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The Porsche M96 Engine and Cracked Cylinder Heads

The cylinder head(s) on any water cooled engine has the potential to crack, and Porsche’s M96 engine in the 986’s & 996’s is no exception. The most common reason for this to happen is when an engine significantly overheats, usually due to the loss of coolant. Overheating can be caused by something as simple as a faulty coolant hose, a cracked or broken radiator or coolant reservoir, or a water pump failure. If the driver is not periodically checking the instrument cluster, they might miss a warning light or a dramatic change in the reading on the temperature gauge (if your vehicle has a temperature gauge). When the coolant flow is lost, it doesn’t take much time for the engine to get really hot.

When the engine overheats a great deal, the aluminum head material can actually distort in shape due to the combination of the extreme heat and pressure (or torque) being applied to the head by the fasteners that hold it in place. If the head material distorts, even a small amount, cracks in the metal can occur.

With regards to the M96 engine, another issue that can cause cracks in the head is based in the coolant water pump. As the water pump ages, pieces of the internal plastic impeller can break off and these plastic bits will circulate within the cooling system. If these plastic pieces become lodged in the many small coolant passages within the head they could partially to fully obstruct the flow of coolant at that location. The combination of debris and loss of coolant flow can create localized hot spots in the head but can yield the same result, cracks in the head.

Also unique to the M96 engine is the fact that the heads can be cracked by improperly attaching components to them. Incorrect tightening of the valve lifter cradle or the camshaft cover (also known as the valve cover) can cause cracks in the head. The use of a torque wrench and the knowledge of the correct tightening procedure are critical to the repair of this engine.

When cracks develop in the head, a number of symptoms can result depending on where the crack is located. Keep in mind that oil and coolant both circulate inside the head. If cracks develop between oil and coolant passages, the result can be the proverbial “chocolate milkshake” in the coolant, oiling system, or both. This is the result of oil and coolant intermixing together. Additionally, cracks can develop in the cylinder head combustion chamber unnoticeably causing a loss of coolant or oil. It is important that the technician think through all the symptoms to insure a proper diagnosis.

Generally speaking, another problem area can be the head gasket. The head gasket fits between the head and the cylinders. A failed head gasket can also exhibit the same symptoms as a cracked head. The good news here is that head gasket problems on the M96 engine are very rare. That is because Porsche uses a multi-layer steel (also called an MLS) head gasket. This type of gasket is very robust when installed properly.

Keeping the cooling system in top operating condition is critical to the M96 as it is to all water cooled engines. Have the hoses and other cooling system components, including the coolant reservoir and pressure cap, carefully checked at every service. It is important to maintain the proper pH level of the coolant, as with age and exposure to aluminum, the coolant can become more acidic which in turn hastens the demise of the rubber hoses. Flushing and replacing the coolant every 2 to 3 years is recommended as the best way to maintain the correct Ph balance. As for the coolant water pump, replacing it every 5 years or 50,000 miles may be the best defense against these components failing when you are enjoying a nice drive.
Tony Callas & Tom Prine

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