How to Diagnose a Head Gasket Failure Causing Oil Contamination in NZ

The Head Gasket — What It Does

The head gasket is a multi-layer steel or composite gasket that seals between the cylinder block and cylinder head. It must simultaneously seal the combustion chamber (against pressure up to 70 bar), the coolant passages (typically 1-2 bar), and the oil galleries (typically 3-5 bar). When it fails between a coolant and oil passage, the higher oil pressure drives oil into the coolant, or the higher coolant pressure drives coolant into the oil — depending on where the failure occurs.

The Five Diagnostic Tests

1. Dipstick and cap inspection — Milky oil indicates coolant in oil. Clean but rising oil level also indicates coolant ingress. A brown oily film on top of the coolant indicates oil in coolant. Both can occur simultaneously.

2. Coolant block test (CO2 test) — A chemical test strip is placed over the coolant reservoir with the engine running. If combustion gases (CO2) are present in the coolant — indicating a breach between the combustion chamber and cooling jacket — the strip changes colour. This is the most definitive non-invasive test.

3. Pressure test — The cooling system is pressurised and checked for pressure drop. A failing head gasket causes pressure loss as coolant leaks into the oil system or combustion chamber.

4. Exhaust visual check — White smoke with a sweet smell at operating temperature indicates coolant burning in the combustion chamber.

5. Cylinder compression test — Low compression on one or two adjacent cylinders with coolant presence can confirm a gasket failure location.

EEK Mechanical's Role

We perform a preliminary diagnostic assessment as part of our oil contamination service. We do not perform the full mechanical repair, but our report documents the contamination source assessment to support the mechanical workshop's diagnosis and repair.