Tune-Up Standards

The technician must use the proper specifications when adjusting the vehicle during a tune-up. The first few years of emission-regulated vehicles were adjusted using an exhaust gas analyzer which measured CO and HC.

In the past few years, manufacturers have produced much cleaner running vehicles. The CO (percentage) and HC (ppm) have become very low, especially when measured downstream of catalytic converter. It has become difficult to measure the effect of fuel and ignition adjustments.

One solution to this problem for vehicles using carburetors requires the use of artificially-enriched propane adjustments. The added propane increases or decreases engine RPM for evaluation of carburetor rich/lean setting. This allows the technician to check carburetor setting quickly and accurately.

As computer-controlled systems were developed, it became possible for the vehicles to adjust the air/fuel ratio, ignition timing and emission control device operation throughout the entire driving range. These computer control systems use a variety of sensors that provide the electronic control unit with information on vehicle speed, altitude of vehicle operation and transmission gear position, along with engine operating conditions.

Fuel delivery to achieve a lean air/fuel ratio is controlled by the computer. The computer controls the on/off (duty cycle) time of the fuel injector(s) or carburetor mixture control solenoid to achieve leanest possible air/fuel ratio while maintaining good driveability.

Although most repair shops have exhaust gas analyzers, computer-controlled vehicles normally do not have a CO and HC specification for tuning. An abnormal exhaust gas reading on an exhaust analyzer may be helpful in diagnosing a problem, but should not be used as a basis for adjustments.

These procedures and specifications are supplied by the manufacturer and may not list CO or HC specifications.