In the old (carb’d) days, we’d slap on a 160 thermostat (or worse) on our Windsors and head out into the wild. Things changed a little with the EECIV managing the EFI on the fox Mustang starting in 86. A “too cold” thermostat will cause more harm than good and it’s hard to go out there and install a 192 or 195 degree thermostat on a performance car, it just feels sacrilegious.

First things first: Make sure your cooling system is running well. Running hot (+215) is a problem with the system, not a bad factory thermostat temperature selection

Important: If you’re looking to dump a colder thermostat in your fox because “it runs hot”, there’s a bigger problem at hand. Is the radiator old and clogged up? Is the fan still spinning like it should? Is the water pump still working like it should? Keep in mind that a thermostat just opens the channel once the water reaches a certain temp, it won’t magically keep your 5.0 running at 160 or whatever. Your best bet is to grab a nice shiny new aluminum radiator and make sure the entire cooling system is working well. Remember 200 degree water is bad for us, but not bad for our 5.0’s! It’s been proven that running too-cold is actually a detriment to the engine. Your focus should be cooler intake air, not cooler engine block.

Why did Ford install a 190+ thermostat in there?

Engines do better at producing less emissions and run more efficiently when running at a certain temperature. So the programming on the EEC was all done under the assumption that the 5.0 will be running hotter than us enthusiasts feel comfortable with. So your engine is programmed and built for tolerances running at 200+ degrees. Changing the values can have adverse effects.

What does the computer think?

Here’s how the computer thinks and hopefully this will help you decide on what to do. When you first fire-up that awesome 5.0 pushrod, the computer goes into open loop operation to get the engine up to running/driving temp. During open loop, it ignores the O2 sensors and waits for the right time and before it goes into closed loop.

When the computer is happy with what’s going on with the engine, it transitions to closed loop.  In closed loop, the EEC does everything it can to reach the holy grail of stoichiometric air/fuel ratio (14.7:1) for better MPG/efficiency and does this by adjusting injector pulse width using information sent from the O2 sensors. Closed loop won’t happen if the computer thinks something isn’t working correctly though. Keep in mind that your ECT sensor will cause a trouble code if it reads below 180 degrees but (correct me if I’m wrong), I don’t think that code will force the EEC into open loop.

It’s worth noting that it’s a time-delay, not a temperature-check for closed loop operation and it could actually go into closed loop even if temps are lower than “factory specification”.  Also noteworthy that the EEC will increase pulse width by 2% for every 10F below 190 and this mode is activated at just under 180 degrees. You do risk having terrible mileage and poor drivability if you go nuts with your thermostat selection.

What’s my ideal thermostat then?

If you daily drive your fox a lot, I’d argue a 192-195 thermostat would be your best bet as you’d get the most efficiency. Also, if you have emissions testing in your area, a lower thermostat temperature can make it harder for you to pass. When the engine reaches operating temperature of 190+, the EEC will also pull 2 degrees of timing, which is why people recommend the 180 as the coolest thermostat you can install for performance gains (however potentially minimal they are). I personally have a 180 thermostat in my black project fox (for now), but my electric fan turns on at 195… I do have a new water pump, hoses and a new 4 core aluminium rad. My other notch has the stock 192~195 thermostat.

EFI stuff giving you headaches?

Here are some articles that can help, at least simplify things