I was driving my wife's Fiesta Ecoboost today and noticed the electronic gauge sitting at 3 bars (instead of the usual 4 that I've always observed it at). Knowing that the gauge is a 'friendly' interpretation of what's going on I plugged in an OBD adaptor and found that the engine coolant temperature was typically sitting at ~73C. I could get it to go above 75C with harsh acceleration (having the cabin heater off helped) at which point the dash gauge would hit 4 bars but back to normal driving it'd drop back to ~73C (and some time later the dash gauge would fall back down).

My symptoms sound like a sticking-open thermostat as I would expect normal operating temperature to be up more towards ~90C, not least on a hot little engine like Ecoboost. I was therefore wondering if anyone has witnessed similar symptoms and fixed them by replacing the stat? Has anyone else observed what their steady-state coolant temperatures are like so that I can see what 'normal' looks like?

I took the stat out today and, somewhat to my surprise, it wasn't stuck open after all - or at least wasn't at that point in time. I manually opened it and did think it felt a little 'sticky' but that could've been a placebo effect in the hope that I'd found the problem.

I've got a spare Ecoboost engine and so I transplanted the spare stat over, rebuilt, and the previous symptom didn't manifest itself. Coolant temperature sat around~88C and I'm sure it'd creep up to the 90's had I tested it for long/hard enough.

So, whether it was the stat or the problem is still present (and intermittent) only time will tell. I'm pretty sure the stat was to blame though as I haven't been able to come up with an alternative explanation as to what else could prevent the engine coming up to full operating temperature (I confirmed that no fans were stuck on). Maybe if I get some time I'll stick the stat in a pan of water and see what temperature it opens/closes at.

Well spotted. I hate this type of 'temperature gauge'! Goes to show it's worth checking actual engine temperature from time to time. I think this can easily be found on 'secret' dashboard display on certain models?

Yes, the true temp can be found by pressing-and-holding the dash-mounted reset button whilst turning the ignition on and keep it pressed for 10s. Pressing the button cycles through various displays, with the coolant temperature appearing around 10 or so presses in.

To be fair to the bar graph, it was actually it that first alerted me to a potential problem as it kept sitting at 3 bars instead of the usual 4. Still, it can reach 4 bars and still be below-ideal operating temperature and so it's usefulness is limited.