Bouncy Idle Fixes
By: Mouchyn / MustardCat / Brian Murray
(From: http://www.preludepower.com/forums/showthread.php?t=184070)

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Before we begin I want to describe why the idle "bounces". Say for example you are driving at 70mph in 5th gear and you let off the throttle, leaving it in gear. The ECU sees that the throttle is closed via the throttle position sensor (TPS) and it also sees that you are above ~1400 RPM. When these conditions are met the ECU cuts fuel to the injectors and keeps the fuel cut until either the RPM drops below ~1400 or it detects the throttle has opened again.

This is what is happening when the idle bounces.

For whatever reason there is excessive air bypassing the throttle body raising the idle above ~1400 RPM. The throttle is closed so the ECU enters deceleration mode, cuts fuel, and the idle quickly drops below ~1400 RPM. The ECU starts injecting fuel again, the idle quickly raises above ~1400 RPM and the cycle repeats forever.

If your lude has a bouncy idle it could be a few things and in no particular order...

1. Air in the coolant lines: If there is an air pocket in the Fast Idle Valve (FIV) once the car is warmed up it will allow excessive air bypass and bump the idle above ~1400 RPM, causing the bounce. There's a bleed screw near the temperature sensors on the thermostat housing. A 12mm wrench will open it. Let the engine heat up and open that screw every few seconds until just solid coolant comes out:

2. Fast Idle Valve (FIV): Coolant flows into this valve and operates a wax ring. The wax ring contracts or expands with the temperature of the coolant. The operation of the wax ring gives you the increased idle when cold. If this wax ring is bad or the valve is just old and loose then too much air will "leak" into the intake manifold, causing another high idle situation. Again the ECU will enter deceleration mode and cut the injectors.

First take the top off of the FIV (2 phillips head screws). Inside you will see a white plastic piece with a slot on either side, use a flat head screw driver in one of the slots and screw the white piece in until it is snug. Put the top back on and check the idle again:

If that didn't work do the following procedure to test the wax ring portion of the valve: Take a cereal box or anything made of the same kind of cardboard. Cut a piece of the cardboard to fit between the FIV and the intake manifold. Place the cardboard piece between the FIV and the intake manifold and bolt the FIV back on. Let the engine heat up. If the bouncing goes away then you know it's the FIV. If the bouncing is still there then it's something else. Once you know what the problem is take the cardboard out and put it all back together the way it was.

3. Electronic Air Control Valve (EACV): This valve is controlled by the ECU. It allows the ECU to pass extra air into the intake manifold when the engine is under additional load, like when you turn on the A/C. If the valve goes bad it could allow too much air into the intake manifold causing a bounce. Test it just like the FIV: once the engine is fully warmed up make a block off gasket for it and see if that solves the issue.

4. TW Sensor: This is the 2 wire sensor located on the thermostat housing that has GRN/WHT and YEL/GRN wires going to it. If you have a bouncy idle while the car is warming up but it goes away after reaching operating temp then it could be this. What can happen if this gives inaccurate readings is the ECU will think the car is all warmed up when in fact the coolant is still cold enough that the FIV is open.

5. General vacuum leak: This could be a break/leak in any of the intake manifold gaskets, the vacuum lines, injector seat gaskets, intake temperature sensor gasket, brake booster hose or check valve, PCV valve or hose, etc. This could take some time to track down but is most often caused by damaged vaccum lines, so start there.

6. Idle screw set too high: Disconnect the 2P connector from the EACV. This will throw a check engine light and cause the engine to run funny. It should stabilize. Then adjust the idle screw until the warm idle is at or near 770. Turn the car off, reset the ecu (pull clock fuse in under hood fuse box), and reconnect the EACV. This is set from the factory so you shouldn't adjust this unless it looks like it's been tampered with or you have investigated the other possible causes: