I recently sold my '04 Lariat and picked up a black '06 King Ranch with 137k. I was real happy b/c the Oasis report showed that the egr & oil cooler were replaced under warranty @ 90k a couple years ago so I thought that was one less worry I had to deal with.
So I take it to this guy that works at the local Ford dealer but fixes my stuff on the side (I piddle around buying/selling, but I bought this truck to keep). He went through it and changed the fuel pressure regulator, egr cooler hose, flushed with vc9, oil/filter change, replaced glow plug harness, fixed some vacuum leaks, etc.
Truck was running great. Went to go on a 1500 mile trip to Oklahoma pulling a 7000lb camper and noticed my coolant was a little low so before I left I topped it off. Just attributed it to some trapped air working itself out after the flush.
Made it the first 650 miles or so with no problems driving 62-68mph. Was pulling up a hill and the truck dinged at me to say the temp was high and sure enough it was so I slowed down to 45 and cranked up the heater. Within half a mile the temp went back down. Got to next town and my degas bottle was empty so I filled it back up. Right as I was coming into my destination it got hot again. Sure enough the bottle was empty again this time after only about 50 miles.
This was in Waynoka, OK which is podunk from the work go. Went buy the only shop in town. They put a little camera scope under the hood and showed me some pipe under the top of the motor that looked corroded out on the bottom. May have been the egr cooler pipe, but I'm not positive. He didn't have time that week to fool with it so I decided to try to limp to OKC at least.
This time I made it 30 miles to the next town and although it didn't get hot the tank took over a gallon to fill. I topped it off and headed out. Within 5 miles I was pulling up a long grade and it got hot so I went back to the last town and had the truck/trailer towed back to Jackson (had a wrecker company owner that owed me a BIG favor I had to cash in).
At no point did the gauge get in the red. It got right up to it a few times, but never pegged out.
Now my guy has checked it out and says he can't find any external leak source. Says he pressurized the system and it held. Says he drove it to lunch a couple times and never saw any water leaks. Back in Oklahoma I noticed coolant leaking onto the ground from the back of the motor when I was running errands in town during the vacation.
Anyway, few days later he says he took it out and ran it hard (whatever that means) and found that it was puking coolant from the degas bottle. His diagnosis is head gasket failure and $1500 labor + parts to fix.
That what it sounds like to you guys? The only thing that really bothers me is that back in OK I saw on the little video screen some silver coolant pipe with what looked like a corroded hole on the bottom. What should I ask to find out if it's reallly the head gaskets or if I'm getting jobbed? I've used this guy many times in the past with good results and more than fair rates. Just not sure how to proceed.
p.s.- oil looks good. no signs of water
Some simple places to start with all 6.0L coolant "leaks" or "coolant losses":
1. You have to find where that coolant is going. Check the hoses, the radiator tanks, the weep hole on the water pump. Check the blue hose (some older ones are red) that goes from the oil cooler to the EGR cooler. Check the back of the engine to see if it looks wet/washed down. Try to see if you can physically find a leak. Its not easy to do on these engines, but that doesn't change the need to give it a good look-over with a strong flashlight to see.
2. Remove and check your EGR valve. If it looks wet or moist, then you at least have an EGR cooler leak. It should be nice and dry and black/charcoal ish.
3. Check your EOT (engine oil temp) versus your ECT (engine coolant temp) and see how well your engine oil cooler is operating. Both of these temps should be about the same... anything more than 15°F difference and Ford says the the engine oil cooler is due for replacement. Make sure you test it out running around and working it a bit. You can't get good numbers just sitting in the parking lot watching the scanner.
4. I know that you had the oil cooler done sometime before you bought it... 47K miles ago. That's prime mileage for having a(nother!) bad/clogged up oil cooler. I've seen them go bad as fast as 10-12,000 miles, even with coolant filters.
5. If you EGR cooler checks out fine and if your engine oil temps are good, then its possible you have a blown HG situation. I would also make sure that your water pump is moving water effectively (the plastic impellers on the shaft like to break, in which case they will move plenty of water at idle, but not nearly enough at highway speeds). I would also make sure that your fan clutch/cooling fan assembly is working and working well.
6. You mentioned that you don't see any water in your oil... good. But check your cooling system to see if you have oil in your coolant. This occurence is fairly common with the OEM oil cooler and indicates an internal rupture in your OEM oil cooler, and higher pressure oil is flooding into your coolant system.
Those are some places to start.