How to Spot a Head Gasket Issue When Buying a Used Car

Head gasket problems can be very expensive to fix. Modern engines often use aluminium cylinder heads, which are lighter but more prone to warping when overheated compared to older cast-iron engines. That’s why a head gasket failure can sometimes turn into a full top-end rebuild — not just “a quick gasket change.”

The Procedure (At a Glance)

Do these steps in order during a viewing:

1. Walk-round cold – bonnet up, engine off

2. Cold start – ideally first start of the day

3. Five-minute idle – watch the temperature and the expansion tank

4. Short test drive – fully up to temperature, gentle pull in a higher gear

5. Immediate re-check – oil cap, dipstick, expansion tank, and ground for drips

Red Flag: If the seller refuses a true cold start or test drive, walk away.

Step 1 – While the Engine Is Stone-Cold

Check coolant and oil levels

Indicator: Low coolant or oil levels
Head-gasket link: Coolant loss with no visible leaks can mean it’s entering the engine.
Other possible causes: External leaks, recent top-ups, neglect, or poor servicing.

Inspect the expansion tank for cracks

Indicator: Hairline cracks, misty residue, dried coolant stains
Head-gasket link: Combustion gases entering the cooling system can raise pressure and temperature, causing the expansion tank to split. Loss of pressure then leads to overheating.
Other possible causes: Old plastic, pressure cap faults, overfilling, or a stuck thermostat.

Look for “mayo” (oil and water mixing)

Check:

• Inside expansion tank: oily film or sludge

• Dipstick: milky streaks

• Oil filler cap: creamy deposits

Head-gasket link: Coolant has mixed with oil.
Other possible causes: Faulty oil cooler, condensation from short winter runs (light film only).

Signs of previous overheating

Indicator: Coolant hoses bulged, gone mushy or sticky; plastic flanges brittle or newly replaced (with shiny clips).
Head-gasket link: Prolonged overheating warps aluminium heads and fails gaskets.
Other possible causes: Poor maintenance, blocked radiator, thermostat failure, broken fan, water pump wear.

Step 2 – Cold Start

Hard starting / slow to fire

Indicator: Long cranking, uneven idle, “chuffing”, shaking
Head-gasket link: Coolant in one or more cylinders lowers compression until it clears.
Other possible causes: Weak battery, old fuel, worn rings/valves, spark plug well oil, coil/fuel injector issues.

Misfire on startup

Indicator: Rough idle, spluttering or noticeable shaking when cold
Head-gasket link: Coolant entering a cylinder quenches spark, causing misfire.
Other possible causes: Faulty plugs, coil packs, oil fouling, coolant leaks elsewhere.

Step 3 – Warm It Up (Idle for 5 Minutes)

Temperature behaviour

Indicator: Gauge climbing fast, fluctuating, over middle, fan always on
Head-gasket link: Exhaust gases entering coolant can overheat and pressurise the system, causing instability.
Other possible causes: Stuck thermostat, radiator blockage, weak pump, trapped air, failed fan/sensor.

Bubbling in the expansion tank

Indicator: Continuous bubbling when fully warm
Head-gasket link: Exhaust gases being forced into coolant channels.
Other possible causes: Air trapped after recent work — but should bleed out; steady bubbling is suspicious.

Hot smells around engine bay

Indicator: Sweet, hot coolant smell or wisps of steam at hoses/cap
Head-gasket link: Cooling system pressurised by gases, forcing coolant past weak points.
Other possible causes: Spilled coolant residue, loose hose clamps, cracked plastic outlets, failing thermostat housing seals.

Step 4 – Short Test Drive

• Get to full operating temperature.

• In a higher gear at low revs, gently apply throttle.

• Look in the mirror for white steam under load with a sweet smell. If seen — walk away.

• After the drive, check the expansion tank for fresh bubbling and the ground for coolant drips.

What Specific Symptoms Mean

Common Lookalikes (Not Always Head Gasket)

Brief steam from exhaust on cold start
Normal on a cold morning if it disappears once the engine warms up — it’s just moisture burning off.
❗ Not normal if the steam continues when fully warm.

Heater matrix leak
Coolant smell inside the cabin, misted windows, or damp carpets — often misdiagnosed as a blown head gasket.

Faulty thermostat
Engine overheats quickly, but there’s no mayo under the cap or bubbling in the coolant tank.

Cracked radiator or coolant hose
May cause slow coolant loss or wet spots after driving — easy to mistake for internal coolant burning.

EGR cooler failure (mostly diesels)
Can cause steam and coolant loss that looks very similar to a blown head gasket, but the fault is elsewhere.

Tip: A block test can detect exhaust gases in coolant — cheap and effective.

Questions to Ask the Seller

• “Have you had any cooling system work done? (Thermostat, coolant housing, coolant pipes, radiator?)”

• “Do you have to top up the coolant often?”

• “Has the oil cooler been replaced?” (Could mask oil in coolant issue.)

• “Any signs of overheating?”

Ask for invoices for radiator, water pump, thermostat, hoses, head skim, gasket set, coolant, etc. A box of sealants or story about “just needs a flush” is not good news.

Safety Disclaimer

Head gasket checks involve heat and fumes. Never expose yourself to hot pressurised coolant, rising steam, or exhaust gases. Do not open the expansion tank when the engine is hot — severe burns can occur from released pressure.

Head-Gasket Buying Checklist (Print / Save)

☐ Cold start from stone-cold
☐ Persistent white steam & sweet smell?
☐ “Mayo” on oil cap/dipstick? Oily film in coolant?
☐ Continuous bubbling in coolant tank whilst warm?
☐ Temp gauge stable? No spikes? Cooling fan cycling?
☐ Heater blows hot once warm
☐ After drive: re-check tank and ground for drips
☐ Reviewed invoices for coolant system / engine work
☐ If unsure: have a block test or pre-purchase inspection done

Quick Answer

Want to know how to spot a blown head gasket fast?
Look for several symptoms at the same time: persistent white steam with a sweet smell, “mayo” on the oil cap/dipstick, bubbling in the expansion tank when warm, unexplained coolant loss, and fluctuating temperature. If two or more appear together, assume head-gasket trouble until a professional proves otherwise — and consider walking away. It could save you thousands.