Electrical

HI and thanks for using 2carpros.com

If the idle is erratic going up and down, most commonly the problem is the idle air control valve (IAC). The IAC is responsible for maintaining the engine idle speed. It is located on the throttle body. I would suggest removing it and inspecting it to see if there is dirt, corrosion, or other issue with it. Here are the directions for replacement.

First, here is a link that shows one being replaced. It may be helpful.

https://www.2carpros.com/articles/how-to-replace-an-idle-speed-control-motor-iac

Here are the directions specific to your vehicle. The attached pictures correlate with these directions.

REMOVAL
1. Disconnect the battery ground cable.

See Pic 1

2. Disconnect the Idle Air Control (IAC) valve electrical connector.

See Pic 2

3. Remove the two bolts and the IAC valve.

INSTALLATION
1. To install, reverse the removal procedure.

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As far as the coolant temperature sensor, it is on the front of the engine near the thermostat housing. Here are a couple links that may help. The first one discusses symptoms of a bad sensor and the second link shows one being replaced.

https://www.2carpros.com/articles/symptoms-of-a-bad-coolant-temperature-sensor

https://www.2carpros.com/articles/coolant-temperature-sensor-cts-replacement

Here are directions for replacement specific to your vehicle. The attached pictures correlate with these directions.

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Sender Unit - 4.2L

Removal

1. Partially drain the cooling system.

2. Remove the water temperature indicator sender unit.
1. Disconnect the connector.
2. Remove the water temperature indicator sender unit.

See Pic 3

Installation

1. To install, reverse the removal procedure.

See Pic 4

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Let me know if this helps or if you have other questions. Keep in mind, most often a erratic idle isn't caused by the temp sensor, however, I guess anything is possible. Keep in mind, an engine vacuum leak can also cause these symptoms. Here is a quick link that shows how to check for a leak.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Let me know if this helps or if you have other questions.

Take care,
Joe

In my experience a hunting idle on a 4.2 engine is 9 times out of 10 a huge vacuum leak from a pcv tube that is collapsed and split open. Idle air control valves do fail, but the usual symptom when they stick is that on a cold start the engine will start and then stall. A coolant temp sensor will not cause this either but there may have been a code for it so that’s probably why they recommended it. Look for vacuum leaks and you might find your problem.