ok guys I have a spare intake manifold I am cleaning up and going to put on my s4 n/a it is the exact same manifold as I have now, my only question is what is that big sensor/switch on the back of the manifold upper plenum?

it is directly on the end of the plenum on the drivers side. what is that and what is it for?

Dave

 

If you are talking about the cylindrical thing about 3 or 4 inches in length; that would be your BAC valve (i think)... It regulates your idle in some way or another... not sure how or why...

 

ah bypass air control valve...

gottcha so in otherwards I can not live without it

Dave

 

i live without it on my car.. it is an 89 TII but if you do a full emission remoal you can eliminate it.

 

yes I plan to remove all the emisions.

cats are gone, air pump is gone, egr will be gone along with anything else I dont need

Dave

 

I have an 88 GTU with BAC and thermowax and an 88 T2 with neither, and I can tell you right now that not having them is annoying as hell. The car barely idles when cold... you will have to keep your foot on the gas for the first 5-10 minutes of driving. Plus it idles lower and so you have less voltage output from the alternator. I'd almost put it back into my T2 if I didn't have a ton of other projects I'm working on for it.

 

it depends on the motor, because my Tii doesnt have it and as soon as you start it it will hold an idle solid.

 

The BAC valve is for idle speed control. It has nothing to do with emissions control, and the two systems don't affect each other. You don't need to remove the emissions controls to be able to remove the BAC valve, and you don't need remove the BAC valve if you remove the emissions controls.

Idle control is a Good Thing. I don't get why people are so keen to get rid of it.

 

I got rid of mine for simplicity of engine bay and mine didnt work in the first place. AS for other people I dont know.

 

I (well the person that did my rebuild) got rid of mine and I plan on putting it back on. There's no reason to take it off. I like not having a lot of my emissions junk on there, but as stated earlier, it has nothing to do with emissions.

However, you can get your car to idle normally (without electrical loads). Mine idles at 900rpm perfectly and it's only that high because the throttle adjust screw is set that way.

 

ok I will prolly keep it then lol...