Chevrolet: Overheating, candy thermometer, temperature gage

Question
I have a 1998 Chevy Cavalier with 68000 miles on it. A few days ago I was driving and the temperature gage was in the at Hot so I pulled and checked the anti-freeze, noticing that it was low I added some stuff that I bought at a local gastation, then found out the next day that there is a special kind to add, so I then drained the coolant and added the Dex-cool GM approved coolant and now with in 5 minutes of driving the car the temperature gage jumps to hot, so I changed the thermostat and the nothing has changed, what do I do?

Answer
Mike,
    There are a couple things to check. The first thing I would be concerned about is where did the anti-freeze go when it was low the first time. I would check over the engine and see if I can visually spot any areas where it might be leaking or seeping out, and check the belts. Then run the car just up to operating temp, don't let it get too hot, and see if it is blowing heat from the heater. Could be your water pump went. While you are doing this feel both radiator hoses to make sure they are both warm and the water is circulating. Then, check the oil to make sure you aren't getting any water in the crank case; you'll know because the oil will be white and foamy. Check to make sure the fans are kicking on. I had a problem with a dual fan setup where one fan wasn't kicking on and it wouldn't cool the motor. Unfortunately when you get these motors hot they have a tendancy to warp cylinder heads or throw head gaskets, which I hope isn't the case. So, If the water is circulating and the hoses are all the same temp and the fans are kicking on and there is no oil in the water, I would wait for it to cool and remove the cap, then run it up to operating temp and physically check the water temp. I use a candy thermometer that goes to 260 degrees to dip in the water to check the temp. It could be the sensor or guage went bad as well. If it is still running hot, let it cool and pull the thermostat and run it up to temp again and see if anything changes. Check all this out, and let me know what happens and I can give you a better diagnosis. Good Luck!