* Head gasket failure: A blown head gasket on that side of the engine is the most probable cause of no compression. This can lead to a loss of compression because combustion gases are escaping into the cooling system or crankcase. A blown head gasket often leads to other symptoms like overheating, milky oil (oil mixed with coolant), or white exhaust smoke.
* Valve issues: Burnt valves, bent valves, or sticking valves on the affected cylinders will prevent proper compression. This could be caused by a variety of things, including overheating, a faulty valve spring, or a collision that damaged the valve train.
* Broken piston or connecting rod: A catastrophic failure of a piston or connecting rod in one of the cylinders on that side will result in complete loss of compression. This is less likely than a head gasket issue, unless the engine was severely over-revved or suffered significant damage.
* Cracked cylinder head: A crack in the cylinder head can also lead to compression loss. This is less common than a head gasket failure but still possible.
Important next steps:
* Don't attempt to start the engine: Cranking an engine with no compression can cause significant further damage.
* Have a mechanic perform a proper compression test: This will confirm the lack of compression and pinpoint exactly which cylinder(s) are affected. A leak-down test will further help isolate the source of the leak.
* Visual inspection: Once the compression test points to a cylinder, the mechanic should do a visual inspection for any obvious damage (cracked head, broken parts etc).
Simply put, the problem is serious and requires professional diagnosis and repair. Attempting a DIY fix without proper knowledge could lead to costly further damage. Have it towed to a mechanic for proper assessment.
