5 Reasons You’re Losing Coolant But Have No Visible Leaks

(Updated on November 19, 2021)

Internal combustion engines turn your wheels using power generated from small, controlled explosions. These explosions generate loads of heat, and this waste heat must be transferred away from the engine block.

If the engine block were to get too hot, materials in the engine could warp, crack, or melt.

Coolant, Radiator Fluid, and Antifreeze… What Does It All Mean?

engine coolant

Coolant, radiator fluid, and antifreeze commonly refer to the same mixture of water and antifreeze inside your radiator and are often used interchangeably.

Excess heat can be transferred away from the engine using water or air. Modern engines use water, and this water is usually mixed with a substance called antifreeze.

Antifreeze is a liquid additive designed to lower the freezing point and raise the boiling point of water, allowing the cooling system to operate smoothly under vast ambient temperature changes.

Antifreeze also reduces the chance of water freezing inside the engine, which would otherwise render the engine inoperable.

Most engines call for a 50/50 mixture of water and antifreeze.

Reasons You May Be Losing Coolant

If your coolant reservoir is empty, you may be slowly losing coolant. It’s important to check the coolant reservoir and radiator if you suspect a coolant leak.

Never open a pressurized radiator or reservoir while the car is hot. Wait until the car cools off before topping it off with coolant.

1) Bad Radiator Cap

bad radiator cap symptoms

A bad radiator cap may not seal properly, which could lead to coolant leaking past the seals and evaporating. Look for steam near the radiator cap, and also check the overflow reservoir if your overflow reservoir is pressurized.

2) Blown Head Gasket

MLS head gasket

The head gasket sits between the cylinder head and the engine block. Its job is to keep combustion gases in the combustion chamber and coolant in the water jackets.

When a leak forms in the head gasket, it is possible for coolant to leak into the combustion chamber and burn off, exiting the exhaust. When you have an internal head gasket leak, you are unlikely to notice any coolant leaking onto the ground. Common symptoms are overheating and white smoke coming from the exhaust.

Early 2000s Subarus with the 2.5 L four cylinder single overhead camshaft engine are particularly prone to head gasket failures if the head gasket has not been replaced since the car was new.

3) Blown Intake Manifold Gasket

bad intake manifold gasket symptoms

Some intake manifolds have water jackets. If these intake manifold water jackets were to leak into the intake, coolant would be sucked into the combustion chamber. The resulting symptoms would resemble a blown head gasket.

4) Leaking Turbo Seal

turbo engine limp mode

Most turbos are cooled using engine coolant. If a turbo seal allows coolant to leak into the turbo, you will lose this coolant out the exhaust.

5) Evaporation

engine overheating symptoms

Even if you don’t see a leak on the ground, you could still have a tiny pin hole leak in one of the hose fittings. If the cooling system isn’t perfectly airtight, some coolant may evaporate slowly over time.

Sometimes you actually have a slow leak that allows coolant to seep out of the cooling system in liquid form. When this happens, you may see white streaks near the source of the leak.

Why Is My Coolant Evaporating?

The water in your coolant can only evaporate when the system isn’t perfectly sealed. The source of the leak can be tricky to find since it happens so slowly.

As long as the coolant loss is very slow (a few drops every month), this is often no immediate cause for concern. Keep an eye on things to make sure the leak doesn’t get worse. If you start to see coolant on the ground, it’s time for a proper repair.

Why Is My Car Losing Coolant but Not Overheating?

low coolant warning

If you’re just driving normally, your car can lose a substantial amount of coolant before you notice the needle move on the temperature gauge. As long as there is enough coolant to adequately keep the engine cool, you may not even notice there’s a problem.

It is possible to damage the engine from running low on coolant, even if your temperature gauge looks fine. Low coolant may form air pockets in the engine where the temperature sensor can’t pick them up.

These air pockets create hot spots where there is not enough coolant to cool the metal in that area. Hot spots lead to weakened, warped, or cracked materials.

As soon as you notice a loss of coolant, it’s very important that you top off the cooling system. If antifreeze isn’t available, you can use distilled water until you can return your cooling system to the nominal 50/50 water and antifreeze mixture.

Once your engine overheats, you run the risk of very expensive engine damage. You can very quickly warp your heads or crack your block, which both require invasive (and expensive) engine work to fix.

Many temperature gauges don’t move off center until they’re already too hot. If you see the needle move toward the “H”, pull over immediately (as soon as it is safe to do so). The sooner you let the car cool, the less likely you are to damage the engine.

How To Stop Losing Coolant

1) UV Leak Detection Kit

coolant uv dye

First, if your cooling system is leaking you need to find the source of the leak. There are UV leak detection dyes you can put into your cooling system.

After these dyes circulating through the cooling system, shine a black light around the engine bay. This is an easy way to determine if your cooling system is leaking externally.

2) Leak Down Test

leak down tester

A leak down test is an effective way to determine if your cooling system is leaking internally, and where the leak is going.

A leak down test involves hooking up compressed air to each cylinder on its compression stroke and measuring the amount of air that leaves the combustion chamber.

Although you’ll see a little leakage on even a healthy engine, excessive leakage on one or more cylinders is indicative of a mechanical problem. 

3) Radiator Pressure Test

radiator pressure tester

A radiator pressure test may help expose small leaks by pressuring the cooling system at or just above its operating pressure.

This simulates normal driving conditions and helps find pinhole leaks that only show themselves when the system is under pressure.

When you do a pressure test, make sure you don’t put too much pressure into the cooling system. For instance, a system rated for 15 psi may not be able to hold 35 psi without breaking something important.

Leave a Comment