* Worn piston rings: These seals between the pistons and cylinder walls wear down over time, allowing combustion gases to escape past the pistons instead of pushing them down. This is a common cause of low compression.
* Worn cylinder walls: Scratches, scoring, or excessive wear on the cylinder walls reduce the seal between the pistons and cylinders, leading to escaping gases and low compression.
* Burned or damaged valves: If the valves (intake or exhaust) are damaged, warped, or not sealing properly, combustion gases will escape, resulting in low compression. This could be due to wear, a foreign object, or previous overheating.
* Head gasket failure: A blown head gasket allows coolant or combustion gases to leak between the cylinders and the cooling system or the outside, significantly reducing compression. You might notice other symptoms like white smoke from the exhaust, coolant loss, or milky oil.
* Broken piston(s): A cracked or broken piston will severely reduce compression in the affected cylinder(s).
* Valve timing issues: Incorrect valve timing can prevent the valves from opening and closing properly, resulting in low compression. This often indicates a problem with the timing belt or chain.
The mechanic needs to perform a compression test on each cylinder individually to determine *which* cylinder(s) have low compression. This will help pinpoint the specific problem. Simply knowing that there's *not enough* compression isn't sufficient for diagnosis. They should also perform a leak-down test to further isolate the source of the compression loss in the affected cylinders.
Without knowing which cylinder(s) have low compression, any repair recommendation would be pure speculation. The cost of repair can vary greatly depending on the underlying cause, ranging from relatively inexpensive valve adjustments to a major engine overhaul.
