سایش کاسه‌نمد ساق سوپاپ

The seals controlling how much oil reaches the valve stems, hardened so oil drains into the cylinder while the engine stands. They are a wear-out item rather than a defect, and their failure is one of the few that is diagnosable from the driveway.

The signature is a puff of blue smoke on start-up that clears within seconds and does not return while driving — oil has pooled on the valve overnight and burnt off at once. Worn rings behave differently, smoking under load and continuously. The consequence reaches the diagnostic systems: oil burnt on start-up fouls the plugs and coats the oxygen sensors and catalyst, so misfire and efficiency codes follow. A cold-start misfire on the same cylinder with clean running afterwards is worth taking here rather than to the coil.