Swaying Temperature Guage

Why does my temperature gauge, and the water temperature, swing like a windshield wiper? 1993 Ford Explorer 171k. It has been doing this for at least 10 years, and I cannot figure out why. I have changed: Thermostat (3 times), water pump, radiator (2), heads, temp gauge, heater core, radiator cap (2). Problem persists. Temp gauge inches up to about 3/4, then the thermostat opens, and the gauge drops like a stone to about 1/4. The process repeats itself. It has never overheated. In the winter, it is very noticeable because the heat disappears when the temp drops. I have been perplexed for years. Any ideas?

Could you be more precise with the description? For example how often does it do this (repeat itself), constantly, once ever hour or so, every few days, when warm or when cold. Does it go between what you would consider normal and either the full cold or full hot? Which one? Are any other gauges involved?

The repetition is constant, like a windshield wiper. All the time, every day. No other gauges are involved. It is definitely the water temperature changing. RE: mechanic put his temp gun to it as it ran. Temp around the thermostat housing went up to about 210, thermostat opened (presumably) and the temp dropped to about 130. Temp gauge for this vehicle has the word NORMAL on it, with the R straight up. Gauge will fluctuate from just below the N to just above the L. Every other car I ever drove, the temp gauge remained steady, rarely fluctuating. I am really noticing this week because of the cold weather, and fluctuating heating in the cabin. I am considering covering half the radiator to try to stabilize the heat.

did you check the temperature guage sender.

The independant readings by the mechanic confirm the gage swings. An 80 degree temperature variation is not normal. Even if his readings are off by 50%, a 40 degree swing at idle is still not normal. What did the mechanic say about this? What temperature rating is the thermostat?

The thermostat installed is 195 degree. And the temperature sender has been changed also. I neglected to mention that initially. Also, the clutch fan was replaced. The mechanic was perplexed and guessed perhaps the radiator was the problem. He said it may be too big for the vehicle, allowing the coolant to cool too much. But the radiator was purchased at a reputable shop, and then a few years later replaced again. His second thought was that the system just needed to be burped. But all the burping he could do did not change it. As I said, this has been going on for about 10 years and I almost do not even notice it anymore. Just on these very cold days, I do notice the heat fluctuations in the cabin. I have driven it on a trip for 5-7 hours in the summer with the a/c running, and for 5-7 hours in the winter with the heat cranking. No difference, just a bit of pinging sometimes when it reaches its peak temp, before it cools back down. Again though, it has never boiled over or over heated. I have never seen the temp swings like that on any car I?ve driven.

Years ago,(back in the 1950’s or early 1960’s) in one of the Model Garage stories in Popular Science, the Model Garage proprietor, Gus Wilson, ran into this problem with a car (I believe a Triumph) that was being prepared for a rally. The temperature gauge was doing the same thing. Gus discovered that someone had incorrectly assumed the location of the thermostat and had put in a thermostat. The car then had two thermostats and this caused the problem with the gauge that you seem to be having.

While I’m certain that this isn’t the problem with your 1993 Explorer, I wonder if the thermostatic valve in the heater may be causing the problem. This may be acting as a second thermostat. As Tom and Ray would say, in the interest of science, why don’t you bypass the heater core by connecting both hoses together and see what happens.

Another experiment you might try is to remove the thermostat and heat it up on the stove in a pot of water where you monitor the temperature of the water with a thermometer. As the temperature of the water rises, the thermostat should open gradually. If the thermostat remains closed until 205 degrees and then suddenly snaps open, this may be the problem. I know you have changed thermostats 3 times, but I suppose there could have been a bad run on thermostats.

I know your Explorer has been doing this for 10 years, but you owe it to science to find out why this is happening.

Well It may be a loose cable or an electrical problem. (sorry about the spelling)