Symptom: Hydrocarbons (exhaust gases or fuel) in the cooling system.
Cause: A leak past the head gasket or a flaw in the cylinder head casting or block.
Solution: While large leaks would be detectable by other means, smaller leaks may be more difficult to detect. Internal combustion byproducts or hydrocarbon leakage into the cooling system can easily be checked by using an exhaust gas analyzer.
Remove the filler cap of the radiator and put the analyzer pickup just over the top of the opening, being careful not to allow any coolant to enter the analyzer.
If the vehicle is equipped with an automatic transmission, you can brake torque it for three to four seconds. This will raise the cylinder operating pressure, which flexes the cylinder head up, relaxing the head gasket. By doing this you might find a combustion leak that won’t show up at idle speed under light-load conditions.
Multi-Layer Steel (MLS) head gaskets require a very smooth finish (30 Ra maximum) to seal properly. A cylinder head that is not torqued properly, or not replacing cylinder head bolts when required, could contribute to this problem, too.
This test will show a leak in the combustion area including head, block and anything else that will let the combustion gases enter the cooling system.