I'm sorry - I could have sworn we were talking about coolant temp here... Wow!!!

 

Yep, and the limper is under the manifold..........2 sensors. That's why the gauge might be pegged but it hasn't limped yet.

 

The sensor under the intake is the Cylinder Head Temp Sensor.

http://ww2.justanswer.com/uploads/eb...-17_125202.png

The one on the top that I pointed out earlier is the coolant temp sensor.

So it may be either of these...
I am still not in the camp that wants to replace the thermostat..

 

I think if you check the OP's original post he said that the gauge pegged AND he went into limp mode.

Odds of both sensors going at the same time are slim..

If the stat was bad (stuck shut) he would have nuked the engine by now.
If the stat was bad (stuck open) he would have had an undertemp condition (which may or may not have set a code / gone into limp mode)
If the stat was bad and not opening all the way, he would have run hot while pulling a load but cooled off at lower speeds / power settings..
There's not a lot of science to this, but the programmers have a number of sensors to check and cross check.

 

Ya, that's a big job replacing that cap and thermo.

 

Might as well replace the serpentine belt and hoses too...

TIC of course...

 

I think air pockets affect the limp sensor under the manifold........could be wrong there but it seems to have a mind of its own.

 

The CHT sensor does not connect with coolant. It just checks the temp of the metal.
Remove the alternator to easier access the CHT sensor. It does not open the cooling system.
Standing by.

 

So what does CHT stand for? That explains why the limp mode actions are so mysterious and why the gauge can be pegged but it's not limping yet. It doesn't take much head metal heating to set it off because I was able to put my whole hand on the valve cover when it limped for about 20 seconds.
Standing by for the OP report.

 

CYLINDER HEAD TEMPERATURE

When it gives a false positive reading it is not actually overheating.

 

I guess it depends on what you call overheating, mine was losing water, the temp gauge was rising, and it was all good after I got the heat down. I don't see anything false about it.

 

I think a key difference between your symptoms and the OP's is that you were boiling over.

Speaking of which...where did the OP go?

 

I never said I was boiling over, if I was I couldn't rest my hand on the valve cover for 20 seconds. Maybe he is loosing water too. I was getting water pushed out but never any sign of boiling...............it was a mystery.

Where is that OP?

 

Wow. I have never seen anyone outright tell everyone else they were completely wrong while especially not being 100% accurate themselves! Many of us have had the temp gauge/limp mode issue and I can tell you a t-stat change was NOT the answer!! Mine was the 5.4L.

Maybe that worked for yours and that's awesome that doesn't make everyone else wrong. Mine never steamed, never puked or lost coolant - so aside from the gauge and the computer forcing the truck into limp mode, there were no other symptoms. But I changed the t-stat as it's the least expensive attempt to repair and they can go bad. It was a Motorcraft direct replacment. It did NOT fix the issue!!!

Here is why I know it's not the t-stat. Once the gauge pegs and goes into limp mode, you can pull over and let the truck run for up to 15 minutes and that gauge will not move (100% pegged). However, you can pull over, let it run for less than a minute, shut off the truck and restart - and the gauge is back to a normal position. This is a not a symptom of a bad t-stat - but a bad sensor.

After extensive troubleshooting, I determined the CHT needed replaced. I did so and the "false overheat" went away... I still took a proactive approach seeing the truck had about 100k miles on it and I attacked the entire cooling system. New double-sized radiator, hoses, water pump and fan clutch.

Btw, for the love of all that is holy - do NOT go by the book to replace the CHT!! IIRC the location on the 6.8L is similar to the 5.4L. Remove the alternator. No normal wrench or socket will get to it so take a 19mm or 3/4 wrench, apply heat to the handle and give it a couple good smacks with a hammer to bend it. You can now easily remove the CHT and replace it. My wrench still has the bend in it, LOL!! Trust me, the cost of replacing the wrench is paid for 10x's over by not removing the entire manifold!

I never ever had an overheating issue again with that truck (just sold it). It performed flawlessly pulling in the hills of W. Va. in 90+ heat hauling 8,000 pounds with the A/C on!

 

Well your're wrong too. As I said; I could pull over and restart it with all back to normal but it wasn't the sensor. I have a feeling you replaced the sensor and other things also at the same time which makes it hard to pin it down. Sometimes I could go for days with all good and other times I could only go a mile in 30* weather and the thermo wasn't stuck. Read those first 4 posts, they are out in left field. Did you change the cap?
Also; I said it was the cap and thermo either/or. I don't think your cht was bad. The cht detects faults in the system but not general heating problems which is why you can pull a big trailer up a mountain in 100* heat.

I never said it was the thermostat, I only said I doubt if it is the CHT.