Tune-Up Standards

When a tune-up is performed, the mechanic must have specifications to use when adjusting the vehicle. The first few years of emission-regulated vehicles were adjusted using carbon monoxide percentage or hydrocarbon parts per million. These are the units measured by an exhaust gas analyzer.

In the past few years, manufacturers have made their vehicles much cleaner (measured in grams per mile). The CO% and HC ppm have become very low, especially when measured AFTER a catalytic converter. It has become hard to accurately measure the effect of turning the idle mixture screws.

One solution to this problem requires the use of artificially-enriched propane adjustments. The added propane boosts the idle speed by a known amount, and makes the effect of turning the mixture screws easily measureable. However, CO and HC can only be accurately measured while the propane is being added.

As computer-controlled systems were developed, it became possible for the vehicle to adjust its own mixture throughout the entire engine operating range, not just at idle. These "feedback" systems use oxygen sensors to measure how much unburned oxygen is left in the exhaust. The computer can then determine when the air/fuel mixture is too rich or too lean, and correct it as necessary. Even if a mechanic incorrectly adjusts the mixture, most computers can compensate enough so the vehicle will still run clean. In fact, newer cars burn fuel so completely that changes in the pollutant level after the catalytic converter are hard to measure accurately.

Although many shops have exhaust gas analyzers which measure tailpipe emissions, computer-controlled engines normally do NOT have CO or HC specifications for tuning. These specifications would be neither useful or possible for adjusting new vehicles. This manual provides procedures and specifications given by the manufacturers and does not necessarily list CO or HC specifications.