The cylinder head seals the top of the engine block, forming the combustion chamber where air and fuel ignite. This metal casting houses the valves, spark plugs, and passages that allow engine oil and coolant to circulate, managing temperature and lubrication. It must withstand the immense pressure and heat fluctuations generated during the combustion process.
A crack compromises this seal, creating pathways that allow exhaust gases, engine oil, and coolant to mix. This fluid contamination and pressure loss diminish the engine’s ability to operate efficiently, leading to rapid component wear and potential engine failure. Recognizing the symptoms of a crack prevents catastrophic damage to the entire engine assembly.
External Warning Signs
The initial signs of a cracked cylinder head are often noticeable while the vehicle is running. Repeated engine overheating is a common indicator, as the crack allows combustion gases to be forced into the cooling system. This gas displacement prevents the coolant from circulating properly and reduces its heat-transfer efficiency.
Another sign is the unexplained, continuous loss of engine coolant, even when no external leaks are visible. The coolant escapes internally through the crack, entering the oil passages or being vaporized in the combustion chamber. This internal consumption necessitates frequent topping off of the coolant reservoir.
Thick, white exhaust smoke billowing from the tailpipe is a sign that coolant is being burned inside the cylinder. This visible plume is steam created when the water component of the coolant vaporizes during combustion. Unlike normal condensation, this heavy white smoke persists long after the engine has warmed up and may be accompanied by a faintly sweet smell.
Engine performance issues, such as rough running, misfires, or a loss of power, are also common external symptoms. A crack breaching the combustion chamber wall allows compressed air and fuel to escape, resulting in reduced cylinder pressure. This drop in compression impacts the power stroke, causing the engine to stumble and perform poorly, especially under acceleration.
Indicators of Fluid Contamination
Cross-contamination occurs when a crack connects the internal pathways for engine oil and coolant. Engine oil that has a milky, frothy, or light-brown appearance indicates that coolant has mixed with the lubricant. This emulsion, sometimes described as “milkshake oil,” can be observed by checking the engine dipstick or the underside of the oil filler cap.
When coolant mixes with engine oil, it compromises the oil’s lubricating properties, leading to wear on internal components. If the milky substance is present only on the oil cap, it may be condensation, especially in cold weather or during short trips. However, contamination visible throughout the oil on the dipstick suggests a serious internal breach.
Conversely, oil may also enter the cooling system, observed by inspecting the coolant reservoir or radiator neck. Oil is less dense than coolant and floats to the surface, creating a distinct, dark, slick layer on top of the fluid. The presence of oil in the cooling system also degrades the coolant’s ability to transfer heat.
Excessive pressure within the cooling system is another indicator of internal damage, often felt as extremely hard or bloated radiator hoses. This over-pressurization happens because high-pressure combustion gases are forced into the lower-pressure coolant passages through the crack. The influx of exhaust gases can overwhelm the cooling system, causing coolant to push out past the radiator cap and into the overflow reservoir.
Methods for Confirmation
Confirming the presence of a cylinder head crack requires specific diagnostic tests. A quick initial check involves examining the spark plugs, as a crack allowing coolant into the cylinder will often “steam clean” the plug. A spark plug from a cracked cylinder may appear unusually white or clean compared to the normal, light-brown deposits on healthy plugs.
A more conclusive test is the chemical leak-detection test, commonly known as a block test. This procedure involves placing a specialized fluid and testing device over the radiator neck to draw an air sample from the cooling system. If exhaust gases are present, the chemical fluid will change color, typically from blue to yellow, confirming that combustion gases are leaking into the coolant.
Professional diagnosis often involves a cooling system pressure test. A technician pressurizes the system to a set level and monitors for a pressure drop over time. A rapid pressure decline without visible external leaks strongly suggests an internal crack. Technicians may also perform a cylinder leak-down test, which pressurizes each cylinder with shop air and measures the rate at which the pressure escapes, using sound to pinpoint the leak location.
Definitive confirmation usually requires removing the cylinder head and sending it to a machine shop for specialized inspection. The machine shop uses non-destructive testing methods to reveal cracks invisible to the naked eye. These methods include dye penetrant testing and magnetic particle inspection, often called magnafluxing. Dye penetrant testing applies a colored liquid that seeps into the crack and is then made visible with a developer. Magnetic particle inspection uses iron filings to highlight cracks in ferrous metals.