How to Check for Water in Oil Before Buying a Used Car in NZ

Why This Check Matters

Milky engine oil in a used car is a major red flag. It indicates a current or recent head gasket failure, oil cooler failure, or flood damage — all of which can mean thousands of dollars in repair costs. Sellers don't always disclose it, and a simple pre-purchase check takes 30 seconds.

The Dipstick Check

With the engine cold and the vehicle parked on a level surface, pull the dipstick and wipe it on a white cloth. The oil should be clear amber to dark brown. Any milky, cream, grey, or foamy appearance means water contamination. Do not buy the vehicle without a full engine assessment.

The Oil Cap Check

Remove the oil filler cap and inspect the inside. A chocolate-milkshake coloured emulsion or grey sludge is contamination. A slight grey film in winter can be normal condensation — ask when the engine was last started and whether it had a long run recently. If the cap was cleaned before showing the car, you may not see contamination — this is worth asking about.

The Coolant Check

Check the coolant reservoir — it should be a clear green, blue, or orange colour depending on the coolant type. A brown or oily sheen on top of the coolant indicates oil is mixing with the coolant — this is the mirror image of coolant in oil, and equally serious.

If You Find It

Walk away, or negotiate a significant price reduction to cover diagnosis and repair. Get a quote from a mechanic for head gasket or oil cooler repair before agreeing any price. If you've already bought and then discovered it, call EEK Mechanical on 0800 769 000 — we'll dispatch a tow truck to collect the vehicle and take it to the nearest certified workshop for a full oil system drain, flush, and refill.