Automotive
Engine Coolant Myths: What Water, the Gauge and the Calendar Get Wrong
Three common beliefs about engine coolant do not hold up against what the motoring clubs say: that plain water is an acceptable substitute, that a steady temperature gauge shows a healthy system, and that coolant either never needs changing or changes on a fixed calendar. In each case the same authority settles the question. It is the vehicle maker’s owner’s manual, which RACQ, RAA and RACV all point the reader towards, with a mechanic for any remaining doubt.
What the cooling system does
Engine coolant circulates through the engine, the radiator and the hoses, moving heat out of the engine. That description is all the detail the argument below requires. It carries no temperature figures, because a number that is right for one engine is not right for another.
Three myths, set against the evidence
| Claim | What the evidence says | Who says so |
|---|---|---|
| Water is enough | Adding water alone will not fix an overheating problem, and refilling a hot engine with cold water can cause major damage | RACQ |
| If the engine is not overheating, the system is fine | Coolant levels should be checked regularly, and any coolant loss should be checked immediately | RACQ |
| Coolant never needs changing | The type of coolant and how often it is changed are set by the vehicle maker, and the manual or service booklet is the guide | RACV |
The second row separates two different things. A temperature gauge reports the engine’s temperature, and a level check reports how much coolant is in the system. RACQ treats the level as something to watch in its own right, which is why a normal gauge reading does not close the question.
The third row explains why this article gives no change interval. Any figure quoted without a make, model and engine attached would be a guess. The calendar is a convenient proxy for a service schedule. It is not one.
There is a fair point on the other side of the first row. Where a driver is stranded with an empty system, water to get the car off the road is better than driving on with nothing, and that is this article’s own position, not a rule quoted from a club. What matters is the step that follows: have the system checked, because the water has not corrected whatever lost the coolant in the first place.
The three rows read together
Taken together, the rows describe a sequence and not three separate rules. The level is checked on a cold engine, at intervals, against the marks on the reservoir. A level that keeps falling is reported to a mechanic, because coolant loss is the thing RACQ says to have checked immediately. The fluid that goes back in is the one the manual names, and the manual’s schedule says when it is renewed. Water enters this sequence in one place only, as a stopgap to get off the road.
A hypothetical case shows how the rows interact. (Everything in it is invented for illustration.) Take a driver whose temperature gauge has always sat in the middle and who has never opened the bonnet. The gauge reading says nothing about the level, so the level is unknown. If that driver tops up with water after noticing a low bottle, the system is fuller, but the cause of the loss is unaddressed, and the coolant that remains is diluted by an unknown amount. The gauge reading would not have told the driver any of those three things.
How to check the level
RAA advises checking the coolant only when the engine is cold, because otherwise liquid can spurt out and burn. The check itself is a look at the plastic flow bottle, where the coolant should sit between the low and high lines, about halfway between them. RACQ adds that the owner’s manual explains how to do this for a particular car.
Signs worth a mechanic’s attention
The sources support a short list, and only that:
- a rising temperature gauge or a warning light
- coolant loss
Neither is a diagnosis. Both are reasons to have the car looked at, and RACQ advises having any coolant loss checked immediately.
What the sources settle, and what they leave to the manual
| Question | Settled by the sources | Left to the manual |
|---|---|---|
| Is water a substitute? | No (RACQ) | The coolant the engine is designed for |
| How is the level checked? | Cold engine, between the low and high lines, about halfway (RAA) | Where the reservoir is, and how to read it on this car |
| How often is coolant changed? | Not settled by any source here | The interval, set by the maker (RACV) |
What these figures leave out
The only measurement in this article is the “about halfway” position of the fluid between two marks, attributed to RAA. It excludes the mix ratio of coolant to water, the type of coolant an engine tolerates, the change interval, and whether a particular product suits a particular engine. All four are specific to a vehicle, and none can be read off an article.
It follows that the sources do not rank one coolant over another and neither does this piece. A reader comparing products should compare them against the manual’s specification, and a mechanic or the service booklet can confirm it.
What the manual decides
The owner’s manual decides the type, the mix and the interval, and it explains how to check the level on that car. Water is for getting off the road, a steady gauge is one reading and not a verdict, and the calendar is a guess at a schedule the maker has already written down.
Sources: RACQ, “What to do if your engine overheats” (https://racq.com.au/articles/drive/2023/1/what-to-do-if-youre-engine-overheats); RAA, “Can your vehicle handle the heat?” (https://samove.raa.com.au/can-your-vehicle-handle-the-heat/); RACV, “Easy DIY car maintenance tips and safety checks” (https://www.racv.com.au/royalauto/transport/cars/easy-car-maintenance.html)