High idle speeds are a little easier to troubleshoot than low idle speeds. Most
of culprits are directly related to the throttle body itself.
On the stock throttle body, there is an accelerated cold-start system that purposely
raises idle temporarily for emissions purposes. Cold-start idle speeds can be
as high as 3,000RPM and should last for about a minute. Make sure the engine
is fully warmed before proceeding into further troubleshooting.
If the upper intake manifold has been removed recently, the throttle cable tends to
bind due to a twisted throttle cable. Always try to remove the throttle cable
when removing the intake manifold. Try to shake and actuate the throttle cable
prior to mounting the cable back onto the throttle body. This will ensure the
cable is not twisted and minimize the possibility of binding.
Sometimes throttle cable tension has been knocked out of spec for whatever reason.
Also check the cruise control cable, as this runs parallel to the main throttle cable.
There should be no tension on either cable to the throttle body when
at idle or at rest; unecessary tension to the throttle cables will crack open the
throttle plates for an elevated idle. The thottle cable tension should be just
loose at idle and should immediately go tight when pressing down on the throttle.
A dirty throttle body can easily cause the throttle plates not to close. Due to
stock intake port timing, reversion occurs in the secondary intake passages that
causes crud to deposit all along the walls and throttle plates. Removing the
throttle body and cleaning all deposits should cure this problem. Make sure
all the throttle plates close properly by snapping the throttle plates several
times by hand.
If the throttle body is stock, the thermowax from the cold-start system is still
active. The thermowax can fail and cause the idle not to drop properly.
Clogged coolant passages can also cause the thermowax not to disengage properly.
Remove the throttle body and clean all coolant passages and check the thermowax
for proper function. An alternative is to remove the thermowax completely
through our Throttle Body Mod...
FC3S Pro - How To's, Throttle Body Mod
On a few occasions, an engine running rich can induce a high idle through an intake
FC3S Pro - Problems #1, Engine, Intake Vacuum Leak
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