1990 90 Quattro idle problems [Archive]

PRY4SNO

07-11-2015, 07:39 PM

ICS = ISV? The ISV can be tricky to clean sometimes, might take a couple tries.

Otherwise, give the idle set screw a 1/4 turn and see what that does. It's on the intake manifold by the throttle body.

If you haven't browsed the venerable 20v.org (http://20v.org) website, you should. Lots of basic info there.

Wandering Idle
Symptoms: Idle will constantly bounce between 1600 and 400 RPM, sometimes stalling. This is a very common problem with these cars.

Idle Stabilizer Valve (ISV)

These typically cause problems because of either a bad electrical connection or some kind of internal gunky build-up.
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

Cold Start/Idle Problems
(Ray Calvo)
Symptoms: After a cold start, the car stalls or threatens to. Also, somewhat erratic idle.
Check the rubber connector from the filter housing/air sensor to the throttle manifold for cracks that would allow unmeasured air to enter. (approx. $50) Also check for any other sources of vacuum leaks.
Idle Adjustment
Unofficial Idle Adjustment (may only work on pre-March '90 cars)
(Bernie Benz) This procedure sets the idle speed to minimize the effects of sudden engine braking when the throttle is slammed shut.

With engine at operating temperature, disconnect the ISV electrical connector.
Adjust the idle speed with the throttle valve air bypass adjustment screw to 700 to 750 RPM. (This is just below the speed at which the ECU computer will control idle.) This sets a minimum idle speed well above zero.
Reconnect the ISV elec. connector, and check the idle speed. The computer should control idle at 800 to 900 RPM. If it's faster, the ISV is not accurately following computer control, the sign of a sticky valve.
Official Idle Adjustment
Warning: Bentley says the idle and CO content are interrelated, and must be adjusted together. The following procedures may be good for fine adjustment; however, they may cause more problems than they fix.
Pre March 1990 production date

Clamp crankcase ventilation hose behind fuel pressure regulator, then remove oil dipstick and cover tube with a lint-free rag to prevent oil spray. (This step is probably more important for the CO adjustment rather than idle adjustment, so can probably be skipped.)
Under driver's side glove/map compartment (near steering column) you should find two diagnostic connectors, with two jacks on each one. Bridge/short the lower jack between each connector (A1 and B1).
Start car, turn off all accessories, make sure car is warm but radiator fan is not running, check idle speed. Should be 800 RPM +/ 50
Adjust idle speed to spec at adjusting screw on throttle body (accel.cable is sticking out of it, adj. screw is pointing approx.to the air cleaner).
Post March 1990 production date
Remove rear electrical connector from carbon canister shutoff valve.
With engine idling and all accessories off, install spare fuse in the top of the fuel pump relay.
Adjust idle speed to spec at adjusting screw on throttle body (accel.cable is sticking out of it, adj. screw is pointing approx.to the air cleaner).