The temperature difference between water inlet pipe and water outlet pipe of automobile water tank is large.

Try to warm the car first, then turn on the warm air. If the warm air is not hot, it proves that the water is not circulating. Then stop and run at idle speed, then feel whether the temperature of the upper and lower water pipes is the same, and then see if the fan stops running for a while. If it keeps turning, it's not normal! Try what I said first and see what the result is. We'll discuss it tomorrow!