Please read this carefully before posting comments or answers. This is NOT an overheating issue. It is just about cooling system behavior under specific conditions.

My car cooling system has two fans which operate in two different modes. The first mode is low speed operation which means the fans run with 6V power at half speed. In this mode, they turn on when the engine temperature reaches 95-96 degrees and turn off when it reaches 92-93. The second mode is high speed operation which involves the fans to run with 12V power at full speed. In this mode, they turn on when the engine temperature reaches 105 degrees and turn off when it reaches 101-102. When A/C is off, high speed mode is never activated because in low speed mode, fans have enough power to keep the engine cool.

Today, I was driving up an uphill highway for 20 minutes (in 4th gear) and the outside temperature was about 39 degrees Celsius (102 degrees Fahrenheit). I noticed the temp gauge reached about 96 °C (when low speed mode turns on). I continued driving uphill and the temp showed 97 and even 98 °C. It didn't go higher and stayed there (between 97-98) and started to go lower (between 93-94) when the slope of the road decreased a little bit.

Why couldn't the fans bring the temperature back to 93 °C when I was climbing the steep road? Is it normal that cooling fans operate but they only manage to keep a steady engine temperature in a steep road? In most cases when low speed fans kick in, they lower engine temp but today this didn't happen. Why?

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  • What was the temperature gauge used? The one on the dash? Notoriously inaccurate... have you used a reader to get the engine temperatures?
    – Solar Mike
    Jun 11, 2021 at 19:16
  • Was the A/C running? There are design limits for most things. The more work the engine is doing, and the higher the ambient temperature, the more difficult it is to keep the coolant temperature down. Engines do overheat. Jun 11, 2021 at 19:18
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    Why would you expect them to turn off if they haven't been able to bring the temperature down to 93? I am struggling to understand the question. As I said, there are design limits to everything. By powering on a long up grade on a hot day you reached them, and the engine did not overheat. In days gone by, you might have expected to have to stop part way up and allow the engine to cool off. Jun 11, 2021 at 19:37
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    No, they are not. At some ambiemt temperature and on some grade of hill at some high speed, and with a fully laden vehicle, they will not do that. If you go to the dealer and complain "Those fans are rubbish. I stressed the car and the coolant was 96 when it should have been 93" they will look at you in a very peculiar way. Jun 11, 2021 at 19:58
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    Something nobody is considering here is, when the vehicle is traveling above 35 mph (50 kph?), the fans do nothing. The speed of the air going through the radiator is faster than what the fans can draw. You could completely unplug them and you'll not notice any difference. Jun 11, 2021 at 20:25

1 Answer 1


Yes, this is normal behavior.

You wrote 'When fans turn on, they have to bring the temp down and turn off and this cycle should go on and on...'

That is not completely accurate. There is a set upper temperature limit and a set lower limit for each fan speed mode. When the temp hits the upper temp limit for the low speed fan operation, the fans will turn on as you know. At this point, most times you will see the temp drop as the fan speed is such that it draws enough air flow to lower the temp and then when the temp reaches the set lower limit, the fans turn off.

The fans are supposed to CONTROL the engine temperature, but they aren't very sophisticated in operation. So, in normal driving conditions if the engine gets a bit hot and low speed fans turn on, you will see the temp drop and the fans turn off IF the fans are able to overcome the amount of heat generated by the engine. If engine effort is maintained then of course the temp will rise again and the cycle will continue.

However, the fan design is such that it draws air at a certain rate. Low speed draws less air than high speed. When you were climbing the hill and the engine was generating more heat than usual, the low speed operation of the fans turned on as the temp reached the upper threshold. The rate at which the fans drew in air at that speed (combined with the natural air flow as the vehicle moved forward) essentially shed the engine heat at roughly the same rate as it was able to generate the excess heat.

This is why the temp maintained a steady, higher than normal temp.

  • Thanks a million for your great answer. So, you believe this behavior is normal but there is something to add. Yesterday, I was the only passenger in my car. What would happen if I had 4 other passengers on board? Could fans keep the coolant temp at a steady 97 - 98 degrees?
    – LFY MP7.3
    Jun 12, 2021 at 7:30
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    Should be fine. They design and test vehicles to take into account many different scenarios. If all systems are working as normal (coolant system, engine management etc.) then it should be more than capable of handling a full passenger load up a hill.
    – NetServOps
    Jun 12, 2021 at 8:04

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