[Christ]

Originally Posted by Daox

I'm not understanding why this is. I'm cruising along at part throttle, so I have vacuum in the intake manifold. Then, I open up the EGR valve a bit. This is going to reduce the vacuum. The MAP sensor should see that.

At full throttle I could definitely imagine seeing positive pressure in the manifold, but I don't see that happening any other time than that. I guess I was also under the assumption that exhaust pressure was always higher than intake pressure as well.

It's a thought... I'm not totally sure about it.

I don't really think that adding EGR is going to reduce the vacuum, because it's just adding mass from an area other than the throttle body. In other words, the engine is still drawing the same vacuum on the manifold, but the EGR is basically a second intake source. While you would show less flow through the throttle body, the vacuum figure should be the same regardless of EGR flow. (Some fluctuation might be noted due to expansion of heated gasses, etc.. )