My 2002 Ecotec Sunfire with 200k is losing a small amount of coolant based on my having to top off an inch or two in the reservoir every other week, but I can't find any sign of a leak. I left it idling for an hour with a sheet of cardboard underneath and nothing. I left the sheet there overnight, and nothing. Hose attachments are all tight and the engine is bone dry. No overheating issues and no suspicious exhaust. It runs perfectly fine.
Any thoughts on where to investigate next?
Cant say for sure but my older cav used to blow coolant out of the overflow while driving. Turned out to be a clogged rad.
As an update, I rented a cooling system pressure tester and my system won't hold pressure at all. Once I pressurize to 15 psi, the needle slowly begins falling and in a couple of minutes all pressure is gone. I couldn't see or hear any escaping fluid during the test. The bottom of my engine is bone dry, as is the radiator, all the hoses, and the connections on the pipe that goes from the thermostat to the back of the water pump. I don't smell or see coolant in the passenger compartment, and I pulled a cold oil sample from the drain plug and the oil looks perfectly normal.
I'm totally stymied.
Head gasket leaking into one or more cylinders?
A small leak might not affect engine performance, and the coolant would go out with the exhaust.
Time to check those spark plugs and find out if they are nice and clean from a steam bath. Seriously, the head gasket may be going and it is just a slight leak at this time. If the spark plugs look normal, then either the heater core is leaking or the radiator. Heater cores are no fun to replace but I would imagine there would be a smell in the cabin if that were the case.
I pulled the plugs and they all look approximately the same, clean. This car gets almost entirely highway miles. I don't see a drop from the radiator, and I don't smell or feel anything in the cabin. I'm changing out the surge tank on the chance that something is leaking there, and I'll see how that pans out.
I had a similar problem with mine. Was a clogged radiator, when it got hot, even a little it would boil out the overflow. A new radiator is fairly cheap.