Idle Mixture Air Bypass Circuit

For proper calibration of the engine air/fuel ratio at idle speed, an idle mixture air by-pass circuit is incorporated into the air flow meter. A screw is used to adjust the amount of air which by-passes the measuring plate. This screw is adjusted and sealed at the factory to discourage improper adjustment and tampering. There are no provisions or specifications for field adjustment.

After factory calibration of the air flow meter, a two-digit number is stamped into the meter casting near the idle mixture adjusting screw. This number indicates the distance from the casting to the flat surface of the screw and can be used as a reference if the idle mixture screw has been tampered with.

The calibration number can be interpreted by referring to the examples in the following chart.

Standard Measurement of Idle Mixture Adjusting Screw

Number on casting

Distance from casting surface to top of screw


13.0 mm


12.6 mm


12.4 mm

Was this article helpful?

0 0
Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

Don't pay hundreds of dollars to find out what is wrong with your car. This book is dedicated to helping the do it yourself home and independent technician understand and use OBD-II technology to diagnose and repair their own vehicles.

Get My Free Ebook

Post a comment