If the air-fuel ratio is very lean at high speed (above 16.1), what is a possible cause of the condition?

When dealing with a very lean air-fuel ratio at high speeds, several factors can lead to this condition. A lean air-fuel mixture occurs when there is a higher amount of air compared to fuel, which can cause engine performance issues and potentially damage components over time.

One possible cause is a stuck closed thermostat. In this scenario, the engine may not reach optimal operating temperature, which can influence the behavior of the engine management system. The engine control unit (ECU) might receive incorrect temperature signals, leading to a lean fuel mixture being calculated to conserve fuel or reduce emissions, especially when high engine loads or speeds are applied.

Low cylinder compression is another contributing factor. Low compression can result from several issues such as worn piston rings, leaking valves, or head gasket failure. This lowered compression can prevent the engine from drawing sufficient fuel into the combustion chamber, contributing to a lean condition, especially evident under high-speed conditions where airflow increases.

Both conditions can indeed lead to a very lean mixture at high speeds. A stuck closed thermostat limits the engine's ability to heat up properly, while low cylinder compression limits the engine's ability to effectively draw in the air-fuel mixture. Therefore, recognizing that both a stuck closed thermostat and low cylinder compression can lead to