Car engines get hot. After all, they are a controlled furnace exploding old dinosaur juice to give you some bit of horsepower and a lot of heat. If left unattended, the heat will wreak havoc. It will make the engine less efficient and even ruin it when it goes very high.
That is why manufacturers give you a vague but handy temperature gauge to keep an eye on things as you drive around.
You should be fine if the gauge is exactly between the Cold and the Hot side. It means your engine is within the ideal operating temperature.
Should My Car Heat Up and Cool When Driving?
Normally, your temperature gauge should be as steady as possible once the engine is up to speed and the coolant temperature has stabilized.
A well-working cooling system will keep coolant temperature changes very tight during normal driving. Your temperature gauge will not report these fluctuations.
However, you could notice your temperature gauge creep up past the middle or creep backward in the following scenarios.
- The temperature gauge will go up but not into the red zone if you push your car hard. For instance, towing up a mountain pass or doing spirited runs at a racing track
- The gauge will go up if you drive out of a cold rain storm or cold snowy area into a hot sunny environment
- The gauge will go down if the weather suddenly changes. For instance, if you drive into a cold rain storm from a sunny place or a snowy place from a non-snowing area.
The changes should be steady and gradual and correspond to the above changes in the ambient temperature or engine load.
If you experience any fast and random fluctuations, chances are something is wrong in the cooling system or the temperature gauge is faulty.
Why is My Temperature Gauge Fluctuating
Coolant temperature fluctuations are different from the steady rise and fall you will experience depending on the engine load and the ambient temperature.
Fluctuations will be sharp rises and fall without no apparent reasons. You can only peg these to a fault in the cooling system and its related sensors.
Some of the most common causes of temperature fluctuations include:
A Faulty Radiator Fan
The radiator fan is a blower that directs a stream of air over the radiator fins carrying away all that heat from the coolant circulating within the radiator. This will, inturn, lower the coolant temperature before the water pump sends it into the engine block to collect more heat from the internal combustion.
Vehicles with a mechanical fan have the fan running constantly, deriving power from the crankshaft through a fan belt. Modern vehicles with an electric fan only drive it on demand when coolant temperatures are high, and you are not driving fast enough for oncoming air to cool the radiator.
Airflow when cruising at highway speeds should be enough to keep your engine temperatures in range regardless of whether the fan is running or not.
However, stop and start traffic or parking on idle means there is no airflow, and the fan has to create it by spinning fast. You will notice temperature fluctuations in this scenario if:
- The fan is damaged and not spinning at all
- The fan is not spinning fast enough to create sufficient airflow
- One of more fan blades is damaged or broken, reducing the fan’s performance
We did a post about damaged radiator fans recently. You can check it out for more if you think you have a damaged or faulty fan in your hands.
A Damaged or Faulty Radiator
The radiator is the center of your cooling system. It handles the heat exchange and helps whisk away all the heat your coolant collected from the engine block.
Any faults in the radiator that reduce its efficiency will reduce how well the cooling system can control the engine’s temperature.
You will see your temperature gauge creep up more often when pushing the engine hard or driving on hot summer afternoons as the radiator struggles to keep up.
While people only get worried when the radiator leaks, other tiny damages will rob it of its efficiency. These include:
- Mud, bugs, or other debris blocking the fins and hindering airflow through the radiator
- Bent or broken radiator fins. Fins increase surface area, making heat exchange more efficient. You could ruin them by directly pressure washing the radiator or sticking things through the fins.
- A bad quality aftermarket radiator that isn’t efficient enough to cool down your engine even though it is the correct fit for the radiator slot
Low Coolant Levels
The coolant handles the heat transport from the engine to the radiator for evacuation before taking back slightly cooler coolant back to the engine block and repeating the cycle.
If you don’t have enough coolant to soak up more heat, your engine will be more susceptible to temperature fluctuations.
However, the levels have to be very low, to the extent of having nothing in the expansion bottle, before you notice any difference.
Make a habit of inspecting your coolant levels and have the system inspected should the levels start dipping.
Your coolant runs in a closed loop and shouldn’t fluctuate unless it leaks or burns. We did a post on coolant level fluctuations. Check it out for more insight on this topic.
A Faulty Coolant or Water Pump
The water pump is like the heart of the system. It keeps the coolant flowing through the closed loop to the radiator and back to the engine block. Without this flow, the coolant will move very slowly, keeping it in the engine block longer than it should.
Instead of keeping coolant temperature homogeneous, you will have some coolant hot enough to boil with the remaining cool.
- If the coolant stops flowing, it will boil and over-pressurize the system
- Cooling lines (hoses), the expansion bottle, and even the radiator can rapture if the pressure goes beyond tolerances
Boiling coolant is terrible at cooling the engine; hence the temperature will increase drastically, leaving you with an overheated engine.
ProTip: The cap on your coolant expansion bottle is designed to release pressure as it goes above the required level. That is why you can hear a slight hiss from it when you open the hood after a long drive. If the pressure is more than it can handle, your cooling system will still overpressurize, causing damage.
The water or coolant pump sits in the engine block and is driven by your serpentine belt. You can quickly check if the belt is on and the pump spins with the belt. Other than this, you can inspect it for other signs of damage like:
- Seized bearing or pump making it not spin even with the serpentine belt running
- Visible corrosion
- Mineral deposits and sludge buildup
- Leaking coolant around the pump
- A broken or visibly damaged shaft
A Faulty Thermostat Valve
The thermostat valve in your cooling system controls coolant flow into the engine block. It remains shut when you start the engine first, keeping little coolant in contact with the engine block. This makes it easier for the engine to heat up to the ideal operating temperature on short notice.
After this, it will open gradually, letting more coolant circulate to keep the temperatures within range as much as possible.
The thermostat will even close slightly if the coolant’s temperature dips below the ideal in case you drive into colder places, making the radiator more efficient.
The result is stable temperature regulation and fewer temperature gauge fluctuations. However, if the thermostat is damaged and stuck open or closed, you won’t have any of these benefits, and you will see your temperature gauge jump around more often.
- If the thermostat is stuck open, your car will heat up slower. It will be more susceptible to ambient temperature changes (especially drops in temperature) since the thermostat can’t exert control over the coolant temperature.
- If the thermostat is stuck closed, your engine will overheat as the coolant isn’t leaving to the radiator for cooling. The temperature will fluctuate as it tends toward the hot side.
- If the thermostat is partially stuck open, temperatures will constantly fluctuate as you drive since only the radiator, fan, and ambient temperature control your engine block’s temperature without the oversight of the thermostat.
A car that heats up slowly on cold days or heats a few minutes after starting it might have a stuck thermostat. Consider getting your thermostat fixed before the extreme heat ruins your head gasket.
A Damaged Temperature Gauge
Finally, there is a remote chance that everything in your cooling system is fine but the temperature gauge and sensor are faulty.
Wild fluctuations that jump between cold and hot could hint at this. An OBDII sensor that can pull up your coolant temperatures is a good way to confirm your coolant temperature.
If you find a major discrepancy between the sensor’s OBDII reports and what the gauge says, then you have a faulty sensor.
ProTip: Some engines have two temperature sensors. One sends data to the ECU, while another (coolant temperature sender unit) sends data to the temperature gauge.
Otherwise, the coolant temperature sensor could be faulty and send corrupt data – especially if it indicates your engine is overheating and the engine has no tell-tale signs of overheating. Or it says your engine is cold, and you can feel it is warm, and your AC heater is heating the cabin to the selected temperature.