- Locate the ISV. On the early 1990 cars, it's on the side of the airbox. It has two rubber vacuum hoses attached and one electrical connector. I have colored the diagram of the passenger side of the engine compartment to the right (ain't I sweet?). In the picture, the ISV is colored yellow, the rubber hoses are blue and the electrical connection is red. The later '90 and all '91 cars have a slightly different arrangement with the ISV mounted on the boot between the throttle body and the airbox.
- A quick test to see of the ISV is causing the problem is to unplug the electrical connector while the problem is occurring. If the idle is now steady (higher or lower speed is normal), then the ISV is the likely culprit.
- Check the contacts on the electrical connector. If they are wet, corroded, or loose, clean them up with contact cleaner (available at car stereo shops or Radio Shack) and reconnect securely. If you're lucky, this will cure the problem.
- If problem persists, remove ISV by loosening the two hose clamps and detaching the hoses, then unscrew the ISV from the plastic airbox lid.
- THOROUGHLY spray carb cleaner into the two hose connections on the ISV and allow to soak inside the unit, then pour it out. Repeat this process several more times until the stuff you pour out looks clean. Better yet, fill a small container with carb cleaner and let the ISV soak in it for an hour or two, then spray it a few more times. When you think you've cleaned it enough, clean it some more!
- If you have an early '90 car with the Hitachi ISV (pictured at left below), then you can try a more thorough cleaning procedure:
- To disassemble, remove (cut off and discard) the crimped-on sheet metal anti-tamper end cover.
- Before unscrewing the brass end plug, mark its position relative to the housing so it can be replaced to the same position, thus maintaining the same spring tension on the valves.
- Disassemble the unit completely and clean all parts with solvent.
- Lube the two shuttle bearings with a drop of silicone oil on each.
- Reassemble the unit making sure the brass end plug is returned to it's original position.
- Reinstall valve on air box and insure no air leaks at the hose connections.
- Adjust idle speed as described below.
- Reattach the ISV and hopefully the problem is gone, because a new ISV costs about $240!
If cleaning the ISV and it's electrical connections don't fix your idle problems, look for vacuum leaks (like the idle screw hole, there is an O ring that might have decomposed) and check out your idle switch before you replace the ISV. A new ISV is expensive. If you are faced with replacing your ISV, please note that there are two varieties:
- Early 1990 cars (CQ VIN <= 13610) have the Hitachi unit shown on the left -- Audi P/N 034 133 455 A
- Later 1990 cars (CQ VIN > 13610) and all 1991 cars have the Bosch unit shown on the right -- Audi P/N 034 133 455 B
