afterburn27

iTrader: (5)

Sorry if this is a stupid question...

After looking through the FSM a bit I am still confused as to why removing the airpump/ACV will mess with the idle and low rpm performance.

The FSM shows four ports on the ACV: airpump input, main cat output, exhaust output, and the blowoff that goes back to the airbox. So the ACV does NOT pump air into the combustion chamber. Correct?

So what causes the idle problems that most people report with a removed airpump? Obviously the O2 sensor will read a richer mixture since the extra air in the exhasut from the airpump is gone. This leads me to beleive that the ECU would lean out the engine, but most people report a very rich idle...

Any input?

*edit*

here is the diagram in the FSM:

DaleClark

iTrader: (55)

From what I understand, the FD uses the O2 sensor as a feedback loop for a lean best idle. The engineers tuned it in with the air being injected into the manifold - without that air in the manifold, the mix gets thrown off a bit and the O2 sensor gives confusing input to the ECU.

Thinking about it further, they probably set the ECU to read a "fake" lean amount at idle that's caused by the extra oxygen from the air pump. Let's say you want to be at, say, 13:1 at idle. With that extra air coming in, it will actually read 15:1, but you *know* it's really 13:1 in the engine. So, you fudge the numbers in the ECU a bit to compensate. Without the air pump, the ECU actually changes the pulsewidth so the engine is really running 15:1 and runs like crap since it's so lean. This also accounts for the part-throttle sloppiness as well.

I guess the ECU just has a function to fudge the O2 sensor's output whenever it's injecting air into the manifold.

Dale