drop across the injectors (regardless of manifold pressure) so the computer can meter out fuel by percentage duty cyle ON vs. OFF, and not get screwed up by the pressure changing the flow rate through an injector.


This is the part that's wrong:
What I outlined is a way to correct for that apparent, but bogus, "drop" in pressure when the engine is running.

The drop in fuel rail pressure (when the engine at idle speed) is very real. The same drop can be achieved with the engine off (manifold pressure = atmospheric pressure) if a hand vacuum pump is used on the fuel pressure regulator's vacuum port.

Remember, when you connect to the test port you are measuring rail pressure. Rail pressure varies with engine vacuum (low vacuum = higher pressure). It is this variation that allows the pressure drop across the injectors to remain constant.

In other words, when manifold vacuum is high (negative pressure), the fuel pressure regulator lowers the rail pressure a corresponding amount in order to maintain a constant pressure drop across the injectors.

While all of the above is a nice technical discussion, it doesn't go very far towards resolving a fuel pressure leakdown problem. Paul addressed that quite well & offered possible solutions (i.e. leaking regulator diaphragm, stuck regulator valve or a leaking fuel pump check valve.)

We try to keep things simple around here. Most folks who visit these forums have a hard enough time following the OEM service manual, let alone advanced theory.

Merry Christmas

 

[updated:LAST EDITED ON 23-Dec-01 AT 02:03 AM (EST)]Why, yes I am! Thank you!

The pressure situation is a "frame of reference" issue. The Fuel Rail pressure does NOT change. It is relative to the injector tip, to be constant "across the injectors" at 39 PSI + -. But we can not easily measure this as the injectors see it, as our pressure gauge uses atmospheric as one side. If we had a special pressure gauge that had a reference port that could be hooked up to the intake manifold (comparing the test port to the reference port), then all would be OK. But we don't, our gauge compares the test port to atmospheric, so it looks like the Fuel Rail pressure "drops" when the engine is started. It's the frame of reference.

Message received on the technical part. It's just that I am against "dumbing down". Aim high, not everyone will understand, but that's OK, that is how we learn (myself included in that!). We are in a worldwide race. People in Japan, Korea, many other places are not dumbing down. They are aiming high. I look at science textbooks of today's kids, and I see why few are interested. It's been reduced to lowest common denominator drivel!

I'm off for the remainder of the year, I'll let the little elves take over ;-) , and I'll get off my soapbox.

Have a Happy New Year