How to Choose the Right Coolant for Your Car and What Happens If You Get It Wrong

Coolant is one of the few fluids where the specification is not interchangeable, where a wrong choice causes damage that develops silently, and where the cost of getting it wrong is measured in engine repairs rather than inconvenience. It is also the item most often topped up incorrectly, because the bottle is on the shelf and the level is low.

This guide covers the specification question and why each part of it matters.

Why Coolant Is More Complicated Than It Sounds

Coolant is not just antifreeze. It is a formulated fluid with several jobs, and each job requires something different. It has to transfer heat efficiently, which means a high specific heat. It has to resist freezing, which means a low freezing point. It has to raise the boiling point, which means operating pressure that can be managed. And it has to inhibit corrosion on aluminium, cast iron, steel, and copper, which is the job of the corrosion inhibitor package.

That last job is the one that causes the most damage when it is done wrong, and it is the least visible. Every engine has a mix of metals in its cooling system, including aluminium heads and blocks, cast iron liners or sleeves, copper or brass fittings, and rubber or plastic components. An incorrect coolant can attack any of them, and the corrosion often happens inside the block where it is not visible until the failure is well advanced.

The Specifications You Will See

The Type

Traditionally classified as ethylene glycol or organic acid technology. The organic acid types use different inhibitor chemistries and are not simply an improved version of the older glycol; they have different maintenance requirements and are not always compatible with each other. Many manufacturers specify one type and prohibit the other, and the reason is that mixing them can produce a precipitate that blocks passages.

The Colour

Colour indicates nothing about compatibility. It is not a specification, and it is entirely possible for two coolants of completely different colour to be incompatible, or two of identical colour to be compatible. Some manufacturers deliberately use unusual colours to discourage mixing with an unrelated product, and that is a reasonable practice rather than a gimmick. Never select a coolant by its colour.

The Concentration

Most products are supplied as a concentrate to be mixed with water, and some are supplied premixed at the correct ratio. The manufacturer's specification will usually state the required proportion, and it is not always fifty-fifty. Mix to the specification using the correct water, and on a vehicle that has a specific coolant requirement, use the specified type of water rather than tap water where the manual says so.

The Approval or Specification Rating

This is the part that matters. Various standards are used across the world for coolant performance, and a vehicle manufacturer will specify one of them, or their own approval. Meeting that specification is what guarantees the fluid performs in that engine, because the test programme for the approval was run against engines built to that standard. A coolant that meets only a general industry standard may be perfectly good and still not be what the engine requires, particularly where the approval includes a corrosion test on the specific alloys used.

What Happens When the Wrong Coolant Is Used

The immediate symptom is usually nothing at all, which is what makes it dangerous. Over months, the wrong inhibitor package does its work quietly. Aluminium is attacked at the hottest surfaces, which produces pitting in the block and the head around the water passages. The coolant then needs to be replaced because it can no longer carry heat, and the aluminium oxide particles it picks up contribute to the blockage.

Low-level deposits in the block passages appear at about sixty degrees, so there is a large portion of normal operating temperature where the wrong coolant is gradually doing damage and the gauge still looks healthy. The water pump impeller, which on many vehicles is a plastic or aluminium component sealed with a resin gasket, is often the first visible sign of the wrong coolant. A thermistor or a coolant temperature sensor can also be damaged, and the plastic coolant bottles crack.

The consequences of prolonged use of an incorrect coolant are significant: a damaged water pump, a cracked expansion tank, a failed head gasket from corrosion weakening the sealing surfaces, and a block that has to be machined or replaced. That is a very different scale of expense from the cost of the correct bottle of fluid.

Mixing Coolants

Mixing two types is not automatically fatal, but it is not advisable either, and the risks depend on the chemistry. Where two organic acid technologies are mixed, the inhibitors can interact and produce salts that precipitate. Where a glycol coolant is mixed with an organic acid coolant, the result is worse because the older glycol coolant depletes the newer inhibitor package, which means the newer coolant stops protecting the system even though it is a majority component.

The safe practice is to treat a cooling system as having one coolant type. When topping up, use the same specification as is already in the system. If the type in the vehicle is unknown, treat that as a reason to flush and refill with the correct specification rather than continuing to add an unknown fluid, because the flushing is inexpensive relative to the failure it prevents.

Flushing the System

A proper flush is worthwhile when the coolant type is unknown, when the vehicle has a history of coolant loss, or when the coolant is being changed at a major service. Simply draining and refilling removes a large proportion of the old coolant but leaves a significant volume in the block, the heater core, and the radiator, and it does not remove the sediment that a proper flush would.

For a cooling system with substantial sludge, a chemical flush product used according to its instructions, followed by thorough rinsing with plain water, is usually effective. The manufacturer's instructions matter, because leaving the product in the system is worse than not using it. On a vehicle where the system contains a heater core that is prone to blocking with sediment, a reverse-flush through the heater circuit may be needed.

Checking and Maintaining

Check the level between services, cold, against the markings on the expansion tank. The minimum and maximum marks are not the same point, and the level rises substantially once the engine is warm, so the check has to be done cold to be meaningful. Where the vehicle has a coolant level sensor in the expansion tank, a warning light means the level has dropped, and the fluid level should be checked both in the bottle and in the engine radiator, because they can lose different amounts.

Service the coolant on the specified interval rather than assuming it lasts for the life of the vehicle. The interval exists because the corrosion inhibitors are consumed as they protect, and once they are depleted the fluid starts attacking the system instead of protecting it. A coolant that has been in service for three years past its interval is not protecting anything.

Safety

Never open a cooling system cap while the engine is hot. The system is pressurised by the expansion tank design, and opening it releases boiling coolant at a temperature well above boiling point. If a cap has been lost entirely and the vehicle is driven, the system may not hold pressure, so an improvised cap is worth carrying.

Coolant is toxic to animals and highly attractive to them, so dispose of it carefully and never pour it onto the ground. Spilled coolant should be diluted with water and cleaned up rather than washed into a drain, and a floor that has been spilled should be mopped rather than hosed, because the floor is normally slippery and a wet floor is a genuine hazard.

For the rest of the maintenance that keeps a cooling system healthy, our vehicle cooling system guides cover the checks worth doing between services. If this guide was helpful, you may also want to read why a car engine overheats and how to trace it for the fault this prevents, and eight reasons a car is losing coolant for what to check if the level is dropping rather than merely ageing.