VAF Question

[Chobobulous]

So i was thinking about how the VAF works and i had a question.

I came up with 2 ways it could work through my thinking and i was wondering which was correct.


Does the ECU tell the Vane to move when it needs more air?

Or

Does the vacuum from the engine move the Vane as it draws more air in and the vane tells the ECU how much air is going in.

[TominMO]

The second one.

[FestYboy]

the latter, kinda... the VAF doesn't see vaccum of any significant value. but the air being drawn into the engine pushes on the flapper and that flapper is attached to a variable resistor which in turn sends a signal to the ECM to indicate how much air is being drawn in. the ECM looks at the intake temp, vane position and Baro sensor to determine exactly how much air has entered the engine by way of it's mass.

[Chobobulous]

Ok. Thanks! Tinkering time

Can someone post the wiring diagram for the vaf. I cant find my book

[Chobobulous]

Festyboy im asking if the vacuum draw of the engine is what makes the vane move.

Example.

When the car is idling there is not that much suction. There for it only moves the vane a little bit. But when you rev it to 4k rpm the engine makes more suction and moves the vane back more.

I hope im making myself clear in what i am asking.



So as the vane moves to the open position the resistance pattern does not follow a straight line like this/ It makes more of a w pattern. Up down up down.

[Movin]

You are right about the pattern, I think of it as climbing stairs.

You are kind of right about what makes the vane move but the kind of not right part can maybe lead you in the wrong direction if you are trying to figure something out.

The intake manifold has a fixed volume. The actual amount of air in there can change a lot. Temperature can change the amount. Decelerating and a high vacuum will actually leave the least amount of air in the intake. A turbo under boost will put more air in. Because the amount of air cannot possibly be figured out by vacuum and rpm alone we need to measure the air like as if we dumped the contents in the intake into a balloon and measured how big it is, several times a second. The best way to do that so far is to measure the amount of air with an air flow meter, like a speedometer.

The advantage of this type of meter is that it does not matter how much vacuum is there, how much boost is there or what the temp is the meter will measure the actual real volume of air going through. This even compensates for altitude.

It does not compensate for temperature changes of the air and the engine and the way that affects combustion. That is why we still use temp sensors. With throttle position and rpm just exactly how much gasoline needs to be injected before opening of the intake valve can be calculated.

[kayweb]

Also when the car is idling it's under maximum vacuum, mine is around 28-30 in/hg of vacuum at idle, ramping up to 0 (equal to atmospheric pressure) when the throttle is open fully

[Chobobulous]

Im slowly starting to understand.

So mr vanes sole purpose in life is to measure, not to inhibit the flow of air?

If it fluctuates like stairs how does the ecu know what .234 that its actually reading?

[Chobobulous]

Also when the car is idling it's under maximum vacuum, mine is around 28-30 in/hg of vacuum at idle, ramping up to 0 (equal to atmospheric pressure) when the throttle is open fully

This is the part that's confusing to me. I would have thought if you are at wot you will need more air in the engine so the engine would suck harder. Uyou described it the other way around.

[sketchman]

The engine can only suck so hard at a given RPM, regardless of where the throttle is. But the more closed the throttle is at a given RPM the more vacuum is present in the IM, because the engine is sucking against more and more of a closed container. Create an open pathway for the air to flow into the engine(WOT), and the vacuum dwindles.

To make it simpler(hopefully), think of the throttle as what it's name says. It throttles the engine. So giving it more throttle is really backwards. The less the engine is restricted(less throttling or WOT) from pumping the more vacuum it creates. The more it's restricted(more throttling or closed throttle) the more vacuum.

[sketchman]

The engine can only suck so hard at a given RPM, regardless of where the throttle is. But the more closed the throttle is at a given RPM the more vacuum is present in the IM, because the engine is sucking against more and more of a closed container. Create an open pathway for the air to flow into the engine(WOT), and the vacuum dwindles.

To make it simpler(hopefully), think of the throttle as what it's name says. It throttles the engine. So giving it more throttle is really backwards. The less the engine is restricted(less throttling or WOT) from pumping the more< (should be less, sorry) vacuum it creates. The more it's restricted(more throttling or closed throttle) the more vacuum.

Had to fix that.