Overheating The temperature guage shows the coolant to be quite hot then it will drop down to normal operating range then back up to hot, down to mid range, back to hot, down to normal. This happens when I am moving. When stopped in traffic or parked the temp gets very hot and stays there. Is this a water punp problem or something else. I recently replaced the radiator and thermostat about 6000 miles ago. Thanks for any ideas.

Don't drive the vehicle until you get the vehicle checked. Running a engine that is overheating can do serious damage. By serious I also mean Very expensive!
Did you replace the thermostat in the cooling system when you replaced the coolant.
Try this simple test.
**Note** You need to put your hands near the engine when it is running. BE CAREFUL you do not touch Moving or Hot parts of the engine.
With the engine Cold, open the bonnet and find the very large black hose going to the top corner of the radiator. -Carefully touch the hose. It should be cold. Start the engine and let it idle for 5 minutes.
-CAREFULLY touch the large hose. It should be starting to get warm. Let it idle for another 5 minutes.
-Again CAREFULLY touch the large hose. It should be HOT to the touch.
If the hose does not heat up the thermostat could be faulty.
-Carefully touch the hose. It should be cold. Start the engine and let it idle for 5 minutes.
-CAREFULLY touch the large hose. It should be starting to get warm. Let it idle for another 5 minutes.
-Again CAREFULLY touch the large hose. It should be HOT to the touch. If the hose does not heat up the thermostat could be faulty.

I suggest changing the cooling thermostat to make sure it's not getting stuck causing temporary overheating and flush the system and put fresh coolant in.
The guage will go hotter then go back to normal every time the thermostat opens to let cooled water into the engine block which is normal. Time for concern is when the guage goes hot and stays there along with bubbling sounds and coolant overflow onto the ground. Which you probably already know.

i would think there is an air lock in the coolant system , have you run the engine whilst at stand still and opened the heater control to the hot position and leave to run for 10 mins or so , let cooldown then check coolant level

You most likely have an air pocket in the cooling system. You'll need to bleed it. The coolant sensor is designed to monitor coolant not steam.

It is normal for your guage to to fluckurate a bit,

Your overflow cap could be your problem. The cap may no longer hold pressure like it should causing small leaks that you don't notice which will make the car run hot. Also it could be the fans not kicking on because of a bad temperature sending sensor. If these go bad, the temp switch don't know how hot the coolant is therefore it will not signal for the fans to turn on to start cooling.

The thermostat may be wearing out. A thermostat controls the engine temperature to keep it at a consistent temperature, The other fan will probably turn on if you turn on your Air Conditioner.

My 2005 Nissan Sentra 1.8s with 55,000 mi. blows cold air from the heater. Coolant not circulating in engine. Lower hose stays cool to touch. Both heater hoses cold. Changed thermostat - didn't fix it. Coolant not leaking or dirty. None in oil - no oil in radiator. Changed worn water pump belt - pump spins freely with no noise or resistance. Cooling fans come on after warm-up. Temperature gauge stays in normal range then jumps to hot and back again. Coolant good to 10 below - didn't freeze here yet. No other problems or repairs to car. Afraid to drive and make worse. Help!

is it ever low in coolant? i agree with emissionwiz sounds like your getting airpockets do you have heat when this starts?

Hi,

If it were a defective water pump, the increase in temperature would not go back down to "mid range (getting cooler)", so most likely it is not.

It is most likely a sticking thermostat as that is basically its purpose, to restrict the coolant flow and allow the engine to reach operating temperature thereafter grant free flow of the coolant. It would seem that there is considerable delay in its opening and therefore the temperature rises above the normal operating temperature.

Hope this be of initial help/idea. Pls post back how things turned up or should you need additional information.

Good luck and kind regards.

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