Originally posted by 96mercury:
You need to remember on our cars there is the fan switch and temp sender. Both located on the thermostat housing. I replaced my fan switch and the car seams to keep a little cooler. If the sending unit for the gauge was at fault it wouldn't always read in the middle of normal while driving and creep up to the a or L when not moving, because the thermostat prevents water from circulating much until it is at the right temp. Meaning that there should be little movement on the gauge between driving down the road and holding still, just seems like the fan turns on late on these cars.



That's incorrect. There is a Temp SENDER for the gauge and the coolant SENSOR for the PCM which is what controls the fan. The fan on the Zetec doesn't come on until almost in to the red, unless you turn on your a/c. Driving down the road you have air flowing through the radiator cooling the coolant. Your car once warmed will usually read the coolest while driving because of that. Idling there is no air circulation cooling the coolant until the fan kicks in. All the thermostat does is stay closed until it reaches the temp it's calibrated for(190 in this case) at which point it opens up and allows cooled coolant in to the engine. It's mainly there to help get your car up to operating temp quicker and to give you heat. Once it's open it's open as long as the coolant doesn't go below 190 (or what ever it is calibrated for). Ofcourse now it helps with emissions purposes also because a warmer car produces less harmful emissions. If you didn't have a thermostat it would take a while for the engine to reach operating temp. In these cars that causes a CE light and rich fuel misture.

From your post that I quoted I'm not sure if I feel that you're actually overheating. If your gague is showing at "a" or "l" while idling, then that is not too far from the average. I wouldn't be suspecting anything wrong. Changing to a 180 thermostat would make the car run cooler while driving, but the fan will still kick in at just under the red.

Tronx....I get what your saying, but I've been working on cars for many many years and it is a sending unit for the gauge and a sensor for the pcm. This goes back many years. Some cars combine both in to one sensor/sender. When you go to buy one for the gauge if you ask for a sensor you will get the wrong part. It's just an old standard that goes back before engine management computers.

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