Idle drops when 710 cap is removed.

[NorCalJalopy]

A coworker of mine bought an 89 3.0 4Runner. When I was looking at it thinking how ratty my 4Runner is I unscrewed his oil fill cap and his idle dropped to where it would barely keep running. We didn’t see any blow by or any smoke at all coming out of the fill. ImAny ideas? Still better than a jk or jl!

 

[rattlewagon]

Yup there is a tube that leads from the valve cover to the intake just before the throttle body to vent the crankcase, and by opening the oil cap it allows unmetered air into the intake, so the idle drops

 

[Bebop]

Yup there is a tube that leads from the valve cover to the intake just before the throttle body to vent the crankcase, and by opening the oil cap it allows unmetered air into the intake, so the idle drops

Shouldn't it do the exact opposite?
More air = raised idle

 

[rattlewagon]

Beats me, I just know it happens.

Maybe some who knows sheet about fawk will chime in. Maybe the computer is seeing the extra air somewhere like through the 02 sensor and dumps fuel in?

 

[NorCalJalopy]

I have almost the same exact rig and set up. Mine doesn’t do it. Mine is def a little more worn out and ratty. The only thing I could think of was the pcv valve but he mentioned that’s what he tried first. Thanks for the input guys.

 

[Etyler2]

You’re letting out the head gasket fluid, leave it closed.

 

[NorCalJalopy]

You’re letting out the head gasket fluid, leave it closed.

Similar to letting the smoke out when you short out an electrical component? Lol