Common Fuel Air Mixture Problems in Cars

Fuel air mixture problems in cars are numerous despite the fact that the computer controls the mixture based on sensor inputs. If any irregularity exists in the ignition system such as a bad, fouled or improperly gapped spark plug, the fuel will fail to burn sufficiently.

If an oxygen sensor becomes burnt or fouled, it will send an erroneous signal, causing a fuel mixture problem. The oxygen sensor is affected by other outside influences. If a vacuum leak happens to appear in the manifold, the amount of air entering the manifold will be higher than represented by the mass air flow sensor, which is the sensor the computer uses to influence the injector duty cycle. An exhaust leak can also cause the same problem. The later model vehicles use a heated oxygen sensor. The older sensors were not heated and they only operate at a certain temperature. It would take quite a few minutes for the oxygen sensor to reach operating temperature--during this time the mixture is rich, similar to having the choke on. By adding a heater, the sensor tends to heat up faster and become more effective.

A defective thermostat will cause a rich mixture. This happens because the engine runs cooler and causes the computer to enrich the mixture, again similar to a cold engine.

A coolant temperature sensor does the exact same thing as the thermostat. It signals the wrong temperature to the computer causing a miscalculation in its decision as to the duty cycle on the injectors.

The throttle position sensor can also cause problems with the wrong indication as to the location of the throttle plate.

Mechanical problems with the engine such as poor valve seating, a worn camshaft, worn valve timing components, leaky head gaskets, a bad speed sensor in the transmission and vacuum hose leaks will give the same result in a poor fuel mixture. 

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