I keep losing coolant in my 99 vert. I have 43k on the motor and have absolutely no leak that I can see anywhere. I leave the car with no mat under the car in my garage to see if I have leakage and running or not I'm absolutely dry. I check the oil and there is nothing but oil so no cracked head or block and no bad head gasket. The car runs flawlessly also. I keep losing a little at a time as the sensor keeps lighting up saying I'm low. I fill it back up and a month or so later the light turns on again.
Where the heck is this coolant going????

You can still have a bad head/intake gasket. If you ain't leaving it on the floor, it's going out the tailpipe. Time for cooling system pressure test as well as cylinder pressure/leakdown tests.

Ya, it's going into the engine. If it gets worse it can fill a cylinder while the car is off and then it will hydralock and either break the starter off the engine or blow out a cylinder the next time you try to start it.

Peter

I had this happening too but I could smell coolant, just couldn't find it. Turned out to be a hairline crack on the top left of the radiator.

You may not see evidence in the garage. Once the system is up to temp any hair line crack will create this effect. Two of the culprits, especially in a car that is 11 years old, no matter what the mileage, is fatigue. These two issues are relatively common in a car this age.. One is the expansion tanks on the radiator are plastic and can crack, also they are just crimped on with a gasket that could be bad. ( very common, evidence of this can be seen on the radiator support under the car, with stains from a leak. The other culprit is a very slight crack in the expansion tank connector ,which is under the tank.
Good Luck
Bill aka ET

I agree. I did replace mine immediately after finding this hairline crack. No more issue for me.

I have a 99 to had the same problem for two yrs and finally noticed a little antifreeze on the floor the other day it's my water pump i guess when i drove the car it leaked from the hole on the bottom of the pump for yrs but justed started to get worse now making nosie but it took yrs to fine this out.Maybe yours don't know just a thought.

Just an FYI, if you do think the tank may be the culprit. The C5 came with different styles over the C5 timeframe. They look the same but the coolant level sensor works differently. just make sure you get the correct model. Good luck