In a hurry? Here are the key points:
A fluctuating gauge means that the coolant temperature varies more than it should.
Check your coolant levels. If necessary, fill them in.
If the problem persists, go to a trusted local mechanic. They will investigate things like the thermostat and coolant temperature sensor and look for leaks.
So your temperature gauge goes up and down as you drive. Is something wrong with your car?
Before you delve into this guide, please know that a small amount of fluctuation is normal. As you drive, the engine is always regulating its temperature. To do this, it uses the thermostat and the cooling system.
As such, you don’t have to worry about small temporary changes (no more than 5 degrees Celsius or 10 degrees Fahrenheit). The temperature should be restored to its “normal” level as soon as the engine has a chance to cool down.
But does your engine temperature gauge constantly go up and down while you’re driving? If so, it’s a sign that something a little deeper is at play.
In this guide, I’ll explain what might be going on.
What is the engine temperature gauge?
On your dashboard, you will find the engine temperature gauge. It will be one of the smaller dials. It has a needle that points to the cooler side when the engine is off or just started. As the engine warms up, the needle will move to somewhere central on the gauge.
This should be the standard operating temperature. It’s actually the coolant temperature. It removes heat from the engine, dissipating it into the atmosphere through the radiator.
It is this needle that will fluctuate. You should always watch this while driving. Rising temperatures are often the first sign of many developing problems.
The temperature gauge sensor is usually located somewhere near the thermostat. However, each manufacturer designs their engines differently. You may need to find or take full advantage of online resources.
It sends an electrical signal to the ECU, changing the needle reading on the dash gauge.
How does the cooling system of a car work?
An automobile’s coolant is made up of antifreeze, a chemical compound that lowers the freezing point and protects the system from corrosion and mineral buildup, and water. It is stored in the coolant reservoir/expansion tank, which is located under the hood.
When you start the engine, the water pump activates. It forces coolant through a series of pipes, hoses, and channels in the block and head.
Once a predetermined temperature has been reached, the thermostat opens. It is a valve made with a synthetic wax. This wax melts when hot enough, opening up the new channel. Once the engine cools down, this wax re-adheres. This keeps the thermostat closed the next time you cold start the car.
The coolant is bypassed through the heat exchanger (known to most people as a radiator). It passes through narrow, honeycomb-shaped channels, where a clever series of tiny fins transfer heat to the air passing through it.
If this heat transfer is not enough to regulate the motor temperature, the fan will activate. It draws a lot more air through the radiator.
The coolant returns from the radiator to the expansion tank with the thermal energy dissipated in the air. This continuous cycle is always happening. It ensures that your engine is not damaged by too high (or too low) temperatures.
The car’s temperature gauge fluctuates – what’s going on?
If your car’s temperature gauge fluctuates, that is, goes up and down repeatedly, you have an interesting problem. Identifying it requires a bit of mechanical knowledge. Make sure you are familiar with how the refrigeration system works so you understand when and why it happens.
A temperature gauge that goes up and down most likely indicates that a component in the refrigeration system is in the process of failing but has not yet failed. It’s not bad enough that the Check Engine light comes on. It’s also not causing the engine heat to get completely out of control.
So it’s safe to assume that the part, whatever it is, works some times but not others.
Here are some of the most likely culprits.
coolant levels
Thermostat
Coolant Temperature sensor (CTS) or Gauge
radiator fan
ecu
coolant levels
Of course, the first thing to do is check your coolant levels. Your engine may have a hard time regulating temperatures under heavy loads if they are slightly low. When he returns to easier driving, he manages to recover.
If you find that the coolant levels have dropped to just below the indicated mark on the reservoir, top up the reservoir. The engine must be cold, so as not to burn yourself when removing the cover.
Be sure to use the correct type and bring it to the recommended range. You will find this information in the owner’s manual. It is not necessary to fill it to the maximum line. However, this can be a good indicator for when you go back to check it out.
Remember, however, that the coolant has gone somewhere. There is nothing to worry about if it is a one-time recharge as part of regular maintenance. But if the levels are back down the next time you check it, there could be a leak. This could be through a hose, seal, gasket, or radiator.
Topping up the coolant levels could stop the problem of a fluctuating temperature gauge. Likewise, maybe not. Take your car for rigorous driving (high acceleration, high speeds and steep hills) to see if you have fixed it. If not, continue with step two.
Note: Slightly low coolant could cause this. However, with poor coolant levels, your engine is more likely to completely overheat. This is dangerous and must be fixed immediately.
Thermostat
If topping up your coolant didn’t work, it’s worth getting a mechanic involved at this point. A bad thermostat isn’t all that common, although the average car will likely need one or two over the course of its life.
The thermostat regulates the flow of coolant to the radiator. If the thermostat is only partially working, it could close or open at inappropriate times or even sporadically.
With the thermostat closed, the engine temperature will rise. This happens because the coolant is not going to the radiator and therefore not actually cooling. You will see the needle drop when open as normal operations resume.
A mechanic might remove the thermostat to inspect it. They could also use an old trick. Removing the radiator filler cap will show if the coolant is flowing. When the engine is cold, it shouldn’t be, and vice versa. If you notice coolant flowing intermittently, you have identified the problem. It’s a faulty thermostat.
Fortunately, this is a simple job. Including labour, it should not come to more than €100.
Coolant Temperature Sensor (CTS) or Gauge
Again, it’s pretty rare for a coolant temperature sensor or gauge to fail. However, it is certainly conceivable!
If the sensor that reads the temperature somewhere around the thermostat stops working correctly, it will be sending inaccurate information to the gauge. The indicator will show this via an up and down scrolling needle.
The coolant temperature and the engine itself may be fine. If the problem is the CTS, expect to pay around €200 or less (including labor).
At the other end of the electrical circuit, the indicator may also be malfunctioning. The needle changes position depending on the electrical resistance in the wiring. If there is a developing problem with the connections or the gauge itself, it may only show temperatures a few times.
This would cause the needle to move up and down. If the gauge has broken, I would expect the maximum reading it reaches to be the correct coolant temperature.
Since a broken temperature gauge is very rare, providing a cost estimate is tricky. Most shops will have to inspect your car and find out more about your wiring systems before giving you a quote. Expect to pay a minimum of a few hundred euros, almost all of which will be labor rates. You may also need an automotive electrical specialist.
radiator fan
The radiator fan cools the coolant as it passes down or through the radiator. It’s not always on, but it kicks in when the temperatures start to rise. They rise in situations like accelerating hard or driving uphill in low gear.
If the radiator fan does not come on at the correct times (or at all), the coolant temperature will rise when the engine is under heavy loads. Then you may find that it slowly returns to normal when you return to flat roads at lower speeds.
Also, if the fan is always on, it may be cooling the refrigerant too much. This would cause the temperature to drop, forcing the thermostat and ECU to work harder to bring it back up. Therefore, the temperature indicator will go up and down.
The problem is likely not with the fan itself and is more likely with the wiring or ECU (see below) or other sensors such as the CTS. A mechanic can also check the fan relay. Expect to pay around €200 to €300, although this could vary.
ecu
The Electronic Control Unit (ECU) receives information from sensors (of which, in a car, there are many). Then translate this into action. In the cooling system, it receives readings from the Coolant Temperature Sensor (CTS), among many others, and transfers them to the gauge circuit.
If the ECU develops a problem, it could misinterpret information from the sensor or send inaccurate data to the gauge. Either way, the needle will show the wrong coolant temperature.
If the fault is still developing, the gauge may go up and down instead of showing an incorrect reading.
Unfortunately, ECU reprogramming can be extremely expensive. If you’re unlucky, it could go as high as €1,000. But, if you intend to keep the car, it needs to be fixed as soon as possible. Otherwise, it could turn into something more damaging and more expensive.
Temperature gauge goes up and down while driving – Conclusion
If the temperature gauge fluctuates when accelerating, driving fast, or going uphill, the problem could be with the radiator fan, thermostat, ECU, or a coolant leak.
If it happens more randomly, such as when idling, when you just started the car, or when driving slowly on flat ground, you should first investigate the CTS, ECU, and other circuitry. Again, check coolant levels for leaks.
Although the temperature gauge that goes up and down while driving can be unsettling, it should be a reasonably inexpensive fix. As usual, some diagnostic work is required.
First, check your coolant levels and top up if necessary. Take your car to a professional to have it diagnosed if that doesn’t solve your problem. Travel as slowly and safely as possible to protect the engine, even if you suspect the problem is only with the CTS.
Prevention is better than cure!
