Would the EGR Valve cause a 1999 Chevy Tahoe to idle hard at stops?

A faulty EGR valve *could* contribute to a rough idle in a 1999 Chevy Tahoe, but it's not the only possibility. A rough idle at stops can be caused by many things. The EGR valve's role is to recirculate exhaust gases back into the intake manifold to lower combustion temperatures and reduce NOx emissions. If it's stuck open or closed, or if it's leaking, it can disrupt the air/fuel mixture, leading to a rough idle, hesitation, or poor performance.

However, other common causes of a rough idle in a vehicle that age include:

* Spark plugs and wires: Worn or fouled spark plugs and damaged ignition wires can cause misfires, leading to a rough idle.

* Ignition coil: A failing ignition coil can also cause misfires.

* Mass airflow sensor (MAF): A dirty or faulty MAF sensor provides inaccurate air flow readings to the engine computer, leading to poor fuel mixture.

* Throttle position sensor (TPS): A faulty TPS can also lead to incorrect air/fuel mixture calculations.

* Vacuum leaks: Leaks in the vacuum system can disrupt engine operation and cause a rough idle.

* PCV valve: A clogged Positive Crankcase Ventilation valve can cause a buildup of pressure, affecting the engine's performance.

* Fuel injectors: Clogged or faulty fuel injectors can lead to an uneven fuel mixture.

* Engine mounts: Worn engine mounts can cause vibrations that feel like a rough idle.

Therefore, while a bad EGR valve *is* a potential culprit for a rough idle in your 1999 Chevy Tahoe, it's crucial to have a proper diagnosis performed by a mechanic to pinpoint the exact cause. They can use diagnostic tools to check for trouble codes and test various components to identify the problem. Simply replacing the EGR valve without proper diagnosis could be a waste of money if the issue lies elsewhere.