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

davidearle1.DeleteThis@hotmail.com wrote:
> 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.

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