Somewhat of an overheating problem

To start off, I had a leak in the bottom hose to my heater core about half a week ago so me and my dad replaced them but with just normal hoses since noone in town had both hoses. I was in a hurry to get to an appointment so I filled my coolant up with water (still in there now, haven't had time to flush it out and put antifreeze in it). Ok so here's how the story goes. My car was basically all the way cooled before I started driving. Then after about 6 miles of driving, my temperature gauge started going up and down. It's weird, it stays at about 150 when I'm driving but when I get to a stop light or slow down it goes slowly up to about 200 and just sits there (doesn't go any higher than 200) until I start going again. Do you think it has something to do with only having water in my coolant and not antifreeze? And another thing, what should my temp gauge be at normally? It should be less than 150 shouldn't it? Sorry if this is a stupid question but I'm only 17 and don't know a whole lot about cars yet.

EDIT: 97 Grand Prix GTP with a 3800

What year Grand Prix & motor are you refering too?
Sounds like the thermostat is stuck open. At normal driving speeds, the higher motor speed & water pump speed with the vehicle speed having more airflow through radiator cools the coolant more than the engine can heat it. At slow speeds or stopped, the slower motor, water pump & vehicle speed means less airflow & waterflow through the radiator and the engine can now heat up a little.
A new thermostat will regulate the waterflow so that the engine can heat up to running temperature and stay fairly close to it by reducing waterflow when the water cools too much and increase waterflow when the temperature gets too warm.
Replacing the thermostat will require draining the coolant again so you can get the antifreeze mix correct...which chemically is much better for your motor, radiator and heater core than water alone.

It's weird, it stays at about 150 when I'm driving but when I get to a stop light or slow down it goes slowly up to about 200 and just sits there (doesn't go any higher than 200) until I start going again. Do you think it has something to do with only having water in my coolant and not antifreeze?



200 is not that bad considering the stock thermostat is 195 degrees. Technically, really not an overheating problem from what I see but I agree you may have a thermostat stuck partially or fully open.

It is not a good idea to run water only in the cooling system. Best to use the proper mix of coolant. But do a proper flush first.

200 is not that bad considering the stock thermostat is 195 degrees. Technically, really not an overheating problem from what I see but I agree you may have a thermostat stuck partially or fully open.

It is not a good idea to run water only in the cooling system. Best to use the proper mix of coolant. But do a proper flush first.

All right thanks. I was going to get a new thermostat anyway and I guess I will now since they're only like $20 shipped. But should I go with the 180° or the 160°?

an engine is cooled by two different methods, air flow and coolant flow, in combination. the coolant and engine compartment are cooled by airflow when the vehicle is in motion by air being forced thru the radiator and engine compartmant. when the vehicle is at rest, and the engine is running, the lack of airflow is augumented by a thermostaticaly controlled fan that kicks in at about 215 degrees. if your vehicle doesnt go over 215 degrees at rest your cooling system is working correctly. a quick check would be to drive around a bit with the a/c off to get the temps up to 195 (where the thermo opens) and then park it it in your driveway with the hood up and watch the temp guage. when it reaches about 215 the first fan should kick in and start reducing the engine temp.

should I go with the 180° or the 160°?

Is this GTP stock or modded? Typically best to stay with the stock 195 unless you plan on modding like going to a smaller supercharger pulley size or plan on getting a re-programmed PCM module which will have modified fan turns on points that support either 180 or 160 degree thermostats. A cooler thermostat helps reduce knock retard (KR) which can damage an engine if it goes unchecked. Also, note that with lower temperature thermostats it may take longer for the engine to reach proper operating temperature. The effect you may get is it will take longer to feel warm/hot air out of the heater system during the cold season or winter months.

Is this GTP stock or modded? Typically best to stay with the stock 195 unless you plan on modding like going to a smaller supercharger pulley size or plan on getting a re-programmed PCM module which will have modified fan turns on points that support either 180 or 160 degree thermostats. A cooler thermostat helps reduce knock retard (KR) which can damage an engine if it goes unchecked. Also, note that with lower temperature thermostats it may take longer for the engine to reach proper operating temperature. The effect you may get is it will take longer to feel warm/hot air out of the heater system during the cold season or winter months.

Right now it's stock. But hopefully sometime (before Christmas I hope) I'm going to get a cold air intake and a smaller pulley. After I do those then should I get a cooler thermostat or still leave it the stock one?

Since you plan on going to a smaller SC pulley go with the 180. It is a good all around thermostat and that way you won't have to waste money or install another one later.