Temp gauge on 97 SOHC stays at hot but fluctuates slightly

I have a 97 Explorer Limited SOHC. A few weeks ago I started up the truck and within a few minutes the temp gauge was all the way at the
top and the computer in the console told me to check the engine temperature. Since then, every time I start it, even if it has been sitting for days, the gauge stays right at the red line but will move to either side just a bit. As soon as it is over the red line the computer tells me to check the temperature. Since the gauge does move a little bit I was thinking the problem is the temperature sending unit. I looked in my Haynes manual and it says that to replace the sending unit I must remove the upper intake! I check it out and I think that if I remove a few hoses and use enough U-joints on a socket that I can probably get it out of there without removing the intake. Has anyone done this before?
I searched this group and I found a few threads that are similar. Seems that some people had a bad thermostat and the engine really was overheating. I don't think this is the case for me as even a cold start keeps the gauge in the red. Another person ended up having a faulty gauge but since mine does move a little I'm leaning towards the sensor. What do you think is the likely culprit here?
Any help is appreciated, Dave
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snipped-for-privacy@hotmail.com wrote:

I'm trying to think of ways to sort this out- whether it is actually overheating or a false indication.
You don't mention your coolant level- are you losing coolant? If not, that's a good sign. Also, I'm not clear on the behavior of your temp gauge- on a cold start, it should indicate cold, at least for a few minutes. Is that the case? If it indicates hot on a cold start, then definitely you have a faulty indication, most likely the sender or possibly the wiring.
You could also pop the hood, remove the radiator cap, and start the (cold) engine. As it warms up the thermostat should open, and you should see the coolant start to circulate thru the cap opening. This will happen when the gauge indicates its normal operating range, if the gauge is reading correctly and the thermostat is functioning properly. The upper radiator hose should warm up and get hot too, as hot coolant flows thru it. (I'm assuming your year has a normal radiator cap on the radiator so you can see the coolant in the radiator)
If it's truly overheating, you should see signs of that- with the cap off, your coolant can't build pressure, and therefore it will start to boil. If your gauge indicates that it's overheating, yet the coolant is circulating normally and doesn't appear to be boiling, I'd say the sender is faulty. I don't think it can be your gauge, because the computer is also indicating overheating.
If the temp gauge rises steadily, and you don't see the coolant start to circulate thru the radiator I'd say your thermostat isn't opening.
You could also try turning on your heater and fan full blast, and see if that drops the indicated temperature at all. The heater bypasses the thermostat. Hope that helps. -Paul
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Paul, The engine has been losing a slight amount of coolant for a while now, not sure where it's going but it isn't much. Every few thousand miles I'll notice it's down an inch maybe.
Since this first started happening, the gauge won't leave the hot zone, so when I shut it off the gauge stays at hot. Then, days later on a cold start, the gauge is already in the hot zone but might move slightly around that area. As soon as it passes the red line at the top the computer in the console will yell at me that the temp is too hot and the "check gage" light in the dash will come on. Because of this it has to be a faulty indication of some type. I'm just not sure if it is the sending unit or the gauge (could be something else too I suppose). I bought the sending unit at autozone for $21 but the book says I have to remove the upper intake to replace it. If that is the case I'm going to bring it in for an official diagnosis rather than ripping off the intake on suspicion.
Thanks for your response,
Dave
snipped-for-privacy@egine.com wrote:

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snipped-for-privacy@hotmail.com wrote:

I think that's normal evaporation from the overflow tank.

OK, that confirms it's a false indication. I don't have a circuit diagram for a 97 SOHC, so I can't pin it down further.
The sender is the most probable, but it could be a wiring problem (pinched/shorted, disconnected) or even the EEC-V computer.
If you can see the sender, check the wiring to it- any obvious problems? If you can disconnect it and reconnect it, do that. Sometimes it's just oxidation or dirt on connections, and removing and replacing scrapes that off.
Good luck with it, -Paul
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