BMW 745Li Cylinder Head Replacement Cost

The cylinder head is a component mounted to the top of the engine block. It houses the intake and exhaust valves, as well as the spark plugs. In an overhead cam engine, the cylinder head also houses the cam shaft.

The cylinder head closes off the top of the cylinders to form the combustion chambers. The spark plugs protrude through the cylinder head into the combustion chambers to spark ignition. Opening into the cylinders are the valves. An intake valve opens to allow a mixture of fuel and air into each cylinder before closing again. The piston moves up inside the cylinder to compress the air/fuel mixture. The spark plug ignites the compressed fuel. And, as the piston moves away from the cylinder head because of the rapidly expanding gases caused during combustion, the exhaust valve opens to allow the gases to exit the engine. The cam shaft makes sure that all of this activity happens in the right timing.

A leaking head gasket or cracked cylinder head will often leak engine coolant into the oil, causing a "chocolate milk" like appearance and the oil level may be higher than normal.

Failed head gaskets or damaged cylinder heads can cause engine coolant to leak into the combustion chamber, producing a white, sweet-smelling smoke from the tail pipe.

Symptoms of a bad cylinder head include coolant seen leaking on the outside of the engine, white smoke from the exhaust, coolant loss, a sweet or syrupy smell from the engine compartment or the exhaust, engine overheating, or bubbles in the radiator or coolant overflow bottle. An engine misfire and rough idle are not uncommon with a damaged cylinder head. A milky residue on the engine oil dipstick is also possible if a cracked cylinder head is allowing engine coolant to get into the crankcase. Some of the symptoms of a bad cylinder head can also be attributed to less severe problems, such as a failed cylinder head gasket or intake manifold gasket.

If an engine is displaying the symptoms of a bad cylinder head - or even a failed gasket - the problem should be addressed as soon as possible. Significant damage to the engine could result if the issue is not taken care of in a timely manner. If the coolant intrudes into the crankcase, or if the engine is allowed to overheat for any significant amount of time, repairs could be costly.

A cylinder head should last the lifetime of the engine. Failure to follow regularly scheduled maintenance recommended by the manufacturer - especially oil changes - can result in damage to the cylinder head, valves, and more. Allowing the engine to overheat can also damage the cylinder head.

Diagnosis of a bad cylinder head depends on the symptoms. Coolant seen leaking on the outside of the engine can be an indication of a bad cylinder head or a failed gasket. It may be necessary to remove the components for a closer inspection. Sometimes the cause of symptoms is not obvious. Coolant loss could be due to a leak anywhere in the cooling system. To check for an internal leak caused by a cracked or warped cylinder head, a technician will perform a cooling system pressure test in addition to observing other circumstantial evidence, such as white smoke emitted from the exhaust, or milky residue on the dipstick.

The procedure for replacement of a cylinder head differs depending on the placement of the camshaft in the engine. In general, a technician will remove any components that are in the way of the cylinder head (i.e. the throttle body, steering pump mounting bracket, air cleaner system, etc.). The spark plugs are pulled and the intake and exhaust manifolds are detached. The valve covers are removed and the entire valve train (valves, springs, guides, lifters, etc.) is disassembled. An overhead cam engine will have one or two camshafts in the cylinder head and require that the timing belt or chain be removed to free up the camshaft. The cylinder head bolts are taken out and the head is carefully removed from the vehicle. Once the old gasket material is cleaned from the mating surface on the engine block, a new gasket is used and the new cylinder head is set in place. The head bolts are torqued and the rest of the components are reassembled to manufacturer's specifications.

To prevent an expensive, recurring problem, the integrity of the entire cooling system should be inspected and tested when a cylinder head/head gasket is replaced due to overheating.