Engine temperature

Welcome to 2CarPros.

The knock sensor is not related to coolant temperature. It is responsible for maintaining optimum ignition timing to prevent detonation and ping.

As far as a rough idle, I have to be honest. If the engine isn't warming up to normal operating temperature, the computer will maintain a richer fuel mixture. That can cause the rough idle. Also, an engine vacuum leak is common to cause this also. Have you checked for any diagnostic trouble codes stored in the computer? If not, start there. Here are two links to help. The first explains how to retrieve trouble codes from a GM OBD1 system, which this vehicle has. All you need is a small jumper wire or even a paper clip.

https://www.2carpros.com/articles/buick-cadillac-chevy-gmc-oldsmobile-pontiac-gm-1983-1995-obd1-code-definitions-and-retrieval-method

Next, I would check to see if there are any vacuum leaks which can cause a vehicle to run a bit rough or even stall.

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

Check to see if any trouble codes are stored. Let me know what they are if you find them. Also, I would recommend replacing the thermostat if you know it's bad. Here is a link that shows in general how it's done:

https://www.2carpros.com/articles/how-an-engine-thermostat-works

https://www.2carpros.com/articles/replace-thermostat

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Here are the directions specific to your vehicle. The attached picture correlates with these directions.

Remove or Disconnect

Drain the cooling system until the radiator coolant level is below the thermostat.

Remove Thermostat and Housing

Install or Connect

Make sure the thermostat housing and water outlet sealing surfaces are clean.

1. Install Thermostat (23) in its housing.

2. Install New gasket (22) into position.

3. Reinstall Water outlet (21).

4. Reinstall Bolts (20).

Tighten Bolts (20) to 27n.m (20 ft. lbs.)

Fill the cooling system.

Start the engine and run with the radiator cap removed until the radiator upper hose becomes hot (thermostat is open).

With the engine idling, add coolant to the radiator until the coolant level reaches the bottom of the filler neck.

5. Radiator cap to the radiator, making sure the arrows line up with the overflow tube.

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

Take care,
Joe