How to test for exhaust gases in coolant

Updated April 17, 2017

Exhaust gases in your engine's coolant are usually a sign of a blown head gasket or cracked cylinder head. Continuing to operate your vehicle in either of these cases will quickly lead to full engine failure. If you suspect you have either of these issues, you should perform this test immediately. The test kit is available at most auto parts stores for less than £32 as of 2010. Testing for exhaust gases in your coolant will take anyone with basic auto repair knowledge less than 25 minutes.

Open the bonnet of your vehicle and make sure the engine is cold to the touch before continuing. Wait until the engine has been off for at least 10 hours.

Remove the radiator cap from the radiator filler neck by hand. Pour the leak fluid into the suction tube, both contained in the combustion leak test kit.

Insert the suction tube into the radiator filler neck manually. Squeeze and release the squeeze bulb on the suction tube repeatedly to draw air from the coolant system through the fluid in the suction tube. The fluid in the tube is normally blue, but if exhaust gases are present in the coolant, the fluid will change colour from blue to yellow.

Remove the suction tube from the radiator filler neck. Reinstall the radiator cap and close the bonnet.


Head gasket and cylinder head failure is often caused by another problem in the vehicle's coolant system, such as heavy blockage in the radiator or something else that prevents the cooling system from operating at full efficiency. If you find exhaust gases in your coolant, have the rest of the cooling system inspected and repaired before performing the head gasket or cylinder head repair to ensure the new parts don't fail due to the old problem.


Opening a vehicle's cooling system when the engine is warm or hot can lead to severe injury and/or death.

Things You'll Need

  • Combustion leak test kit
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About the Author

Allen Moore's career includes awards in poetry and creative fiction, published lyrics, fiction books and nonfiction articles as well as a master certification in automotive service from the Ford Motor Company. Moore is a contributing writer for and various other websites, a ghostwriter for Rainbow Writing and has over a dozen works of fiction currently in print.