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Discussion Starter #1
My S3 has been popping quite a bit on deceleration, and really struggling under a load. In reading threads about how finicky the L-Jetronic system can be with air or vacuum leaks, I decided to see if I had a vacuum leak.

I un-screwed the oil filler cap on top of the valve cover while the car was idling. Absolutely no change in idle, which suggested that I probably did have a vacuum leak somewhere.

I started checking the obvious places, like the 26 year old vacuum lines attached to the plenum and the large rubber air intake sleeves. I found a couple of hoses that spun easily on their mounts and I tightened their clamps.

When I got to the port shown in the first photo below (which I believe is the by-pass hose for idle r.p.m. adjustment) the hose was firmly clamped to a copper tube that entered the plenum chamber there, but that copper tube spun freely within the hollow bolt that held it to the plenum.

After removing the hose, I tried to get the copper tube to quit spinning by tightening the sleeve nut that held it in place, but the shoulder of the nut was already bottomed out on the spot face of the plenum and would not tighten any further.

I unscrewed the nut and removed it along with the copper tube. The threaded hole was countersunk, and sitting on the countersink ledge at the bottom of the hole was a small fat O-ring that the flange of the copper tube was apparently resting on. The three parts of this assembly are shown in the second photo, and the third picture shows the three parts in a "dis-assembled" or "exploded" view.

My question is this: Is the copper tube supposed to seal tightly on the small rubber o-ring so that it cannot spin and thereby make a good "seal"? If so, is this assembly missing something that would make it seal better, such as a washer or spring, or is the O-ring just worn out?

I can't imagine that this copper tube is supposed to be loose enough to spin and let air get by.

Thanks, in advance, for your esteemed advice.
 

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That is the idle speed adjustment. As you tighten or loosen the adjuster the O-ring is supposed to squeeze/unsqueeze to vary its ID & thus the amount of air allowed to pass varying the idle speed.

When they get old the O-ring gets hard & inflexible. Replace the O-ring and let us know if that solves the issue. (I just took the old O-ring with me to the hardware store and bought a new one that was about the same size.)
 

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Discussion Starter #3
how to adjust?

Thank you Eric.

After I replace the rubber o-ring with a fresh one, and re-install the brass tube, the adjusting nut and the lock nut, can I determine the correct amount of o-ring compression just by setting the idle with it (with car hot, idling in neutral, and no accessories operating)?

Will the amount of o-ring compression actually allow me to set the idle without making other adjustments, i.e., to other devices?

Is it correct to assume that the more I compress the o-ring (and decrease the o-ring's I.D.) that the r.p.m.'s will increase?

Do you think I can I trust the tach display in the cluster enough to set the idle at 950 +/- 50 r.p.m.'s by it, or should I use something more accurate or trustworthy? My tach appears to work fine, but I have not calibrated it.

Thanks again,
 

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The O-ring sits in a cone shaped area, As you snug down the adjuster the ID should get smaller and allow less air to flow. Less air = lower idle speed. There is no other 'regular' adjustment needed. But idle speed is also dependent on all other parts operating correctly (valve clearances, ignition itiming, no vacuum leaks, etc).

There is a stop screw for the throttle butterfly. But that is not normally adjusted - it should be set at the factory then idle speed adjusted with the idle air adjuster. If you suspect the throttle stop has been messed with then you can back it all the way out then in until the throttle is just barely cracked open. With the engine running the difference between backed out and cracked open is 25-50 rpm.

IME, Alfa gauges give a general indication of things but are not always 100% accurate. A modern timing light will also have an rpm function. You can use that to see if your dash gauge is accurate. But even if your dash gauge is off 10% then the difference at idle would only be 100 rpm or so. If it sounds good and idles smoothly & reads 900 to 1000 rpm that should be fine.
 

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vacuum leaks

Hi. Concerning vacuum leaks, the fix on the idle adjustment orifice "O" ring will take care of that but you may have another vacuum leak. I would ask, if you would look at the Auxiliary Air valve. With the engine at operation temp the valve should be completely closed, if it is not then you have a vacuum leak of sorts. It's not like an open air leak but it does allow to much air into the engine. It's just a thought and something to check if you still have a problem.

Chief.
 
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