Misfire After a Valve Cover Replacement: Recheck These Items
When a new problem appears immediately after a repair, inspect the work area before assuming an unrelated component failed.
Electrical connections
Confirm every ignition, sensor, and ground connector that was unplugged is fully seated and undamaged.
Spark-plug wells
Check for oil, water, debris, damaged coil boots, and plugs that were cracked or disturbed.
PCV and vacuum routing
A pinched, disconnected, or cracked hose can create a lean condition and rough running. Some engines integrate PCV components into the valve cover.
Gasket and sealing
Verify the cover is seated correctly and fasteners were tightened in the correct sequence and to the manufacturer's torque specification.
Bottom line
Diagnose the cause before ordering parts. Vehicle configurations vary, so verify specifications and procedures for your exact year, engine, and model.