Fuel injectors get dirty through an accumulation of deposits caused by heat degradation of the fuel. When an engine is turned off, the injectors experience significant heat soak because of the engine heat and lack of cooling fuel and airflow, through and around the injectors. The fuel that remains in the heat soaked injectors degrades and creates varnish deposits.
When the injectors get dirty, the spray pattern is disrupted which causes poorer atomization of the fuel. Total fuel flow is also reduced because of the deposits. Symptoms of dirty injectors include reduced engine power and fuel economy, difficult cold starts, and in extreme cases, noticeably poor engine operation. Fuel injector deposits have an even more dramatic effect on direct injection engines because the designed fuel spray pattern is critical to proper combustion and engine operation.