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
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rynniki wrote:

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Is the coolant all the way to the MAX line in the reservoir? That's the first place to look.
'curly'
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no its not the reservoir fluctuates
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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
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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
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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
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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

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"DGB" <dgbtfsatcomcastdotnet> wrote in

Bad electrical grounds?
--
Jim Yanik
jyanik
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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
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was turned on in the morning, so it has to be a false reading.
Mike
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thanks guys for your help i will keep an eye on it
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