The engine control and emissions sub-systems all rely on good mechanical condition of the engine to operate normally and effectively. Mechanical malfunctions effect exhaust emissions and driveability, both directly and indirectly:
• Directly, any mechanical malfunction will likely cause significant increases in exhaust emissions by causing misfire, allowing combustion gasses to escape past exhaust valves or piston rings, by altering air/fuel ratios, or any number of other possibilities.
• Indirectly, mechanical malfunctions change the composition of catalyst feed gas, preventing the catalytic converter from operating efficiently.
Examples of mechanical problems that can increase exhaust emission output include; low cylinder compression causing poor combustion and/or misfire, worn oil control rings that allow excessive engine oil (HC) to be consumed during combustion, etc. Remember, always check the integrity of basic engine mechanical systems before moving on to more complex engine or emission sub-systems.
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