I got a 93 F150 with a 4.9 eng and 5 speed trans, 2 wheel drive, it has duel tanks. When the truck sits over night it loses all fuel presure on the fuel line, and each morning i must sit and change from tank to tank untill the fuel pumps cut off,(after it sits the fuel presure will read next to zero, after i get the pumps to presure the line back up it reads around 43 psi, give or take) then the truck will start. I have installed new pumps in both tanks, new fuel filtes and a new fuel presure regulator. I also have checked for leaks in the fuel lines and ect..

 

I am trying to read between the lines on this one.
Zero pressure over night is normal.
Sounds like the selected fuel pumps run until it will start (a lot more than one second).
So to me it sounds like a bad computer.

 

Yes it takes like 20 mins of running the fuel pumps in the morning before it will start. And as time goes on it takes longer and longer before they kick off and the truck will start. So you think it might be a bad computer?

 

I don't understand what you mean by this. How are you "running the fuel pumps" for 20 minutes?

When you turn the key to the on position (and not rotate further to start), do you hear the fuel pumps run for 2 seconds and then shut off?

 

When u go to start the truck up u can hear the fuel pump come on but it doesnt go off for a long time. i change back and forth between the 2 tanks and pumps untill the pumps go off. when they shut off is when the truck will start. So yes key on and off untill the pumps get the presure high enough on the fuel lines for it to start. If you try to start the truck before the fuel pumps cut off it will not start. And turning the key on and off ( and changing tanks ) untill the pumps kick off can take as long as 20 mins.

 

Nothing to do with the fuel pumps or fuel pressure. Your computer is failing.

The computer is designed so that once it receives power it automatically turns the fuel pumps on. Once the computer has started up properly, it turns the fuel pumps off after about ~1 second. Constantly running fuel pumps means the computer is not starting up.

 

Ok.. And thank you very much!!

 

Precisely what I was about to say. The only other possibility I'll add is that it could be a marginal EEC-IV relay. I'd probably try that first since it's even easier to swap and I always have a spare in the glovebox any way. It's one of the 3 or 4 relays that sits above the driver-side wheel well under a cover.

 

Yes it is a bad computer.

There are no pressure sensors for the fuel system. The fuel pumps or the computer has no idea if there is pressure in the fuel system or not.

When the key is turned on the timer in the computer grounds the fuel pump relay for one second and the then drops the fuel pump relay back out.
When the engine is then cranked the computer will see a PIP pulse from the distributor it will start the one second timer all over again. The distributor will send more than one PIP pulse per second when the engine is cranking or running so the the fuel pump will run all the time the engine is cranking or running.

When the fuel pumps run for a long time with the key just turned on and the engine is not cranking or running the clock in the computer is not running.
Could be bad Electrolytic Capacitors or a bad crystal in the computer.

Some posters have changed these and then the computer works OK again.

 

I have the same truck except auto trans. Same problem 3 month ago. It's a bad computer, a computer going bad. I fixed mine myself. Can you solder or have a friend who does? The capacitors are leaking or popped. Mine were leaking and I found corrosion under one which ate through a track. I replaced all caps and jumped the broken track with a wire out of a cat5 cable I had.

Pull the computer out of the truck. Disconnect the main connector from the firewall area beneath the brake cylinder. Go inside and remove the kick plate. You will see a metal plate and look up and behind it u will see the computer. There is a tray that it sits in and a lock tab holding it in. I had to bend the tab metal out of the way and wrangle it out.

I think there are 2 screws maybe not (writing this from memory) anyway the cover is not hard to open. Find the caps and replace them with the same values and the same voltages. U can go higher voltages but not lower. Also the higher voltages may be larger in size but as long as they fit in the case. I scavenged some computer boards, namely older modem and one motherboard. Radio shack might still have an assortment or any local electronics dealer in your area. Mouser electronics on the web is another good source if u can wait.

Reinstall and happy driving.

 

The computer should be taken out from under the hood. You will have to take the screws out of the inter fender and pry it back some. Then the computer comes out through a gap between the fender and the inter fender. You do not have to do anything inside the cab.
You will have to back out the center screw of the wiring plug, and two small nuts one above and one below the computer plug.
You do not have to pry anything back in the cab.

Center screw in plug:


Computer location:


/

 

[QUOTE=subford;12377512]The computer should be taken out from under the hood. You will have to take the screws out of the inter fender and pry it back some. Then the computer comes out through a gap between the fender and the inter fender. You do not have to do anything inside the cab.
You will have to back out the center screw of the wiring plug, and two small nuts one above and one below the computer plug.
You do not have to pry anything back in the cab.

I guess that's why mine was a bear to take out.

 

I thank you all for your help and the info of where the computer is.. I had no clue where it was hiding on this truck..lol

Thank you all very much

 

That's what we are here for. Your Welcome