For me, mileage whent from over 30 to under 3mpg almost overnight.
I never let it go below 1/2 tank after being warmed by the dealer early on about lift pump failure. I left the house with a full tank, about 3 miles from a fill up where I super topped the tank. Drove 10 miles to work, parked the truck and it sat there for 8 hours. It never left my sight all day, as I was theonly one on site that day and the truck was parked in full view of where I was working 50ft away. I noticed the tank was half empty when I got back in the truck. Thinking it was a recurrance of a fuel guage issue the truck had when new I went to a fuel station next door to the site when I left and refilled the tank, it took 9.5 gallons.
I then drove the 10 miles home. About a mile from the house I noticed the gauge was down to almost half tank again. When I pulled in, a neighbor walked up to the drivers door to ask if I could help him unload something from his truck, he said he smelled diesel. I got out, left it running and looked high and low for a leak but found nothing. The smell though was STRONG as if it were spraying fuel into the air. There was no smell of fuel back at the tail pipe, only at the drivers door and out ahead of the truck about 8ft or so. I couldn't detect it up close or under the hood. I refilled the tank right then, it took 11 gallons.
I backed the truck into the garage and put cardboard underneath figuring I'd look closer in the morning. The next day the cardboard was dry, and I could smell no fuel with the truck not running. The bottom of the truck was spotless and dry.
Its way too much fuel gone to be burning it, and like I said, I don't smell it at the tailpipe and there's nothing wet. Its been like this now for about 6 months. Two dealers have looked, neither put any time into it other than a short road test, computer scan and a look at it on a lift. They said there's nothing wrong.
The fuel gauge has always had a delay action thing going on, if it was low on fuel I'd top it off and the gauge wouldn't show the tank as full for 50 to 100 miles or so. they changed the tank, the sender, the dash and put a harnet in the cab when it was new. Nothing fixed it.
Then we got two trucks at work and they did the same exact thing. I later figured out that where ever the gauge reads when the engine starts, is where it stays for the first 1/2hr or so, then it re-calibrates and registers. If I refill the tank, cycle the key a couple of times each time letting the gauge creep up to full, it works fine.
I've known 7 trucks, all 2003 models that did the same thing.
f this were some high mileage old work truck I'd be less surprised, or if it were running poorly or smoking I'd say it was burning it but its blown through 5 tanks of fuel in 100 miles now with no trace of where it went. It gets parked in an alarmed, video camera monitored garage when at home, and its only seen work twice. Its never unattended to to where someone could have stolen that much gas, and then there's the obvious odor of fuel when it runs. What I've not been able to figure out is how the odor is only noticeable way out in front of the running truck, the fan is drawing air toward the truck not away, so the fan isn't pushing the smell forward, and its absolutely not detectable behind the truck.
I really thought I was going to find dried up or cracked orings at the fuel filter or regulator but they were fine. I even taped a piece of newspaper to a stick and pocked around down in the valley of the motor to look for leaks but its dry.
I have a coolant filter on the truck, someone said it was need on these 6.0L trucks. I change the filter every 1000 miles or so. I've gut it open a few times and never found any metal in it and I smell no diesel in the coolant.
Besides, if it had pushed that much fuel into the coolant I'd have it bubbling out the cap long ago. Keep in mind this truck has under 25k on it and I've owned it since new. The only vehicle I have with fewer mildes is my '88 LTD Wagon with 12k since new.
I'm actually looking at another 2003 diesel almost identical to mine, there's an old guy who bought ne nearly identical to mine around the same time around the corner, he's in his 90's and never much drove his either. I think it read 3,600 miles on it sitting in his attached garage. Its still got the factory floor mats and new smell. (I was there helping him change the batteries in it last month, he said someone keeps stopping and offering to buy it, he said they offered him $3,500, he was livid that they would take him for a fool like that because he was old. He told me if he sells it, it'll be to me since I've kept it running for him all these years. Its an XLT 4x4 super cab like mine but with less miles.
(He's got a 67 F250 too with 18k on it but his son has dibs on that. The 'kid' (67), hates the smell of diesel and is pushing his dad to sell the '03.
I have no clue what its worth thought but I do know that the dealer here had a 230K model on the lot for $22k and it sold in a week or less. Even with fuel at $7/gal. My buddy sold his crew cab last Friday because of the cost of fuel with 300k or more on it for $19k.
I won't price the neighbors truck, I told him and his son to come up with a price and we'll talk. I'd likely only use it till mine is sorted but I also have two other daily use trucks here to use. "Need" is never the reason I buy another truck. If it were, I'd have not bought any of my newer trucks. I do regret letting my 1986 go, my current '03 took its parking place in the garage and it wasn't doing me anygood parked outback in the unheated garage for months on end. I was just replacing tires and changing oil for no reason, it got 100 miles a year and I was throwing away tires that rotted with less than 500 miles on them.