On the front of the intake where the t.b. mounts there is a small hole that lines up with the IAC on the t.b. This is not open to the intake plenum but instead is a cast passage that goes where exactly?
Also, next to the passage, on the floor of the plenum there is a hole on each side. Where do they go?

 

There is a second plenum underneath the main one. Each intake port has a small hole that enters this. This was done to create consistent airflow to each cylinder at idle with such short runners.

 

Thanks, BC. I never read that before. Any idea about the other two? EGR passages maybe? They where very dirty, like carbon.

 

Oops, didn't catch that part

I remember those other holes now, but I'll be damned if I can remember where they went. For some reason I want to say they go absolutely nowhere, would you confirm for me?

 

Both the idle air and the EGR combine in the space under the base of the intake plenum, then flow through the little holes drilled in each manifold intake runner.

I'd have to look at my intake to remember where the other two holes you mention are even located.

 

They are immediately behind the TB openings, on the plenum floor. I'm almost positive they serve no function other than added volume to the plenum. Then again, I haven't looked at my intake for a year now

 

Hey! This discussion motivated me to post my "Intake Exploration" pictures.

http://web.camaross.com/forums/showt...hreadid=138756

It will explain all the holes you see in there!

 

Excellent job! I've done the same thing, but I missed where the EGR ended up in the front of the intake. I though it combined with the IAC, and might explain why the IAC valve gets so dirty.

One minor point.... only the 95+ models (and certain earlier emissions variations) use the port under the TB for the PCV system. The 93/94 models have a pipe plug in that hole and connect the PCV directly to the drivers side of the intake plenum with a u-shaped hose.

 

Thanks, Fred! I'm wondering ... does that u-shaped hose attach to the hole in this pic that leads to the IAC passage?
http://z28.reanimator.org/stuff/intake/side.jpg

I'm thinking it should since the later models connect to the IAC passage, just through the front instead of the side. On my car this hole was used for vacuum to the EGR solenoid.

 

Good stuff Mikey

 

Yes, that's where it connects....

Hard to see here

In 94, left is EGR vacuum, upper right is brake booster, and lower right is PCV

 

That is a great write up, very good info there. Im fairly convinced somehow I am siphoning oil into the intake, especially the IAC passages after seeing this write up.

http://web.camaross.com/forums/showt...hreadid=138127

Its worse at idle and it's becoming very frustrating.

 

Something didn't look quite right, then I realized being a '94, you added the vented opti. Hmmm, maybe that would make for a better/more effective vacuum source than the IAC vacuum for the opti vent line.