My Civic's thermometer on the dash has been fluctuating very abruptly from medium level heat up to red and back down. It happens almost instantly and is starting to worry me. Just today it appeared to be running mostly hot near the red.

I looked at the coolant and it is dark, as if the engine oil were mixing in. I read this could mean a cracked head/gasket. Also that it could be a bad thermostat or clogged radiator? Could anyone help me find a solution? Will replacing the thermostat and flushing the radiator potential fix my problem?

 

Check your alternator belt and make sure it is still tensioned. I had the same thing happen to me where the temp gauge would jump with the RPMs...Freaked me out to say the least but it was just a voltage problem....sounds lie that is what you are experiencing. normal overheating wouldn't make the needle jump up and down that fast. Also check the engine temp sensor...may be failing or the wires may have started to come loose.