Need help with cooling system issues.

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I've been having some trouble with my '95 Toyota Corolla. It looks like it leaks coolant, but it only seems to do this overnight when it's cold. I've checked every bit of that radiator, the hoses, and the water pump, and they're all clean. It doesn't make sense to me because I thought cars should leak more coolant while they're hot and running, since the coolant will be under pressure and circulating through the system. The only thing I haven't had a chance to check is the engine block, but that too should leak more coolant when the engine is running. Does anyone know of a situation where a car would only leak coolant while it's cold?

-Thanks
 
When the car is running, the coolant might evaporate instead of drip on the ground, so you might not see the problem then. I had a similar problem on my 1990 Protege; coolant would leak out, hit something hot and vaporize, causing steam to waft out from under the hood, but no puddles would form when the engine was running. The radiator did have a small crack, and it finally let go in the school parking lot one day (I came out to my car after class, and there was a giant green puddle underneath it). It was sort of funny in a way, but I had to be uber-careful getting it home.

Anyway, if there is a leak somewhere, you're losing coolant whether you can see it or not. Try to figure out where the coolant is leaking from. You say you've already checked around the radiator itself, but it could be something else. Our 1985 Mazda mysteriously lost coolant, and it was because there was a crack in the heater core rather than the radiator.
 
Could be hose issues... it's awfully difficult to detect pinholes in hoses because they don't always leak... only when the hose has expanded or contracted a little, or when the engine is running hot... But then again, it's best to have everything taken out and checked.
 
Try putting a hose (ie: a garden hose) over your radiator, hoses etc. If the coolant is pooling at all it should get washed out and you'll see green int he water washing out. This might help to narrow down where the leak is coming from.

Also, it's ofter easier to tell from underneath than on top, so try and get under your car and have a look around. A friendly mechanic with a hoist, or just some ramps/jacks?

I think the evaporation issue is on the money though - it's likley the coolant is leaking while your car is hot, it's just evaporating and not dripping to the ground.
 
A good check from the underside would be a start. Two other possibilities are that the heater core is dead, or that (possibly) the head gasket is leaking. Antifreeze residue on the windshield/windows (inside) is a telltale that the heater core is leaking or rusted.
 

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