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When your engine is warm and at idle in park or neutral, it will vary between 700-900rpm.
When you put it in gear the idle will drop a little to between 650-750 rpm maybe a little higher when A/C kicks on.
Common causes for a high idle would be a vacuum leak, it would also cause the engine to act funny when in gear, rolling idle or hesitation. You'll also hear a sucking noise.
The most common area for a vacuum leak was the pcv tube. If you find the pcv valve follow the tubing all the way to where it meets the intake. The rubber components will degrade over time causing a hole to form. This will create a vacuum leak. Look and listen for the leak.
The other common problem for high idle on 4.2L was intake gaskets leaking. The gaskets where the plastic intake meet the metal lower intake would leak. Also the bolts that would hold the 2 together would weaken causing a leak so you would replace the gaskets and the "isolator" bolts at the same time.
A way you can test for this is to buy some brake
clean from the autoparts store (flammable). Get the engine to operating temp and then spray where the 2 intakes meet with brake clean. If the idle starts going up or down, you found your problem.
Be very careful not to spray it on the exhaust
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