Where is it and how do you know if it's working OK? My engine seems to run on the hot side and the temp gauge needle is near the 'p', but this could be the temp sending unit too. The temp gauge is new.
Sounds like it's 3/4 way across the gauge, normal temp would put it near center or just right of center I believe. The thermostat is mounted inside the housing on the front of the intake where the upper radiator hose attaches. Mustangs Unlimited - The Premier Source for Mustang, Shelby and Cougar Parts and Accessories
There are 2 bolts that hold the 'water neck' on and the Tstat is under it between the neck and the intake. You would have to remove it and boil it in a pot with a cooking thermometer to see what temp it opens at.
Someone may have removed it or swapped it for too low (160*) or it's not working anymore. The best temp range Tstat to run is usually 180-195. There are many other factors that can cause a motor to run hot, improper timing, clogged cooling system, bad radiator, collapsing lower hose, wrong antifreeze mixture (50/50 is most used) etc.
It only costs a new gasket ($1) and some permatex (have to drain cooling system to below the intake level) to take your old one off and swap for a new one.
Jon brings up a good point; sometimes previous owners or bad mechanics seem to think the way to solve an overheating issue is to remove the thermostat, but this does more harm than good. The engine never heats up properly or evenly, and the back cylinders can actually overheat as the thermostat serves to restrict and force coolant flow around the back cylinders.
If it's your gauge that's got you worried, bear in mind these Mustang gauges were probably inaccurate the day they rolled off the line, and add 40+ years of corrosion and fatigue to the system and you will get some interesting readings. Mine reads about 1/2 to 3/4, depending on how hot it is outside. Instrument voltage can also affect the reading, as well as wiring integrity, etc etc.
Our Mustangs ran on the hot side from day one, so if you have an original style 2 core radiator with a 4 blade fan and no shroud, consider upgrading these items. They're relatively cheap to replace. I'd start with a fan and shroud upgrade (lots of threads here on the subject), and move to a radiator upgrade if the problem persists.
One last thing, Joe is right on the money about proper temp range of 180* to 195* for the new thermostat, and cooling system and coolant integrity.
If your thermostat is more than 2 years old, replace it anyway!
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