How to Check for Exhaust in Coolant

A combustion leak detector, sometimes called a block tester, checks whether exhaust gas is getting into the coolant. You draw air from the radiator or tank through a fluid that changes color if those gases are present. If the fluid changes color, stop driving and see a shop. Don't pour in a sealer and hope it goes away.

These steps are general information, not a repair procedure for your car. If you're unsure, the engine is hot, or the test result is unclear, take it to a radiator shop.

What The Color Change Means

The test fluid reacts to combustion gases. If it changes color while you draw vapors from the cooling system, exhaust is getting into the coolant. Common causes are a head gasket, a cracked head, or a cracked block. Those need a shop.

A test that stays the original color doesn't prove the engine is fine. Some leaks only show up under load, or after the engine has been hot for a while. Milky oil, white smoke that doesn't clear, or coolant that disappears with no puddle still needs a shop even if this test looks clean.

How To Use The Detector

1. Read the kit instructions before you open the bottle. How long the fluid lasts, and the exact color change, vary by brand.
2. Work on a cool engine when you remove the cap. Never open a hot radiator or pressurized tank.
3. Set the tester over the radiator neck or tank opening the way the kit shows. You want air from above the coolant. If you suck up liquid, the test is no good.
4. Start the test only as the instructions describe. If the kit calls for a brief idle, keep your hands and face clear of the fan and belts.
5. Squeeze the bulb slowly for the number of times the kit specifies.
6. Compare the fluid to the kit's chart. A clear color change toward the "fail" color is a positive test.
7. Shut the engine off, cap the system, and don't keep driving to see if it clears.

Don't inhale the fumes, and don't reuse fluid the kit says is spent.

A Positive Test Is A Shop Job

Stop leak and a coolant top-off won't repair a gasket that's letting exhaust into the cooling system. Keep driving and the overheating can turn a gasket job into a bigger one.

Take the car to a radiator or engine shop and tell them the detector changed color. A cooling-system pressure test can show whether coolant is also leaking outside. It doesn't replace this test.

Search for shops near you.

Related Reading

• Radiator Leak vs Head Gasket Leak
• How to Pressure-Test a Cooling System
• Can a Bad Radiator Damage Your Engine?

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General information only. Not a substitute for a diagnosis. Follow your owner's manual and talk to a qualified technician when something's wrong.