I'm experiencing a couple problems with my 2003 F250 4x4 7.3L Automatic with 68k on it. It has 4 inch exhaust w/down pipe and muffler and a K&N filter, but I don't know if it's chipped cause I bought it used 1 year ago with 48k on it. Last week after driving 3 miles around town, my truck started running a little rough, like it was missing on 1 cylinder. Within 2 blocks it was barely running. I pulled over, shut it off and let it sit a minute. I then restarted it and it ran rough but not as bad as when I shut it down. My fuel lite had just recently come on so I was at about 1/8 on the fuel gauge. My friend lived close by so I drove it there. Checked the fuel filter and it was very black so I changed it. Still ran rough but I was able to drive it 2 miles to home. I then put about 4 gallons of fuel in it to see if my gauge was lying and it still ran the same but I didn't drive it. I pulled codes on it and there was none. No history codes either. While doing the KOER self test I was standing outside the truck so I could here the fuel pump and it seemed to be making more noise than normal. I made an appointment with the dealer since it still had some warranty on it. Of course it started running fine(I had added fuel at this point) while driving 3 miles to my local dealer. They pulled codes and nothing. Tested fuel pressure several times and said they got it to drop pressure once for a few seconds, so they put a pump in it under warranty. Picked it up and ran fine until the next day when the fuel lite came on again. Started doing the same thing again at 1/8th tank. Pulled over, shut it off and rocked the truck back and forth a couple times incase the fuel pick up was loaded up and plugged. Started it up and babied it to a gas station. Put fuel in it and it ran fine after that. Dropped the tank to look inside and it was clean as a whistle. I only had 1/4 tank at this point which was just above the bypass valve on the side of the fuel pick up. I pulled the fuel pick up off and checked the bypass valve which is working but I don't know if it is opening too easily and maybe sucking air when the fuel drops below it. It has run fine so far as long as the fuel level was up, but I also have a second problem. I filled the tank(29 gallon) after dropping it and noticed my mileage has dropped alot. I put 40 miles on it and dropped 1/4 tank. I usually get 100 miles on 1/4 tank. I don't see any obvious smoke behind me, no fuel leaks and my engine oil is not over full. If anyone has any ideas I would appreciate your feed back. I am a mechanic, but with very little deisel experience.

 

Have you tried running any kind of additive to the fuel. try running a injector cleaner made for diesel's put it in at half tank and run it down the highway about 70mph for 30 min to get it cycling through. you could have sucked something from your tank up into your injectors. its just a thought but might as well try the little things first. i suggest something like diesel power supplement its called something like that its in a white bottle with red lid

 

I have added both Stanadyne and Redline additives but not recently. I would think that if I had an injector hanging open to the point that I was getting 6 MPG I would notice something in the way that it was running.

 

Roostonu, welcome to FTE.
Those bypass valves are notorious for gulping air with low fuel levels.
If you had an injector hanging open there would be clouds of smoke accomanied with a very loud knocking noise similar to what a spun rod bearing sounds like.
If you truly are only getting 6 mpg and the engine isn't burning it check for a fuel leak at the filter drain valve. It's a simple repair, just o-rings inside the little housing.
Now it's time to give you some homework.
https://www.ford-trucks.com/forums/s...d.php?t=278697
This one especially when it comes to the fuel supply.
https://www.ford-trucks.com/forums/s...d.php?t=260173
https://www.ford-trucks.com/forums/s...d.php?t=437805
https://www.ford-trucks.com/forums/s...d.php?t=440448
There's more, I'll go dig them up.

 

Here's some more.
https://www.ford-trucks.com/forums/s...d.php?t=443804
PSNut did his own version, and a very good job.
https://www.ford-trucks.com/forums/s...d.php?t=446151

 

Thanks for the links. The first link with the glow plug wire problem says it was running funny but no real explanation. The problem with the fuel pick up bypass valve could very well be my problem when it starts running poorly but doesn't explain the bad mileage. There aren't any external leaks. The mileage is a guess concidering I went through 1/4 tank in 40 mostly freeway miles. I usually get around 100 miles per 1/4 tank.

 

Don't be fooled by the gauge vs. mileage. I tried that and found that my '01 PSD would go about 135 miles before dropping off full. I constantly check my mileage and usually get btwn 18 and 21 at 68-71 mph. This seems to be the sweet spot for this truck. BTW it runs great. 153K miles.
The most I've gotten on a tankful was 755 miles. Took 37.334 gallons to refill.

Bill in KY

 

I figured out why the mileage looked so bad. When I got fuel, I prepaid $60. The pump shut off when the counter slowed down .20 cents before the $60 mark. I assumed without thinking that the tank was full and thats why it shut off. The gauge also read full. Normaly I go about 50 miles before the gauge moves off full and then there is another 40-50 miles to the 3/4 mark. So it turns out that I had just filled it to that point where the needle starts dropping. I refilled the tank and it did take 6.6 gallons so if I hadn't figured out why the pump shut off I still would have thought the mileage was that bad. Now I have driven another 50 miles and the needle has just started to move off full, just like normal.
I also think the rough running at 1/8 tank is the relief valve on the fuel pick up, due to the symptom showing up twice only after the low fuel lite came on.
Thanks for all the help and thanks for giving me the bump I needed to not assume my mileage was what I saw on the gauge. I should have known better.