* Cracked Cylinder Head or Block: Even with a new head gasket, the cylinder head or engine block itself could have a crack. This crack might only be visible under pressure. Pressure testing the cooling system is crucial to diagnose this.
* Cracked Engine Block: This is a more severe version of the above. A crack in the block itself can allow coolant to leak into areas not easily visible. The pressure test is vital here too.
* Faulty Radiator Cap: A faulty radiator cap might not be holding the correct pressure, allowing coolant to escape into the overflow tank and then evaporate or be lost through a small leak. Replace the cap – it's cheap and a common problem.
* Warped Cylinder Head (despite replacement): While unlikely if it was *properly* machined and installed, the cylinder head could still be warped, leading to microscopic leaks. This is less likely than a crack.
* Porous Cylinder Head or Block: In rare cases, the material of the head or block can become porous over time, allowing coolant to seep out slowly. Pressure testing is essential for this as well.
* Leak in a Hose or Connection (hidden): Though you say there's no wetness, a very small leak, perhaps in a hard-to-see hose connector, could still be the culprit. A thorough visual inspection, including under the vehicle, is necessary.
* Coolant Leak in the Heater Core: A small leak in the heater core can lead to coolant loss, but it would often be noticed by a sweet smell in the cabin or reduced heat output. Check the passenger compartment carpet for wetness.
Troubleshooting Steps:
1. Pressure Test the Cooling System: This is the single most important step. A mechanic will pressurize the cooling system and look for pressure loss, indicating a leak.
2. Thorough Visual Inspection: Carefully examine all hoses, connections, the radiator, the water pump, and the engine block for any signs of leaks, even tiny ones. Look under the vehicle as well.
3. Check the Radiator Cap: Replace it. It's cheap insurance.
4. Inspect the Exhaust Manifold: Check the area around the exhaust manifold as coolant leaks there can also evaporate quickly.
5. Exhaust Gas Test: A more comprehensive test can check for combustion gases in the coolant. This would further confirm a head gasket issue, but a positive test doesn't rule out a cracked cylinder head/block.
If you've recently had the head gasket replaced, go back to the mechanic who performed the work. They should be able to diagnose the problem properly. Continuing to drive the vehicle with a coolant leak could lead to serious engine damage.
