The fuel delivery and injection control system delivers fuel to the engine and meters the amount of fuel which is injected into the intake manifold. There are two factors which, under normal conditions, should determine the air/fuel ratio; fuel pressure and injection duration. In the event that either of these factors is incorrect, normal air/fuel ratio will be upset.
One factor which can upset the normal air/fuel ratio is unmeasured fuel. Leaking injectors, a leaking fuel pressure regulator diaphragm, crankcase oil diluted with gasoline, or a saturated evaporative emissions system can all cause an excessively rich air/fuel ratio.
Finally, the air fuel ratio can also be upset by restriction in the injector nozzle or problems with the injector spray pattern. Symptoms caused by fuel injector spray pattern and restrictions are similar to those caused by intake valve deposits; stumble, hesitation, loss of power, etc.
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