FUEL MIXTURE Residual AIR This illustration represents the air-fuel mixture inhaled by each cylinder of a typical 6-cylinder engine. The fuel injectors are specifically designed to match the air displacement with the precise amount of fuel for optimum performance.

FUEL MIXTURE Residual AIR In order to meter the AIR-FUEL-RATIO precisely, a small amount of residual air generates enough oxygen to toggle the O2 sensor between lean and rich for the most economical fuel consumption.

This marvel of modern ingenuity is subject to failure due to no fault of its own! There are several excellent ways to check electrical functions… Reduced fuel flow due to a restricted inlet screen. Sluggish plunger movement due to contamination. Improper seating of the pintle valve due to dirt particles. Deposits on the nozzle affecting the spray pattern. However… no easy way to test mechanical failures like…

FUEL MIXTURE Residual AIR If all injectors are equally contaminated, the injector “on-time” would increase, but the fuel volume remains the same resulting in no change of AIR-FUEL-RATIO. The FUEL TRIM may indicate a higher reading but there is no significant detrimental effect to the engine.

FUEL MIXTURE RESIDUAL AIR However, with only one injector restricted, fuel distribution per cylinder is uneven and the computer compensates for the leanest injector.

FUEL MIXTURE Since all the required oxygen to make the O2 sensor function now comes from one cylinder, the computer drives the other cylinders rich. Over time, this results in carbon deposits on valve stems, O2 sensor, EGR valve, converter, etc. To identify which is the lean injector, we trick the computer in subtracting fuel until this injector misfires.

This will drive the computer to take more fuel away, and this lean cylinder is affected most. Pushing this button artificially adds small increments of fuel… …to one of the injectors.

FUEL MIXTURE When continuing to add more fuel… …this cylinder eventually misfires.

By consulting the scanner we can tell which injector is the faulty one! When this cylinder misfires we know that we diagnosed the problem.

Since the computer compensated with a richer fuel mixture, a restricted injector may cause long term carbon damage to engine and delicate components. The Fuel Trim Generator is the easiest way to detect the restricted injector. In addition, this method also identifies the number of lean injectors and whether or not it is a problem. Reading the fuel trim data, the scanner can indicate a potential problem but cannot pin-point the source. It is important to know!

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