2000 Discovery II - Gauge reads in the red but the truck is not overheating - it reads 201 degrees in coolant tank. Upper radiator hose before thermostat is hot, below thermostat is cold. Does ECT switch also control the gauge?

 

ECT drives computer (ECU). ECU drives gauge pointer. ECU also drives OBDII port, so you can hook up a scanner. I would trust ECT/ECU rather than coolant tank. But temp of tank may not be on the money of what is inside the motor. Temp of upper hose or the metal pipe that feeds it? You could be in big trouble, if sensor is correct. You could also have a shorted or grounded wire to sensor. You may want to pull sensor (between AC compressor and alternator), and put it in hot water of known temp, and see what kind of readings you get.

 

Thank you for the info. With scanner, readings are 190 degress at mid-gauge and 253 degrees at edge of the red. Have done chemical dye test, no hydrocarbons in cooling system. Have changed thermostat and bled. Have changed ECT. Upper radiator hose at thermostat very hot, hose leaving thermostat very cold. Any help would be greatly appreciated. Thank you!

 

The D2 gauge (if you want to call it that, it is really an idiot light with a pointer) is controlled by the ECU, and maintains a 50% reading over a wide range. The scanner will be more accurate, certainly when looking for small improvements.

Original coolant in many models was Dexcool, which when exposed to air or topped off with other coolants makes a sludge called "Dex Mud". Even if regular coolant was used, the factory nonsense about lifetime coolant and 150,000 mile coolant leads people to not flush and replace (lower cost of ownership). The resulting buildup inside the tiny radiator pathways makes the radiator smaller than it needs to be, in terms of the ability to transfer heat.

A sludged up rad will be cool on the bottom (blocked or restricted passages) and hot on top. My indy rad guy uses 10F as a rule of thumb, top to bottom on the fins. Too much difference then ask why. IR pix of a sludged up radiator. If cool spot is large enough, it can fool viscous fan to not come back to full power. Trash in the system can plug the little holes in the thermsotat top opening, they sample hot water to the thermostat to make it operate.

Also, at idle the viscous fan, if weak, won't provided needed cooling. At 60 mph, with 5280 feet per minute passing through there is not much need for a fan. So do you get good cooling at highway speed?

Coolant cap needs to be holding pressure, and there should be no coolant loss or bubbles sounds under dash.