Gents,
I have a 2015 GT Premium (automatic, just in case it matters) with just under 74k miles on it. On the way to work this morning, the display (behind the steering wheel) showed me an engine overheat warning. I looked down at the temp needle and it was pegged on hot. I pulled over, opened the hood....didn't seem very hot (had been driving for about 20 minutes). I gently unscrewed the cap to the overflow tank and let the pressure off. Coolant came up into the tank, but not boiling or anything scary-hot. I screwed the cap back down, got back in the car, started it up, and temps showed normal. The warning light was gone and the needle was back where it normally stays after the car warms up. What's the most likely culprit?

 

Cars these days have so much electronics I don't even know where to start. If it reset itself hopefully it was just a fluke, and won't rear it's ugly head again. However, if these things still have a thermostat like in the old days, that would be where I would start. Good luck. Hopefully someone else will come along with some better ideas than me. Welcome to the forum.

 

I'm convinced that it was an electrical / electronics issue, as the car was never actually overheating, it just thought it was. If I understand correctly, the temperature sensor uses a resistor to control the voltage that gets to the computer (the hotter the engine, the more voltage the resistor allows to flow). Maybe the resistor was stuck all the way open or something. Hopefully, someone with more experience than me can throw me a bone.

 

Sometimes temp senders can fail. Probably worth having it checked. I love the picture!

 

Thanks! We have a really cool old / restored gas station in the little town where I live, and it's great for taking photos.

Just a follow up - man, was I wrong about the diagnosis. My coolant was a full gallon low. The part I still don't get is how letting the pressure off the overflow tank caused the gauge to read normal temperatures again. At least it's squared away now.

 

P1/T1 = P2/T2, for a constant volume. Since the volume of your cooling system remains constant, for the P/T ratio to remain constant, lowering the pressure lowered the temperature.

 

That makes sense, although if someone asked me if the cooling system incorporates a valve to release excess pressure, I would have assumed it did. My relic / bane of my existence 1972 Corvette overflows when it gets hot and has excessive coolant. I guess it's safe to assume the Mustang does not have such a valve?

 

The pressurized cooling system allows higher coolant temps without boiling.

 

So why was it a gallon low?

 

That's a very good question, Marco, and I spent a 3 day weekend getting the answer. The short version is that the water pump failed.

I learned some things in the process, not the least of which was that a water pump can most definitely fail in 74,000 miles. But before I discovered that the water pump was the problem, I ran across a service bulletin (TSB 16-0019) that recommended the replacement of a metal tube that connects the water pump to the heater hose (BR3Z-18663-A). Mine was leaking at some point, as the engine cover had telltale signs of sprayed coolant on the bottom side. I also replaced the idler pulley and tensioner pulley, neither of which eliminated the squealing noise that developed pretty quickly after the overheat thing came up. Ultimately, I put the breaker bar back on the tensioner pulley, gave the belt some slack, reached down to the water pump pulley, and shook it like a maraca. Literally, the pump shaft was flopping around like someone left the bearing out when they put it together, yet not a drop was coming from the weep hole.

Taking the pump off wasn't too bad. It has 4 bolts (not counting the 3 that hold the pulley on), and was pretty straightforward. Next stop was the parts house, where I bought a new one for $99 and change. It went on quickly, and I didn't have any problems until I noticed that the pulley had 3 holes, but the pump hub had 4. So back off the new pump comes, and back to the parts house we go. I finally got the right one, and it was downhill after that. I cleaned up the messy residue where the coolant had been, and later noticed a slow drip. I looked under the front of the car to make sure I didn't have another coolant leak (which I didn't) and immediately noticed that my right front tire's inside corner had belts showing (which is always great news). So I also got 2 new tires, totaling $559.00. So yeah, if you love spending copious amounts of cash and roughed-up knuckles, it was a spectacular weekend.

So going forward, I'll put a new pump on mine every 60k or so, and I'll keep a spare tire, though I'm hopeful that an alignment will eliminate the excessive wear that I'm getting on the inside corner.