I've had the feeling like my car was leaking coolant. It never over heated though at first. I had a friend of my mine who is a mechanic look over my car and he said I was a little low on coolant. Which was fine I filled it up and it was fine. About a month later I was under the hood looking to fix a vacuum leak and I notice there was no coolant in the resivor so I filled it up. I've noticed that my car heats up pretty quickly but wouldn't really get past 220, except if I left it idle, it would heat up. So I figured was just low, so I went to add more but this time there was still a small amount in the tank and there was rust. Like the coolant looked like muddy water. It then recently started over heating and I keep adding more coolant to it but it's a temporary fix. But the weird part is, I've never seen a puddle under the car or anything. Ever. So where's it going?

 

Any smoke in engine bay or out of the tail pipes?

If not, look at the water pump. There is a little hole on the underside of it. You may see a few drops or a small trickle coming from this weep hole. Check your passenger side floor and see if it's wet. That would point to a bad heater core. Another thing, look at the underside of your radiator, look for small water paths, and check the petcock. Sometimes the seals go bad and you'll have to replace it.

Check your thermostat too.

 

The past two days I haven't noticed it leak at all. Its weird. I've had to put in over 3 gallons of coolant in it over the past 3 days. I was thinking it's a blown head gasket but now it's not leaking at all.