Fuel Pressure Question 1999 AWD 5.0

[HeavyEX]

Hi,
I have been having the p0171 p0174 code for a while now and have done tons of work assuming there is a vacuum leak. Since I can't find one anywhere I have moved on to testing the fuel pressure.

I do not drive this car much, but I replaced only the pump and not the regulator about a year and a half ago because the pump totally failed. I replaced it with the Airtex Master Pump that Autozone shows for this model. Now that I have tested the fuel pressure...

If I just turn on the key the fuel pressure goes to about 20-25 psi

When running ad idle pressure is 35 psi

at 2000 rpm it holds pretty steady at 35 psi

at 4000 rpm it drops to 30 psi

when the key is turned off it starts dropping immediately to about 25-30 psi and then gradually to 20 psi

I just checked it again after about 15 minutes it is down to 17 psi

After sitting for days it was at 0.

So it seems to me that it should not leak down like this but I am not 100% sure where the fuel is going since I can't see any leaks. I assume back into the tank. Also I keep finding 2 different figures for where the fuel pressure should be while it is running. Some say 35-42 some say 56-72. Would the pump I put in be the wrong one? Since it is the old regulator does the pump matter? I could use a little guidance before buying a pump for 200-300 dollars. Thanks a lot in advance.

 

[koda2000]

i'm not sure what the correct pressure for your vehicle is, but what you're reporting sounds low to me. i'm sure someone else will reply shortly with more specific info, but one problem i've read about is that there's a small piece of hose between the fuel pump and the pipe that carries the fuel to the line. this hose can leak where it's clamped to the pump or pipe. as the leak is internal to the fuel tank, you have no way of knowing if it's leaking but it will reduce fuel pressure at the rail. of course there are many other things which will cause you to have low fuel pressure (the pump getting weak, fuel pressure regulator going bad, fuel damper going bad, clogged fuel filter).

 

[HeavyEX]

That's interesting...I think I do remember that hose. I have changed the fuel filter already 2 times so I don't think that is the issue.

Just cheked AutoZones website and the pump is listd as having a max pressure of 100 psi...I guess it is possible it could be weak but would the regulator be underregulating the pressure for some reason?

 

[99SportX]

On a 99 you should be seeing about 65psi. The pressures you are seeing are indicative of the 98 or earlier system I believe.

 

[HeavyEX]

Ok. I checked Autozones website and they show the same part number for the pump on a 98 and a 99 however if you buy the whole assembly it has different part numbers, so I think the pump I put on is ok.

About an hour later I went out and the pressure had dropped even further to 8 psi.

So I guess my main question is, what can cause the car to run at a fairly constant pressure of 35 psi if that is not where it should be? If the regulator fails, can it just be weak and run at 35 psi or would it be a total failure? is there some other sending unit that could cause this? I hear the damper at the fuel rail is just for smoothing out the pulses, so it doesn't seem to me like it would affect the pressure this way.

 

[kevinspann]

Sounds like it needs a fuel pump to me. Low pressure, check valve not holding pressure. Replace it.

 

[HeavyEX]

Ok, I think this time I will replace the whole thing with the sending unit and everything so I don't wind up having to do it twice. However, I have no idea which brand would be the best, or at least not the worst. Which brand do you guys recommend...besides OEM.

So far I have found...

Herko
ADP
Spectra
Bosch
Pierce
Carter
Airtex

Which should I avoid?

 

[96eb96]

When there are multiple fuel pump failures it may be a good idea to verify the pump is getting a generous voltage signal. Maybe this is not exactly your case but the first thing is to verify you have VBatt at the fuel pump connector.

Also, Put a voltmeter from battery (-) to the fuel pump neg terminal. Theoretically this should read zero(or very close) if it is a perfect connection. If you have any voltage drop, there is some resistance in the circuit and it is robbing the fuel pump under demand. Motors can actually burn out from low voltage.

See the video for more info. Also, I believe Carter is the OEM so you are ok with that choice.

 

[bigcrunch]

Ok, I think this time I will replace the whole thing with the sending unit and everything so I don't wind up having to do it twice. However, I have no idea which brand would be the best, or at least not the worst. Which brand do you guys recommend...besides OEM.

So far I have found...

Herko
ADP
Spectra
Bosch
Pierce
Carter
Airtex

Which should I avoid?

Airtex !

 

[99SportX]

You may very well have a leak in the hose between the pressure regulator and the pump. As a result, you will never get to full pressure since fuel is spraying out the hose when the pump is running. You need to make sure that the hose used is rated for submersible type use, otherwise it may degrade and leak. I am curious as to what you find. I have a fuel pressure leakdown issue on my xlt. I get 65psi all day long when the pump runs, but once it shuts off, it slowly leaks down, but faster than it should.

My sport is dead right now because the fuel pump just wont prime, but that is a whole different issue. I think the pump is just plain dead as the relays and electrical are fine.

 

[HeavyEX]

That video was amazing! I never would have guessed. I think I will go ahead and order a pump, since I have the leakdown issue too. I guess it could very well be the hose, but I want to have the pump to replace it just in case it is not performing well. I would hate to have to drop the tank twice. Last time I did it was a huge pain in the butt. I will check the ground while I have the tank off so it is easier to get at both ends. I will report back. Thanks guys.

 

[turboplz]

One quick question, when you are checking the fuel pressure, is vacuum connected or disconnected to the FPR?

 

[96eb96]

That video was amazing! I never would have guessed. I think I will go ahead and order a pump, since I have the leakdown issue too. I guess it could very well be the hose, but I want to have the pump to replace it just in case it is not performing well. I would hate to have to drop the tank twice. Last time I did it was a huge pain in the butt. I will check the ground while I have the tank off so it is easier to get at both ends. I will report back. Thanks guys.

This is another part of the returnless fuel diagnostic - worth watching too:
He has many other great videos that are definately worth watching.

 

[HeavyEX]

One quick question, when you are checking the fuel pressure, is vacuum connected or disconnected to the FPR?

I am not sure I understand the question...if by FPR you mean Fuel Pressure Regulator then I thought the Regulator was all electronic. I didn't know there were any vacuum lines running way back to the tank. If there are though, I did not disconnect any of them before checking the pressure at the rail.

 

[koda2000]

there's no vacuum line running to the tank for the FPR. i don't think the '99 5.0 has a vacuum controlled FPR on the fuel rail. i believe the FPR is part of the fuel pump assembly in the tank and it is electronic. sees to me i read there may be a fuel "damper" on the engine, however.

 

[99SportX]

The FPR isn't electronic. It is just an inline relief valve just after the pump inside the tank. Once the line pressure gets to 65psi it starts to dump excess pump flow inside the tank, maintaining 65psi. The pcm doesn't modulate the pump. It's always on when the truck is running. The fuel damper shouldn't need to be disconnected for checking pressure.

 

[HeavyEX]

I finally got a day off to change out the fuel pump. Got a Carter off Ebay...I think I fixed the problem. Truck accelerates better and no engine light so far. I am keeping my fingers crossed. Thanks a lot for all your help.