Vacuum

[CG10DET]

Ok, so I not relating to Forced Induction at the moment, however, I have connected my MAP sensor to the vacuum port on the throttle body.

Now, what is strange is that the sensor reads atmospheric and not a vacuum as I expected, and when I free-rev the car lightly, the vacuum increases, usually this vacuum should decrease with a wider throttle.

Now, I believe there is a system of putting the port slightly above the level of the butterfly valve, so that it won't read the vacuum while the throttle is closed, but as it opens the port becomes effectively below the butterfly valve giving a reading.

Now, if this is correct, am I right in assuming that I should see the vacuum increase as I drive with light throttle, and then decrease as the throttle becomes wider as it should?

[Accord2nrz]

Not quite. As you drive the vacum should be approaching zero and here is why.

As your car is moving your engine is not sucking in air as hard as it should becuase the vehicle is in motion and the forward motion of the car is actually moving air into your car. when you are sitting at idle the engine is having to suck the air in becuase there is none being forced into the car.

THis is why vacuum gauges always read -20 at idle, or somewhere close to it. That is a near ideal vacuum. I think... at least it is on cars I have seen.

you know what, I just realized I read your question wrong. are you saying that when you push the throttle your vacuum increases into the negative?

[CG10DET]

It was starting at atmospheric and then goes into negative, then goes back to atmospheric at full throttle.

So I'm going to assume that this vacuum position is ported, so will only read when throttle has opened, causing it to read atmospheric when the throttle is closed.

I have found the vacuum hose from the inlet manifold that goes to my Fuel Pressure Regulator, so I've took my Map sensor from there, and the readings are as expected now.