Cold start rough idle

Hi and thanks for using 2CarPros.com.

Since the rough idle seems to be related to temperature change and there is no check engine light on, my first suspect is the engine coolant temperature sensor (ECT). There is a designated temp sensor for the computer only. Its function is to let the computer know the coolant temperature. Based on what the sensor tells the computer, air and fuel mixture is determined. Now, if the sensor is telling the computer it is 90 degrees F and in reality it is 60, the fuel mixture will be too lean and cause a driveability issue.

What I suggest is this. If you have access to a live data scanner, connect it when the engine is cold. Review the live data and determine if the ECT is accurate in the information it is providing. For example, it is 60 degrees outside. The sensor should indicate approximately 60 degrees + or - a couple degrees. If it is way off, replace the sensor.

If you determine the sensor is bad, it is simple to replace. Here are the directions. The attached picture shows where it is located.

Engine Coolant temperature (ETC) Sensor - 4.6L and 5.4L

REMOVAL and INSTALLATION

1. Remove the Engine Coolant Temperature (ECT) sensor.
1 Disconnect the electrical connector.
2 Remove the ECT sensor.

To install, reverse the removal procedure.

Also, an engine vacuum leak can cause a rough idle. Here is a link that shows how to check for leaks. Try this too if the sensor is good.

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

Hi:
Actually, most times there is no code. It is because the sensor is still sending a signal, but it may be the wrong signal information. As a result, the computer thinks it is working fine.

Yes, the coolant temperature sensor for the computer is different than the one for the gauge.