So- I was hunting a possible vacuum leak using starting fluid the other day. No, nothing burst into flame haha. What I did discover though, is that even a relatively small amount of starting fluid sprayed into the intake snorkel would stall the engine out and kill it. I asked a friend about it, and he agreed that it seemed odd.

I started looking for the leak because of my idle, which likes to bounce, regardless of operating temperature. It has a new TPS, set correctly. I'm at a loss here. Sometimes it idles very smoothly, but half the time around 1400 RPM, and other times around 800 RPM. It seems to bounce in different ranges depending on how i set the screw on top of the throttle body. it seems when I tighten it down, it stops bouncing but also idles too low once it settles. I have also tried to adjust the idle with the little screw that has the 8mm nut on it, since moving the piece it stops seems to adjust the idle as well. Been fiddling with them both but I cannot seem to get rid of the bounce and get a solid idle. Once again, car has no vacuum leaks, and a new, correctly adjusted TPS.

 

When it idles at 1400 rpm try pressing upwards on the throttle linkage and see if this drops the idle speed to an acceptable level. If it does then make sure the throttle cable, cruise control cable, Fast Idle Cam are set properly as two of these items could be sticking. You could also have unmetered air entering the engine due to a sticking BAC for example. And where is the location of the 8mm screw you speak of?

 

I will have to post pics and video to give you the whole picture. The screw you asked about itself isnt 8mm; the nut AROUND the screw (which is a slotted screw that only fits a very small flathead driver) is 8mm.
I feel silly not knowing what the names of the things I'm adjusting are. I hear/read things like BAC and whatnot here in the forums but if you asked me to point to it on the car i would be like WUT LOL...

 

Is the small screw on the front of the throttle body or the rear?

 

front, however the piece that it stops also is represented on the back of the TB. as if it adjusts a butterfly inside the TB.

 

Don't pay attention to the arrow... lol. the screw I was talking about is holding the resting position of the part just above and to the left of the arrow. You cannot see much detail of it, but its a set screw with a nut on it at the top to lock its position. that piece above and to the left of the arrow pivots, and the arm on its left goes from the pivot point towards the intake of the throttle body and turns 90 degress to the rear of the car, where the screw adjusts its resting point. WTF is that screw, and how do I know where it should be?

also this pic is not of my TB, so never mind the missing linkage.

 

It sets the secondary plates to the proper closing position. It is not to be messed with and the proper setting or position from the factory is the screw is tightened all the way.

And when you play w/the Idle Adjust Screw, located on top of the Dynamic Chamber, the Initial Set Coupler "must" be jumpered.

 

Well ****. Too late! I've been messing with it. How to set it properly is still in question. I was under the impression that you were never to run the engine with the initial set coupler (the green wire near the shock tower, yes?) jumpered.

Also, according to the way my idle likes to rest up at 1500 sometimes (esp. after applying throttle then letting it fall all the way back) I am going to test my MAF later.

 

You're advised not to "drive" the car w/the coupler jumpered but for setting the idle it is imperative because w/o doing so you will fight against the BAC and it prevents you from being able to set it properly. Having the car idling w/the jumper in place for idle adjustment is completely different from driving the car.

The plate on the AFM might be sticking causing the different idle speeds. You can actually disconnect the AFM to see if this is the issue. W/o the AFM connected though I believe the engine will run very rich which can cause the exhaust manifold to get hotter than normal so if you do this just quickly play w/the throttle to see if the idle speed can stay consistent or still fluctuates.

 

Is the AFM the same as a MAF?
Also, the instructions I found for testing a MAF only account for three wires... Positive, Negative, and MAF info. I'll have to search for what the wiring is on mine since it has five on the harness that plugs into it.

Thanks for all your help satch, I adjusted my BAC and once the engine is warm, the idle is great. However, it idles a bit lower (~850rpm) than I like. I would like it to idle higher since when it drops below 1Krpm I get voltage fluctuations when applying throttle from idle. I suppose this is just a grounding issue though, since the FSM states that it should be idling at 750rpm +/- 15.

 

Your Pressure Sensor has three wires (some refer to this as a MAP sensor). The AFM is located under the air box and has a series of wires and is a different item altogether. And how did you adjust the BAC? This sensor is "only" adjustable on a turbo BAC as the NA version is not adjustable (the allen screw at the end "is not" to be tampered with.

 

BY BAC i meant the screw on top of the TB... jeez I still don't know what things are called LOL. I need to make a diagram or something hahahaha =/

 

OK so "idle adjust screw" it is.

Also, the proper setting or position is tightened all the way? I'm not sure that there is an "all the way".. it seems like if you wanted to, the screw could be turned all the way through and past the threaded area is sits in... do you mean that the top of the screw and top of the nut are flush when it is set correctly?

 

If you look at the pic closely all there is is a flat headed screw as there is no nut. If you want to call it a bolt then so be it. The head of the ???????? is tightened completely.

 

I see a flathead screw and a nut around it....

 

so how do I set that correctly?

 

On my S4 it doesn't look like that and my car is as it was when it was purchased new. They either changed it for the S5 or it's not stock, but either way the lever that rests upon it should be level, and from the pic shown it appears to be that way.