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mechanical newbie (85 720)


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EGR at idle will fail to ignite the mixture properly leading to worse emissions than without it so systems now use a ported vacuum source just like the vacuum advance that comes in as the throttle plate rises up above idle. At full throttle the ported vacuum is zero so no EGR. Later systems have a transponder that senses exhaust back pressure and more accurately adds or deduces EGR content. All EGR are controlled by a thermal vacuum valve that only passes the ported vacuum signal once the engine warms up. So no EGR when engine cold.

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