Bought an 84 f250 IH 6.9 engine - ran great first day. Front tank,
bad gauge, ran out of diesel the second day.
Switched to rear tank and ran fine, went and filled up front tank.
On the way home, switched back to front tank a mile
from home. Parked overnight at home (driveway is on a slight uphill).
Next day, started up, ran for 30 seconds and died. Wouldn't restart
after 3 fifteen second cranks.

I did not want to burn out the starter, kill the batteries, etc. (based on
previous experience with a chevy 6.2). I hooked up a 2 gallon tank and used
a small electric pump to connect to the standard fuel pump inlet. Bled air
out the valve on top of the fuel filter. Started up, ran good for a few
minutes and died. Diesel fuel started leaking out of the front tank fuel
cap. Opened the fuel cap and about a half gallon of diesel poured out
under pressure. And after cleaning up the mess, it won't start.

Is it just flooded? I don't see fuel in the tail pipes or anyplace else.
I didn't think the injection pump would backfill the fuel tank with
pressure. Oh no, what did I do now?

Thanks for any troubleshooting tips or suggestions.

 

Where is the fuel tank selector valve? Some were located on the floor near the tunnel by the driver's feet.
All diesels have a return line for unused fuel to return to the tank. Could you be pulling from one tank and returning to the other tank? Also diesel fuel expands quite a bit when warmed, from running because it is also used as a lubricant and coolant to remove heat generated from the engine as the heated fuel is returned to the tank.
Fuel systems for diesels must have a vent in the cap or on the tank to allow for this.

 

The fuel tank selector is on the dash to the right of the heater
controls. I can hear an electronic 'thump' when I select from
front to rear and back so I believe that is working fine.

 

Most of the time the dash switch was only to show which tank the gauge was showing.
You might want to get under the truck and confirm what is switching when you go from one tank to the other. As long as you are getting fuel to the injector lines (all air removed) the engine should eventually start.

 

"All diesels have a return line for unused fuel to return to the tank"
"Fuel systems for diesels must have a vent in the cap or on the tank to
allow [for expansion]"

I'm trying to get an understanding of where the fuel goes. If I am parked
nose up on a hill, do the above comments mean the fuel drains back to the
tank by gravity? That would empty the fuel lines and introduce air, if I
understand correctly (but I must be wrong). There are no backflow
prevention mechanisms (like in our blood veins)? So what keeps fuel
in the line between the (bottom of the engine) fuel pump and the
(top of the engine) IP and keeps out air?

Am I overthinking? maybe I should coast backwards downhill to a level spot
and let things normalize. I am just fearful of having to replace expensive
batteries and/or starter. I had a 65 chevy gas truck that wouldn't start
if I parked it on a hill more than a couple of days.

 

If there are no leaks or holes in the lines for the system to suck in air , the fuel stays in the lines and pump(s). If air was able to get in on the suction side you would have a performance problem because that air would get to the injector lines. I don't believe that is the problem you are seeing at this time. There has to be a switching valve/assy so the returned fuel goes to the tank providing fuel. I will need to check if the diesel system is the same as a gas system as far as possible automatic switching is concerned on the older vehicles. Did you get a chance to look under the truck for what is happening when you switch tanks?

 

I believe the diesel set-up is the same as a gasoline engine.
After rereading your earlier post where you substituted an addl tank and fuel and got the engine running the unused fuel was being sent to an already full tank and that is why you had the pressure and excess fuel problem. There still has to be some kind of vent otherwise when pumping any fuel from any tank the tank would collapse.
If the previous owner put the wrong type fuel cap for a gas engine on your truck you will have fuel problems.
I'm pretty sure I can assist you to get this thing squared away.

 

I coasted down the hill of the driveway and let it sit over night
on a flat spot.
It started up after three 10 second cranks.

Scary. Gravity caused and solved the issue? Don't feel good
about this but I'm running for the time being.
Thanks for helping with some key info.

Greg