Originally posted by: Paperdoc
I agree with the advice to check your dipstick for signs of milky or foamy oil. That would indicate significant coolant leakage into the oil through a cracked head gasket. Let me clarify the "white smoke" comments. One reliable indicator of a cracked head gasket is this. Wait until the car has NOT been running for several hours at least. (During this time a bit of coolant can leak into one or two cylinders.) Start the engine (helps a lot to have someone else do this, so you can...) and watch the tail pipe IMMEDIATELY. If a big puff of while smoke comes out in the first 5 to 15 seconds, then disappears, you probably have a leaking head gasket.
Nobody mentioned another possible culprit. Most water pumps have a "weep hole" so that, if the seal on the pump shaft starts leaking, the coolant can drip out the hole. It is relatively common on older cars for this shaft seal to start leaking. Replacing the water pump solves that problem. A mechanic can tell you if that is your problem, unless you happen to be good enough to figure it out yourself. Now, on some cars that problem can be "lived with" for a while - you just have to keep topping up the coolant system. On mine, the water pump is inside the timing belt enclosure, and excessive coolant leakage there can contaminate and damage the belt, causing it to fail. So it's not a good idea for me to let such a problem go on for very long.