Mazda 2 engine fan starts after turning engine off

Question

Apologies for some unclear terms, I have no idea how some of the components are correctly called in english.

I have a 2003 Mazda 2 DY gasoline with about 77k KM driven. Since a couple of months, the fan in the front of the engine bay will start as soon as I turn off the engine. I often read that this is normal, however in the past 9 years I have the car, this only happened very rarely. Since it started in summer, and I've driven a lot Autobahn, I figured it was just cooling down, but it has continued to do so until winter. The fan runs almost exactly for 2 minutes and is pretty loud.

It seems to be somewhat connected to the heater. The car has AC, but it doesn't work and is turned off. I turn the dial to warm when I start driving (it's cold outside!), and it takes a good 5 to 10 minutes until it really warms up. If I leave the dial on max heat, the fan will reproducibly turn on after the drive. If I put it in the middle (between cold/warm) a couple of minutes before I arrive, there's a 50/50 chance it will start.

There are no warning lights on or fuses out. Engine temp is always normal. The fan and the radiator are not obstructed.

Before I drive to the workshop, I at least wanted to do some self-diagnosis.

1. Could it be connected to the non-working AC?
2. Is it possible that I need to refill the engine coolant? I thought the tank looked kinda empty, but I'd have to check again.
3. Other ideas?

Answer 1

Great question. It sounds like the problem is linked to the coolant level, and not the AC.

I'm not familiar with the plumbing in this particular cooling system, but it is definitely normal for the fan to kick on after you turn off the car to continue pulling air through the radiator to help cool the coolant, but usually only when the outside temperatures are hot, as you mentioned.

If your coolant is low, it's possible that when you shut off your engine (and subsequently the water pump), the coolant level drops below the thermostat. When that happens, the thermostat is exposed to ambient air, which is much hotter (due to radiating engine heat) than the coolant. This would make the car think the engine is hotter than it actually is, kicking on the fan for a few minutes.

This would occur more frequently when you are using the heater, because more of the coolant has been diverted into the heater core, meaning there is less coolant to cover the thermostat. This would seem to explain why it almost always happen when you have the heater on hot (max coolant diverted to heater core), and not as often as you move the setting toward cold (less coolant diverted).

At this point, you may have introduced air to the coolant system, which can cause cooling issues down the road. As long as you can still see something in the coolant reservoir, you probably didn't cause any permanent damage. At the very least you should top the coolant back to the intended level, but I would also recommend having the coolant replaced to ensure there is no air in the system.

EDIT: As for the low coolant, if possible, you should inspect the coolant hoses and water pump for leaks, or have a mechanic do the same. Considering the age of the car, it could be as simple as a cracked or loose coolant hose (fairly cheap), or a failing water pump (not very cheap). If anything is leaking, ignoring the problem could risk overheating the engine (very not cheap). Good luck!