Alza radiator reservoir tank always less coolant after engine off few hours

 An interesting experience that took me 5 experiment to test making sure there is no leak in the car radiator cooling system. What happened was when the coolant filled in and starting on the engine everything looks fine no leak on the major parts of the cooling system. However after a couple of hours engine is off the coolant spare tank (reservoir) is less. 

I have tested this at least 3 times and still some coolant in the reservoir tank reduce by a few mililiters see pictures below (before and after). It is very significant reduction that worries me. I've checked all the following

1. Water pump leak under near the mid section near the front - no leak

2. Check radiator - no leak

3. Check radiator hose - no leak

4. Check radiator cap - no leak

Where did some of the coolant goes to then? This lead to the major worries that it could sip through the head gasket and flow into the engine - which may result in engine damage. But there is no way for me to check without opening the engine.

So still proceeding with adding more coolant and continue the experiment. -- this time I came to realize the reservoir tank cap never been put in place in snugged fit position. I use to just put it in and push a little bit just to make sure it is closed. Therefore i decided to push the cap this time until it make the tap sound and snugged fit.

Then turn on the engine and drive a few round - then back and off the engine leaving it few hours. Walaa the coolant level didn't reduce. So I've found the solution - is to ensure there is no holes or gap in the coolant overflow tank. This is because the coolant will evaporate through any small holes / gap when it is hot and it will not happen immediately. It took few hours for it to evaporate and result in less coolant in the reservoir tank.

I've tested another time to confirm this and it really does - so this is one uneasy experience that make me learn something new about the car cooling system.

NOTE: The car cooling system including waterpump, radiator, radiator hose, thermostat, and radiator cap, coolant are among the most important parts of a car beside the engine, power steering, brake, and automatic transmission. If any part of the cooling system damage it potentially cause engine overheat and recklessly neglect it may cause major damage to the engine that will result in huge financial losses or physical injuries. The car cooling system is practically one of the part which keeps the engine running - without it engine will fail miserably. --- NOW I NEVER ALLOW THE CAR TO RUN WITHOUT PROPER COOLANT AND ENSURE ALL THE PARTS WORKING AT ITS POTENTIAL

coolant less in reservoir tank | coolant kurang dalam spare tank | alza coolant kurang dalam overflow tank selepas engine off