Why does coolant leak only when it cools down?

First of all, did it have the leak when it was new? If so, then you should have brought it up with the dealer. If not, then it was good when it was new, and now something has changed. Second, you might want to establish where the leak is located. It might be in the cap gasket, or it might be otherwise. No sense in going on a witch hunt until you know.

---Bob Gross---

i have the same problem on my '99 olds alero. either it's a really slow leak, or it only leaks during that cool-down period. it could be the cap gasket, like bob mentioned. i guess i should change it anyway since the cap's "twist head" side is totally seperated from the seal side, and the seal side will not release from the expansion tank unless the car is in a heated garage.

please post your solution when you come across it,

yofa - snipped-for-privacy@hotmail.com

I had the same problem with my 92 K1500. Seems it would always leak from the upper hose at the gooseneck. Like you said, only after it had sat overnight and cooled down. The truck leaked like that for about a year and a half before I finally replaced the upper hose and thermostat while I was there. I had tried cutting the upper hose and reseating but it didn't help. You might try removing the hose and cleaning the connection with some Scotch-Brite, I found that there was a bunch of crud there when I replaced mine. Good Luck.

When you turn the engine off it builds up more pressure, and can cause a leak. When my water pump seal blew at a fast food restaurant it poured out when I shut the car off. I started it up to drive home - less than a mile - and the leak slowed to practically nothing.

========= Harryface =========

I'm sure it leaks as well when the engine is running, but with the engine heat it evaporates rather quickly because it is a small leak. When the engine cools, it quits evaporating it drips to the floor, until the pressure on the cooling system is down to atmospheric pressure.

My 81 El Camino leaked when cold. * Never * when hot. Problem was steam vent valve in water pump. Pump was quiet hot or cold. Usually when water pumps leak, it's the front bearing, which you can see. In my case, the vent hole would leak a good quart overnight. Start it up, and leak is gone. Figured it out when I changed timing chain, and decided to replace pump too.