Hello I have a 98 Civic ex. My problem is the temp gauge goes up and down while idling and driving. It reads normal temp (in the middle) then all the way down (cold). It does not read hot, all the way up, or overheating. (good, i guess?)

I have changed the thermostat, my fans do work on time with the a/c, and I have filled my coolant up. I'm just stuck as to what could be the problem?

Also I've noticed my car has been getting crappy gas mileage lately. It used to be around 300 miles a tank and now it's barely 200 miles a tank. It almost makes my 4 cylinder not even worth keeping. Other than minor problems it's been a bulletproof car and I've definitely got my money's worth, hopefully I get these couple issues ironed out.

 

the gauge is going up and down because the sensor is either bad or the connection is loose. Check the single ping (its like a 3.5mm plug) on the side of the head, that is where it connects and those connectors are notorious for connection problems.
And be lucky, because I only get like 200 miles a tank.