I think I found the real problem with my old 87 F250, so help me out guys. I need my toy hauler on the road so I don't miss out on a whole summer of use.

Long story short, the truck might run for an hr, or only one minute and shut off, it may not start again for one minute, and sometimes it will start right back up and die again.

I think the real problem has surfaced with my F250 quitting at random. After trying the system of elimination, the problem was still there. I replaced the high pressure frame mounted fuel pump, EEC and fuel pump replays, EGR valve and wiring thingy on top, full tune-up, and a brand new starter since I fried the last one just to get the truck home.

Well, I had thought something electrical was the problem, but hadn't got a chance to start checking everything. Last weekend after putting the new starter on, I fired up the old 240k mile F250 and smoke started coming out of the dash. I quickly shut it off and started grabbing the wires and parting them to find where the short was. The whole harness to the tank selector switch was HOT and also the two wires to the fuel inertia switch were too. I took the tank selector switch off and checked it and pins #1 and #2 were so hot it had melted the plastic around them. The very next (3 inches away) connector was melted together and I couldn't get it unclipped. I pulled the tape off the wires between the tank switch and that clip and the same two wires coming off pin #1 and #2 had melted together.

This truck has had a short in the fuel gauge, when on the front tank, that makes the fuel gauge needle go up and down like crazy. When I look at the wiring diagram, the #2 pin/wire goes right to the fuel gauge. The #2 also splices in before the fuel inertia switch and the high pressure inline pump. The book says all power has to go through the inertia switch.

Wires #1, #2, #4, all go to the front intank pump when switched on.



I know I've got to replace the tank selector switch, and the harness as far as it's melted/damaged, but what do you guys think caused this? A bad ground, Inertia switch short, tank selector switch bad, front intank pump wiring short, or the wiring to the high pressure pump?

I’m thinking where ever the short is causing this, that it is cutting power to the high pressure pump and/or the front intank pump.

 

Oh yeah, where is the best place online to buy a new wiring harness and front tank fuel pump assy?

 

...Welcome to Ford Truck Enthusiasts! Please read the Guidelines, it will help you to navigate through and understand this site.
Stick around someone will be by shortly to answer your question.
We are happy you have chosen the best source for Fords!

Enjoy FTE....and
JOIN CLUB FTE -SUPPORT THE FORUMS!
…..See you on the boards.

 

My guess is that the wiring in the front tank is shorting to the tank or the feed pipe. This will kill power to the front lift pump, which will eventually stop the engine. This could cause wacky operation of the front fuel sender due to extra ground current.

When the short is bad enough, it heats up all the wiring in the circuit -- the selector switch, inertia switch, fuel pump relay, etc.

Try a temporary fix of bypassing the fuel selector switch and running off the rear tank only. Turn the key off. Disconnect the battery. Use some crimp connectors and some spare wire (For extra credit, use fusible link wire). Route circuit 787 direct to circuit 789, bypassing the selector switch. Set the selector switch to command the rear tank.

Turn the key on. Monitor for any heating of the bypass wire, or in the area of the selector switch. Start the engine and continue to monitor. It would be a good idea to keep a fire extinguisher in the cab, at least until you get this sorted out.

 

Thanks for the help! I'll try this out and report back.

 

Fe,

I haven't got a chance to work on the truck yet (can't until I get back home on the weekends) but I recall my fuel pressure being at 31psi at idle and 39psi when I wicked it WOT real quick. Could a bad groud make the fuel pressure low? I put in a new high pressure pump.

Just my random thoughts as I go over it a million times in my head. lol

 

From one of your other posts, I see you have a 5.0 V8. The fuel pressures you cite are correct for a V8.

The way to test it is key on engine off. This will give you the max fuel pressure since the manifold vacuum is zero. Then start the truck and the fuel pressure should drop about 8 psi as the regulator reduces pressure in response to the vacuum line.

Hammering the throttle to WOT should get you back to KOEO pressure, but there may be a bit of a lag in the regulator and guage responding to the drop.

It seems to me your grounding is all too good, since you have a short circuit somewhere, not an issue with too little current flow.

 

So since I replaced the high pressure pump I know that's not it. It's got to be that front tank pump, right?

 

You (apparently) do not have a fuel pressure problem. Little inference can be drawn from this. The pump in the front tank (for pre-1989) is nothing more than a "lift" pump. It helps supply fuel to the high pressure pump. But with a tank almost full, and with a low demand on the high pressure pump, the fuel pressure would probably be OK even with a non-functioning pump in the front tank.

You DO have a problem with excessive current draw, at least when the front tank is selected. You can rule out the high pressure pump and the common wiring if you can confirm that everything is OK if you stay on the rear tank. That's why I suggested jumpering the selector switch to route power to the rear lift pump.

If the high draw is confirmed as only happening with the selector switch on the front tank, then the problem lies in the wiring between the selector switch and the tank, or within the tank, or the fuel pump in the front tank is shorted or siezed. My money is on a wiring problem, but you will probably have to drop the tank/lift the bed to inspect it anyway, so you might as well have a look at the pump.

 

OK, I'll jumper the wires and test it out this weekend (praying for no rain). Thanks for all the help, fefarms!

Oh, and my dad mentioned he seen the battery discharge off and on when on the front tank.*

Btw, are you a Ford service guy, or what? You sure know a lot about these trucks! I only know modular 4.6L and 5.4L Fords because I play with a Mustang. And as you can see, electrical isn't something I know much about.

 

I didn't get to work on the F250 much since I spentthe weekend working on my sisters vehicle, but I did make some pictures. I also checked to see if any of the wires were burnt or melted beyonf what is pictured and no where else was it messed up.

Shown, #1 and #2 got hot.

http://home.comcast.net/~nx330mustang/11SelSwi.jpg

 

Here is the harness that the tank selector switch plugs to. It looks to me like the red/yel wire was the one that got hot and then melted in to the red wire. It also melted the connector, but beyonf that the wires are fine. The only other wire that got how was the one that plugs in to the fuel inertia switch.


http://home.comcast.net/~nx330mustan...ness_small.jpg

 

One more of theharness to selector switch. Would this mean the selector switch was the problem, or whatever the red/yel wire leads to? The selector switch was fine on the inside, nothing burnt lookin inside. Only the plastic around the those #1 #2 pins melted.

http://home.comcast.net/~nx330mustan...ness_small.jpg

 

You can see where the wire got hot on the fuel inertia harnes. It was touching the other wire where the hot spot is.

http://home.comcast.net/~nx330mustan...etia_small.jpg

This is all I've found messed up so far in the wiring. What do you all think caused this (one more time after seeing the carnage)?

 

I see two things that give me some concern:

1) The R/Y wire coming out of the selectror switch. does not match the color code in the schematic. It should be PNK/B. Maybe this was a factory change, or the schematic is "generic" for several years? Or did somebody replace part of the harness?

2) The last picture appears to show splices made with solder and weather-proofed heat shrink tubing. Decent quality work, but probably not factory. Again, it looks like somebody has been messing with the wiring some time in the past. "Farmerized" wiring can be a challenge to straighten out.

Beyond that, it still looks like there is an intermittant short circuit somewhere between the selector switch and the front tank. All of the wiring in the path from the battey to the short will get hot. The red 18 AWG fusible link shown at the top left of the schematic should have blown and prevented the melted insulation. I wonder if our mysterious "splicer" replaced or eliminated the fusible link some time in the past.