I know that this has been brought up numerous times but my temp gauge seems awfully low even once truck is fully warmed up. Before anyone asks, I do have heat and it gets VERY HOT. Here is a pic after I've been driving for a good half hour or so.
If heater air is "VERY HOT" then the temp sender for the gauge may be off, if you have had this truck awhile and it used to run warmer on the gauge then I would spend the $8 and replace the sender, but check the t-stat first.
You can check if the t-stat is going out in a couple of ways.
An engine should get fully warmed up in under 10 minutes, 5 minutes is usual, even in cold weather, if it is taking longer then t-stat may not be closing all the way when cold.
With engine cold, start engine and pop the hood, feel the upper rad hose near the engine, where the t-stat is, is should be cold, matching outside temp.
Open the rad cap as well, no there should be no pressure until engine heats up.
As engine idles the upper hose should stay cold, and there should be no circulation in the radiator, coolant should be cold.
If upper hose starts to warm up and you see coolant circulating in the rad then t-stat is open slightly, which it shouldn't be.
When engine coolant gets to about 190deg the t-stat should start to open and upper hose should heat up fast, coolant in upper rad should go from cold to hot very fast as well.
If upper hose and coolant in rad warm up slowly with the engine then t-stat needs to be replaced.
Fords use 192 or 195deg t-stats, don't use 160 or 180deg t-stats.
The point of the t-stat is to warm up the engine coolant to 200deg ASAP, 200-220deg is the prime coolant temp for the best gas mileage and best lubrication, all oil "weight" is measured at 212deg, that is the temp it works the best at.
So the t-stat stops circulation to the rad, the waterpump only circulates coolant through the engine(and heater core) until it gets hot enough and the t-stat opens, then the rad coolant is allowed to circulate to prevent overheating, but the t-stat may only open partially to maintain the 200deg if rad is cooling too much, like on a very cold day, so no matter what the outside temp is doing engine temp should be normal.
There are two coolant temp sensors on fuel injected engines, a two wire sensor and a one wire "sender".
The two wire sensor is used to "choke" the engine, since there is no carb or jets a choke plate is not possible, so this sensor(the ECT) tells the computer the engine is cold so to run rich with advanced timing, if this sensor fails then engine will barely run when cold or gas mileage will go to heck because you are running rich all the time.
The one wire sender is the one that is for the dash board temp gauge and the one you would change.
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