I can't figure out why you didn't take my advice about the thermostat in the first place.. Why would a coolant temp sensor cause the failsafe?

 

Well, EXv10,

I couldn't take the suspense any longer, and rushed to the store to get a thermostat + o-ring (not really, it just turned out to be a little cooler today than usual... ;o)

I changed the thermostat, backfilled the coolant, put the old ECT back in, and took it for a 15 mile spin.
The temp gauge worked again as it should. About 10 miles into it at 70mph the temp gauge started to go up above normal, and a minute later it pegged again. No limp mode though. I stopped the engine, waited 5 seconds, started it back up, and the temp gauge indicated everything was just peachy. I drove home at around 45 mph and the temperature gauge stayed at normal.
The heat did blow hotter than usual though.

 

What will you try next?

 

So it never went into limp mode as usual? Did you replace the cap? Was that a Motorcraft thermostat? Did you put the o-ring on the top?

 

No limp mode, but it didn't go into limp mode before either, only occasionally. Motorcraft thermostat, o-ring on top of it. I did not replace the cap. When I opened the cap after it had cooled off it still gave a short hiss so it doesn't seem to leak.
So it appears that the ECT drives the gauge, and the CHT drives the coolant light and pegs the gauge when hot. Since both happen, it does seem both the SCT and CHT are working properly, and the temperature is actually elevated. So at this moment I am not sure what to do next.
I'll give it some thought and am open to suggestions.

 

The limp mode system defects in the cooling system. That cap should hold pressure overnight so I would listen for a hiss many hours later. The water pump could have eroded vanes also (remote chance) but it happens. When it goes into limp the oil light comes on also. Maybe you should hook up a permanent water temp gauge but that seems like a "cop out" to me. You can borrow a cap and cooling system tester at O'reillys. I would listen for that morning hiss next.

 

You really need to find a way to look at live data. In your situation the first thing I would have done would be to monitor the CHT and ECT sensor data to see what was going on. If both where showing an overheat condition then I wouldn't worry about sensors anymore and focus on what was causing your engine to overheat.

But if only one of them was showing an overheat while the other looked completely normal, which I think is HIGHLY likely in your situation, I would look to see which sensor was spiking and test it. Thus far you haven't done a thing with the CHT sensor, and you have absolutely no idea what it's telling the PCM.

Being able to read this data can cost just a couple dollars over what you paid last night for your thermostat. STOP throwing good money after bad and see what the PCM is seeing. Do you have a smartphone of some flavor? That's the cheapest way.

 

Brent,
I will be using the truck a little today, so after it has cooled off I will look for the hiss.

Tom, iPhone here, how will that read the PCM data? Is there an adapter available?
Thank you.

 

After reading Brent's comment I went out and shot a little video, but thought better of posting it. When the system cools completely down there is no hiss; it's a red herring. Perhaps in Brent's heyday that was common, but it isn't today. I'm trying not to get into another argument over it though, as I think we've crapped all over your thread enough.


Yes there is. Unfortunately it's not as seamless of a process as it would be with an Android phone because the iPhone can't communicate that way via Bluetooth. But here you go:


And you'll need an app like this:

https://itunes.apple.com/us/app/engi...591557194?mt=8

I love technology. SO much cheaper than any of the professional tools that were required before this kind of thing was common.

 

That's not true, why would I say something that was true 30 years ago? Mine does hiss in the morning when I remove the cap and does hold pressure overnight because I have a new cap that was hand selected at Oreilly's and my system is airtight. I don't just guess at things in here like some do. Also, while live data is nice it costs money unless you own a shop or have that stuff. I cured my own problem by a lot of research and thought, and if he doesn't want to buy a new cap I don't think he is going to buy a scanguage or whatever it is.
OP: What gets hotter now?

 

Update; I just went out and removed my cap after the truck was driven yesterday about noon and it hissed big time and the water raised up about 3/8".

 

Geeze Brent...
I give up.. You know it all...

 

Yes, I do know a lot.

 

 

OK Tom,

I ordered the Engine Linc unit, says it has 17 days shipping. Hopefully it will come sooner.

I drove yesterday pulling a boat to and from the lake (~3500 lbs). Staying below 50mph (much to the displeasure of the cars behind me) it didn't overheat. Once the temp started to go up, so I turned off the air, opened the windows, and set the heat to max, and the temperature came back down again.