Blown Head Gasket Symptom Checker

Is it a problem? A little puff of vapour on a cold morning is normal condensation. However, if you see continuous, thick white smoke after the engine has warmed up, it's a major warning sign.

Your Next Step: A combustion leak test is essential. The LeakLogic kit will detect the exhaust gases that are pushing coolant out, confirming the diagnosis with a clear blue-to-yellow colour change. This is one of the most common signs we cover in our full guide to the 7 signs of a blown head gasket.

What it means: Blue or grey smoke indicates that engine oil is being burned. This can be caused by worn piston rings, valve seals, or in some cases, a head gasket that has failed between an oil passage and a cylinder.

Your Next Step: A combustion leak test is still a valuable diagnostic step. If the test is positive (yellow), it suggests a complex head gasket failure. If it's negative (blue), you can rule out a coolant-related breach and focus on rings or seals.

What it means: Black smoke means the engine is burning too much fuel (running "rich"). This is almost never a head gasket issue.

Your Next Step: You do not need a head gasket test for this. Instead, investigate common causes like a clogged air filter, faulty fuel injectors, or a bad sensor.

• The temperature gauge on your dashboard is in the red zone.
• Steam is visible from under the bonnet.
• You smell a "hot" or sweet antifreeze odour.
• You hear unusual ticking or knocking sounds from the engine.

Continuous overheating can lead to catastrophic engine damage, including:

• Blown Head Gasket: The primary risk. The heat and pressure warp the head, breaking the seal.
• Cracked Cylinder Head/Block: Severe thermal stress can physically crack the engine's metal castings.
• Scuffed Pistons: Pistons can expand beyond their tolerance and scrape against the cylinder walls.

If your engine is overheating but you're not sure why, check these common culprits first:

• Low Coolant Level
• Visible Leaks (hoses, radiator, etc.)
• Faulty Thermostat or Radiator Fan
• Blocked Radiator

Your Next Step: If all of the above seem fine, the overheating is likely being caused by a head gasket leak. A combustion leak test is the most efficient way to confirm this.

If you are losing coolant, the first step is to identify if the leak is external or internal.

• External Leak: You will see puddles of coloured fluid under the car, or visible drips from hoses, the radiator, or the water pump.
• Internal Leak: The coolant level drops, but you can't find any visible evidence of a leak. This is a strong sign the coolant is leaking *inside* the engine.

An internal coolant leak is a hallmark symptom of a blown head gasket. The coolant can escape into a cylinder and be burned off (causing white smoke) or leak into the oil (causing milky sludge).

Your Next Step: If you suspect an internal coolant leak, a combustion leak test is the definitive diagnostic tool. It will confirm if exhaust gases are entering the cooling system through the same breach that coolant is escaping from.

What it means: A creamy, tan sludge under the oil filler cap is a sure sign that oil and coolant are mixing. This only happens with a significant internal breach, and a blown head gasket is the most common cause.

Your Next Step: Stop driving immediately. This is a critical symptom. A test will confirm the head gasket failure, but this visual sign is already a near-certainty of a major problem.

If you look in your coolant reservoir and see a dark, oily film floating on top, or the coolant itself looks like dirty sludge, it's a sign that engine oil is being forced into the cooling system.

What it means: This happens when a head gasket fails between a high-pressure oil passage and a lower-pressure coolant channel. It's a clear sign of cross-contamination.

Your Next Step: Like milky oil, this is a serious symptom. A combustion leak test is the best first step to confirm the head gasket is the source of the breach before you commit to a major repair.

The bubbles you see in the coolant tank of an overheating car are rarely the coolant actually boiling. It's usually high-pressure exhaust gas being forced into the cooling system through a head gasket breach. This process is called "gassing" and it violently displaces coolant and causes rapid overheating.

Your Next Step: What you are seeing visually can be proven chemically. A LeakLogic test samples these exact bubbles. If they contain CO₂ from combustion, the test fluid will turn yellow instantly, providing an undeniable diagnosis.

A head gasket that fails between two cylinders will cause a loss of compression, leading to a misfiring, shaking engine and poor performance.

What it means: This can be caused by many things (spark plugs, ignition coils). A combustion leak test helps narrow it down. If the test is positive, the loss of compression is almost certainly related to the head gasket breach.

What it means: A metallic rattling or pinging sound, especially when accelerating, can be a sign of detonation. This happens when fuel ignites prematurely in the cylinder. Overheating—often caused by a blown head gasket—creates hot spots that trigger this detonation.

Your Next Step: Engine knock can cause serious piston damage if ignored. A combustion leak test is a crucial step to determine if the overheating that's causing the knock is due to a head gasket failure. If the test is positive, you've found the root cause.

Sometimes the gasket fails on an outer edge, causing coolant or oil to visibly weep from the seam between the cylinder head and engine block.

What it means: A visual inspection is key here, but a combustion leak test is still wise. An external leak can often accompany a small internal leak, and confirming this gives you the full picture of the damage before starting repairs.