Hello, i have a 98 f150 4.6 2wd. I have been chasing a problem with intermittant rough idle and stalling for some time. Today i was prepairing to drain the fuel by way of disconnecting the fuel line before fuel filter and then jumpering the relay. Well i had hood up and went to move truck for better position and hear a relay going crazy whenever the rough idle set in. I went to the relay/fuse box and the fuel pump realy was pulsing very fast and loud. I put my finger on it and could feel it. I turned off truck and swapped it with another relay from that box with same p/n and problem stayed at fuel pump relay socket. What would cuase this? Where do i go from here?

 

Ok, so im thinking bad ground or the pump is failing. Disconnect electrical connector to fuel pump, pins like new. Where does the pump ground? Four wires come out, go thru that connector, then into budle towards front of truck via left frame rail.

 

Dropped tank to get to pump, vent bung broke off. So new tank, new fuel pump. NO fix, it has to be the wireing. Do i just run a new set of wires on the outside of the bundle or...? I have looked at the wire diagram in the haynes manual but have no clue where the ground wire goes. Anyone know?

 

Ok, key on eng off, relay goes intermittant so does fuel pump. Disconnect wire bundle connectors under air filter and pump stops relay still intermitant, also hear some random solinoids or something actuating near intake assy. I now think its a power feed issue not a ground, oh yeah took off cleaned and reassembled 3 ground points left hand eng firewall. So, my new question is;
What logic does the computer use to tell the fuel pump relay to send power to fuel pump?

 

DELETED

Sorry I thought you had a 96

 

Removed, cleaned, reinstalled wire connections at both mega fuses, starter solenoid. Wiggled every connection i can find, moved every bundle bend i can find, no fix and no clue as to what is causing problem. Its beyond me at this point without help. I need to sell truck. Delemia is do i drop a big chunk of money and have it towed to dealership to see if they can fix it? Or do i just try to sell it as is?

 

Hey there. If your truck was the previous generation, I would say you may be looking at an ECU problem. In this case, I am unfamiliar with your truck, but it is logical to look at the ECU here. The click you may be hearing near the intake could be the IAC. Also, the relay may ground to the ECU, as I say with this truck I am unfamiliar. Try pulling codes and see if you get anything strange. Also, try resetting ECU. With the way you describe it I am leaning toward an ECU problem......

Since you aren't getting much help, I will be happy to see what I can do. Please bear with me as I don't know this generation well, but I am well versed on the previous. In order to better understand what is going on, can you tell me:

When you turn key to position 2 do pumps prime? Do they run constant or not prime at all?
If they prime, how long do they prime for?
what happens when you go to start it?
Does it start?
Do the relays work as they are supposed to?
If not what are they doing?
Remember, the ECU controls the pumps so this is a key.....

Lets start here and continue after this....

 

Thanks for reply. Yes, pump primes, sometimes intermittantly when relay is pulsing, but completes prime cycle. Truck will start and run, but of course stumbles and dies occasionaly when relay pulses. As i stated earlier, truck has had issue for a while, when it first started had dpfe code. Changed dpfe, ran good for a month, then code returned. Disassebled intake found 1 of 2 ports between egr and aic area 100 percent clogged. Cleaned intake, cleaned tube that feeds egr dpfe from exhauhst, new aic, new dpfe, new egr. Now no codes but now found relay issue.

By reset pcm, do you mean dissconect batt? If so did this several times, a couple for several days, no fix.

Do you think the program could be messed up or would it be more likely the pcm hardware? Can they reprogram a pcm off truck?

I just ordered a factory manual cd, off course the sent the wrong year, so waiting to hear back from them so i can get correct manual to help my shoot the wiring.

Thanks for your time

 

As for relay, best i can say is yes, its working, have swapped it with the other relays, problem stays at pump relay slot. When relay is not pulsing truck runs like new. Then relay pulse, rpms fluctuale, sometimes recovers, sometimes dies. Very audible, can feel it when touched.

 

If you don't have access to an oscilloscope you can use a digital meter to find out if power loss is on primary side or secondary side of relay. Once you've determined this you can troubleshoot further.