Idle drops when oil cap is off..raises when put on?

[Raginstang]

whats up with this? I just realized that when I pull off my oil cap while the engine is running my idle drops and when I put it on or cover the oil filler hole it dramatically raises?? whats the deal?

 

[Black1987Stang]

Prob because since you are mass air when u do that air us going through that tube up to the throttle body and into the engine and the mass air flow sensor isnt metering the air, (its called unmetered air BTW) so basically its running lean when u do that cuz its making the proper air/fuel mixture for the air going past ur maf but if air is getting into the motor any spot before that it doesnt read it and it runs leaner

 

[Raginstang]

makes since! thanks

 

[Black1987Stang]

no prob

 

[87GTSnake]

If you pull the tb tube off of the valve cover and cover the end of the tube with your thumb it should have the same effect you get when putting the oil fill cap back on.