[Porsche944]

Here is a test: Unplug the Throttle Position Switch and see if the idle still hunts.

If the hunting stops then I would suspect either the TPS is out of adjustment, the throttle plate mechanical stop is out of adjustment, or both.

Typically the throttle stop is the culprit, particularly if the engine had a low idle and a non-Porsche literate "mechanic" got his hands on it.

To check this out you need to pull the throttle body and loosen the TPS hold down screws. With the TPS loose, use an 8mm wrench to loosen the throttle stop jam nut, and then turn the stop screw out until it does not touch the throttle stop arm.

Clean the throttle bore, throttle plate, pivot points, throttle plate seat, idle by-pass port, and both vacuum ports with some carb cleaner, and then verify that the throttle plate will close completely. If you hold the throttle body up to a strong light you should be able to see a very small amount of light between the plate and the seat.

Once the throttle is clean and seating properly, turn the mechanical stop screw in until it just touches the stop arm, turn the screw an additional 1/2 turn, and then tighten the jam nut. This sets the throttle plate to its proper "fully closed" position.

Now you can adjust the TPS. To do this, hold the throttle plate closed and turn the TPS until you hear it click. Tighten ONE of the hold down screws. Test the adjustment by slowly opening the throttle and listen for the switch to click. You should hear the click just as the throttle plate comes off the seat. Verify that the mechanical stop screw is touching the stop arm. Now you can tighten the other hold down screw and the throttle body is ready to install.

The final step is to hook up a dwell tach, start the engine, and use the idle by-pass to set the idle at 950 +- 50 RPM.

What is causing the idle to hunt is the fact that when the throttle plate is closed, the engine is SUPPOSED to be at idle. If the DME sees a throttle closed signal from the TPS, but the RPM is above a certain value, the DME assumes the engine is in an over-run, or "trailing-throttle" condition, like when you are using the engine as a brake.

Since a trailing-throttle condition wastes fuel, the DME turns the injectors off until the RPM drops to a more "normal" value. If the mechanical stop or TPS are holding the throttle open too far, the idle will hunt.