CIS question

[deerwd]

I am reading How to Understand, Service and Modify Bosch Fuel Injection & Engine Management by Charles O. Probst. Here's a quote. Help me understand.

Chapter 5, pg 11, Idle Speed, 2nd paragraph. "The (idle air bypass)screw is downstream of the air-flow sensor. Any change in bypass air results in a corresponding change in the amount of fuel injected, so idle speed adjustments do not affect the mixture adjustment."

My question is this. Since the idle air bypass screw is downstream of the air-flow sensor, how would this change the amount of fuel injected? The position of the air-flow sensor would not change, regardless of how much air was bypassed downstream. Thus I would assume the control plunger position would not change and the amount of fuel to the cylinder would remain the same. I would think that the only thing that would change is the air/fuel mixture ratio.

I am very novice, so please indulge me. Thanks in advance for the help.

[deerwd]

I may have found the answer to my own question. Please feel free to correct me if I am wrong. The reason the amount of fuel injected is changed is this. When you open the air idle bypass more, you lower the air intake pressure, thus allowing more fuel into the cylinder. Am I close?

[stormcrow]

When you adjust the idle air bypass screw this allows more air to bypass the throttle plate. More air will then enter the engine and at the same time cause the airflow sensor plate to lift thus allowing more fuel to enter the cylinders. This will then increase engine rpms.

Steve

[wckrause]

Steve's right, the idle screw bypass allows air to flow around the throttle plate. It has the same effect as slightly opening the throttle plate (or pressing a little on the gas pedal).

The Aux air regulator has a similar effect at cold start to cause the car to high idle for a few minutes.

[deerwd]

Thanks guys. It's good to know that SOMEBODY (certainly not me) knows what they're talking about. I'm learning. Thanks again.