To start i have dual tanks and run off the rear tank only, as if I try to run off front tank it fills the back one.
Now I got a fuel pressure gauge hooked it up, and turned key on and nothing, no pressure. The pump does its 2 or 3 second cycle. even tried it on front tank and nothing. Try to start truck and pressure comes up and engine starts but never goes above 32lbs on either tank. Remove FPR vacuum and pressure goes up to 40lbs.
Then shut the engine off and gauge goes to zero in about 5 seconds.
Ok, now where should i start? FPR, or looking for a leak like injectors? Not happy right now.

 

My truck does the leak down like that as well I was told leaky injectors or bad fdm check valve.

 

What year is that truck? I wonder if we get the fuel pumps to run continuously (like they do when the engine is running) if they'll ever build pressure. That would help point us in the right direction.

Now before we do that, I noticed you said you run off the rear tank only but were trying to test pressure from the front.
Is there any fuel in that front tank?

 

1992 5.0,
Now if i keep on cycling the fuel with the key but not starting the motor, it will build up to 35lbs after about 6 or 7 cycles, but if i stop cycling the pump and in about 3 secounds goes right to zero lbs. Now that works exactly the same on either the front or back tanks(same results either)
Yes there is fuel in front tank, I use that tank as my reserve, when back tank get low I run off front tank just for 5 or 10 minutes and it fills back tank.

 

Should i change out the FDM on the rear tank seems that is the tank that receives fuel from the front one or should I change the entire pump and FDM? Maybe that will increase fuel pressure and maintain fuel pressure after key off for more then 3 seconds as it does now.
Or should I lean toward rebuilding all the injectors?
I believe maybe one or both of these are the cause of my little rough idle i have, and longer cranking to start??? As i find if I cycle the key 3 or 4 times then the truck starts right up, but still a little rough idle.

 

Pull the plugs to see if any are wet, if they are then they are leaking but if not then your problem is elsewhere.

 

Question: If the FDM is not operating properly and allowing the front take to pump fuel to rear tank, would that not create a relief opening for the fuel pressure to not build up quickly and to bleed off quickly as well?

 

If running the front tank is filling the rear tank, you have a problem with the tank selector valve. Replace it.

If you want to test your pump, ground the fuel pump test connector at the EECIV test connector. It will engage the fuel pump relay continuously if the key is on.

Yes, a leak at the selector valve could bleed off fuel pressure.

 

I thought my truck did not have a tank selector valve?

 

If you have 2 tanks, you have a selector valve. Yours will be electrically operated, but it does actually switch between the 2 independant fuel pumps in the front and rear tanks. It also switches the electrical power between the front and rear tank pumps as well as the gas gauge signal.

Good explanation of it here;

https://www.ford-trucks.com/forums/1...tor-valve.html

 

OK, where is this selector valve located at. I looked on the frame rail drivers side, but did not see an electrical valve.

 

No, the later trucks did NOT have a standalone selector valve. Each FDM has a shuttle valve and a check valve for controlling fuel flow in/out of itself. These valves are hydraulically operated by fuel pressure alone.

 

Ok thanks,
So back to my previous question. If the FDM/ check valve is not closing on my rear tank allowing front tank to fill the rear. Will that cause my fuel pressure to not build up quickly and also bleed off real quick when you turn key off?

 

Yes. It absolutely will, and can also cause low fuel pressure even while running. This is what was happening with mine.

 

Thanks, thats what I was thinking. Kind of looks like I may be dropping my rear tank soon. I think i may change the entire pump/FDM as long as the tank is out.
I really hope this will fix a few problems I have been having since I bought this thing six months ago. I have had a little rough idle and slight hesitation under light throttle only. And also long cranks to start unless you cycle key and fuel pump on and off 3 or 4 times quickly then start ups are quick.