The purpose of this page is to supplement the information in our application help file regarding the Idle Air Clamp tab with a concrete example of how to use this function. The purpose of this function is to work around various airflow setup issues that tend to cause stalling or bogging of the engine when pushing the clutch in or simply free rev'ing. This problem is usually the result of an inflated airflow signal reported to the ECU on throttle lift conditions. This might caused by an aftermarket intake snorkel or a vented BOV, for example.

The following clip from a customer's log file shows this problem very clearly.

You can see that as the throttle was quickly closed, the idle switch went active and engine speed (RPM) started to fall, as it's supposed to. But airflow per rev actually ROSE! If engine speed and engine load is falling, airflow per rev should fall as well.

The problem here is that the MAF sensor is being disrupted in some way under these conditions and the ECU is injecting far more fuel than it should. The ECU “sees” more airflow and believes the engine needs more fuel as a result. But the engine does not need more fuel…it needs less. The ECU just has no way to know that. This excess fuel can basically flood the engine and cause it to bog (as seen in this example) or even die entirely.

It's hard to say if this is really a solution or a work around, but either way, it does solve the problem at hand. What you basically do is to tell the ECU to clamp airflow under these conditions to some value that you know is more reasonable. So when the ECU sees the idle switch go active, it knows the throttle is closed and that airflow really should be rather low.

You can configure the Idle Air Clamp function to do this for you.