I observe my AC chills only early hours and evening times but during the day the Cool is not enough I have to keep the fan on the highest

What might be the cause and remedy?

 

What year and engine do you have?

Could be simply low on refrigerant - it is a closed system, but many leak slowly. If it's the newer 6 cyl engine - the compressor used on that engine has a variable displacement valve that can go bad and cause poor cooling.

 

My car year is 2007
6 inline cylinderThe gas level is very much ok with no leaks
Do you suggest I change the variable valve on the compressor?

 

Secondly it cool to chilling point early hours and evening times but from 11am till 4pm Is low cooling

 

start with the simple and check the system pressure. Keep in mind the pressure reading when you first start up will vary based on ambient temperatures so have a chart handy when measuring to know what is the correct pressure on the low port. Also, does the AC start cold then go completely warm on hot days? If so, that may be pointing to the compressor clutch gap being too wide (clutch heats up due to slippage, looses grip as the solenoid heats it generates less clamping power. If you turn off the AC to let the clutch cool, it will start working again...)