Short answer: Whatever the journey or conditions, the temperature gauge needle should stay steady on the normal mark throughout. If it doesnt, something is wrong. Either the engine is developing too much heat, or the cooling system is not doing its job properly.
If the variations are not extreme, the problem might be minor so you can continue motoring without undue concern. But dont ignore the warning. The fault will not fix itself, and it is likely to get progressively worse. The list of possible insipient weaknesses is not short. Heres why:
Long answer: On any trip, your engine is generating heat like a fireplace through combustion of fuel and added friction. If it was not cooled, the metal would expand and the moving parts would jam solid, scratching, burning, bending and snapping all sorts of components in the process.
The first and last line of defence is engine oil, which reduces friction and absorbs and dissipates some of the combustion heat. If the oil level is too low, or its condition is weak or dirty, more frictional heat can overwhelm even a good cooling system (and burn the oil itself).
Next, further and essential cooling is provided by a robust flow of water with or without additives and technically known as coolant pumped through a jacket of channels inside the walls of the engine casing (where it collects heat). It then circulates through hoses to the radiator (where it is cooled by the passage of air) before returning to the engine again. The temperature gauge measures the heat of the coolant at the radiator.
There must be enough coolant (check) and there should be no leaksanywhere. It must be pumped round at the right speed; not so fast that it doesnt have time to cool enough in the radiator; not so slowly that it spends too long in the engine and vaporises. The flow is generated by the water pump (check) which is driven by a fan belt which is carried on pulleys which have lubricated bearings (check), and flow is controlled by a self-adjusting valve called the thermostat (check) fitted in the coolant cycle between the engine and radiator.
It constantly varies the flow so whether the car is cruising in cold weather or working very hard in hot weather, the temperature remains the same all the time. If the needle goes higher or lower, something is happening that the automatic control mechanisms cannot fully cope with.
To enable complete control, coolant must flow well through all the channels, which can get bunged up with rust and dirt. Using only very clean and pure water and designer additives can help prevent that.
The coolant must be kept under the right amount of pressure enough to stop water from turning to steam in places where it reaches temperatures well above 100 degrees centigrade, but not so much that it bursts hoses and radiator seams. The radiator cap has a pressure spring that organises that balanceif it has the right rating and is working properly.
And there must be plenty of air passing through the radiator fins and over the engine to carry the captured heat away. The cars own motion is enough when cruising, but at low speeds or in traffic or under extreme load, extra air flow must be provided by a fan - driven by a belt on more pulleys (check) or a temperature-triggered electric motor.
And the radiator itself should not be blocked by dirt inside or out or by fin damage or ill-placed accessories obstructing the airflow of cooling air getting in and out.
So checking your cooling system is not one or two items. Its more than a dozen. And add another: develop a habit of regularly checking your temperature gauge (eyes) when driving, and be alert to changes in smell or sound (nose and ears), too. Driving is a multi-sensory process.
If the temperature reading goes below normal, it means the thermostat is not regulating flow properly, and the engine is running cooler than design intends for optimum performance, economy and durability. If the temperature needle goes to zero, stop as soon as possible. It means no coolant is flowing, either because there is no coolant or part of its circulation channel is blocked most probably by a broken thermostat that is jammed shut.
In an emergency, faulty thermostats can be removed and you can motor on to your destination (and far beyond) without one.
However, your temperature needle will often read well below normal so your car will run better and last longer if you fit a replacement as soon as practicable.
Is it okay to use water instead of coolant? Plain water is a coolant,
