temperature gauge going all the way up when the car is not running

my gauge was all the way up to hot then it would drop all the way to
normal, the car was not overheating it seemed like the guage was stuck and
it would go all the way up when the car was on but not running ,so they
changed my temperature sensor,it was fine for 2 days then it did it
again,it shot up when i started the car and after the car was runnng for 3
monutes or so it dropped to normal. what could be wrong??? could it be my
gauge? water pump and radiator and thermostat is fine..thanks guys

 

To clarify - the car could be sitting overnight, then as soon as you start
it the guage is at max but comes down as the car warms up? Also, when it
drops is it gradual or sudden? Finally, what model and year is this?

If that's the case (pretty much regardless of whether it drops gradually or
suddenly) it almost has to be the guage. Some guages - don't know for sure
about yours - are balanced coil types. Instead of the guage operating
against a spring the returning force is another coil energized with the same
voltage as the voltage that feeds the sensor, so the guage isn't voltage
sensitive. Balanced coil types can do that if the ground is missing from the
guage or if something is wrong with the balancing coil. If the shop has a
sensor simulator (a variable resistor or a "decade box") they can see if the
guage is being wacky at all temperatures or only when cold.

Working on guages isn't cheap because of the difficulty getting to it.
However, if the guage isn't reliable at normal and high temperatures you may
not have a choice. Not knowing if your engine is starting to overheat can
lead to some shocking expenses.

Mike

 

no its not the reservoir fluctuates

 

1994 honda civic ,it does it when siiting overnight and also after i get
off work it did it,but this morning i checked it and it didnt do it.and
its a sudden dropping not gradual.but even if i dont start the car and
just turn the key so the radio and all come on it shoots up and then drops
to normal

 

If the guage works when it isn't being weird - reading cold at first then
following the engine as it warms up - you may be best to leave it alone.
Intermittents are notoriously hard to fix and I'm sure this will be
something in the guage. After all, there really isn't anything but the
sensor and the guage (unless something really odd with the instrument
voltage regulator is doing that) and the sensor has been replaced. The guage
is a pain to get to. It won't misbehave predictably enough to be sure it's
fixed until you've had the car back a while. Trying something else means
going back into the instrument cluster, and even replacing the guage is not
a guaranteed fix because it could be a ground problem. Maybe this is
something you will need to live with until it gets intolerable.

Mike

 

My wife's 95 Civic's temp gauge died a while back and I still haven't been
able to locate the prob with it. Two days ago however the check engine
light came on and then went off again and the speedo then died and came back
about 10 minutes later.

DGB

 

Bad electrical grounds?

 

It could very well be a sticky thermostat. Two approaches: 1st: Take the
radiator cap off. Let the engine warm up; you can tell when the thermostat
opens because the hose feeding the radiator will suddenly get very hot. You
need a thermometer that will give you a good reading in the 200 to 250F
range. When the hose gets hot have the thermometer in the neck of the
radiator and note the temperature. Well over the stat opening temp would
place a lot of suspicion on the stat. 2nd: Just replace the stat if you
can.
MLD

 

Although in another post the OP mentioned it would do it the moment the key
was turned on in the morning, so it has to be a false reading.

Mike

 

thanks guys for your help i will keep an eye on it