* Incorrect Timing: The most common reason. During head gasket replacement, the timing belt or chain might be disturbed. If it's off by even one tooth, the engine won't start. This requires careful re-alignment to the manufacturer's specifications.
* Damaged Components: The head gasket replacement might have revealed or caused other issues. For example:
* Warped Cylinder Head or Block: If the head or block was warped from overheating (the reason for the head gasket failure in the first place), even a new gasket won't seal properly. These require machining or replacement.
* Damaged Head Bolts: Over-torquing or improper tightening sequence can damage the head bolts. This leads to leaks and an inability to maintain proper compression.
* Cracked Cylinder Head or Block: A crack in the head or block, often hidden until the gasket is removed, will prevent the engine from starting or running properly.
* Damaged Valves: Valves might have been damaged during the head gasket repair process. Bent valves will prevent proper compression.
* Interference with Sensors or Wiring: During the repair, sensors or wiring harnesses might have been disconnected or damaged. This can disrupt the engine's ability to start. Things like crankshaft position sensor or camshaft position sensor issues are common culprits here.
* Cooling System Issues: An improperly bled cooling system can lead to air pockets which impede coolant flow and can cause overheating, even preventing a start. Air in the system prevents the coolant from properly circulating and can lead to overheating even with a correctly installed head gasket.
* Fuel System Problems: While unrelated to the head gasket itself, this work could have accidentally affected fuel lines or the fuel pump.
In short, a head gasket replacement is a complex repair. If the engine doesn't start afterward, it's crucial to systematically check all the above points rather than just assuming the new gasket is faulty. A mechanic needs to diagnose the specific problem to resolve it.
