IdleI Issue

Hi, I'm one of the auto experts on RepairPal's Car Genius team! Great question, and it sounds like you've already done some solid troubleshooting! On your 1997 Lexus ES300, a fluctuating idle that dips down to around 500 RPM at stops is a pretty classic symptom of a failing or dirty idle air control valve (IAC). The IAC is responsible for maintaining a steady idle by regulating the amount of air bypassing the throttle plate, and when it starts to wear out or get dirty it can cause exactly this kind of hunting or dipping idle. Before replacing it, though, it may be worth attempting to clean it if you haven't already. Sometimes a good cleaning can restore proper function. That said, there are a few other possibilities worth considering such as a leaking intake manifold gasket, a vacuum leak or even a faulty coolant temp sensor, both of which can affect idle quality in a similar way. If cleaning the IAC doesn't resolve it we'd recommend having a qualified shop do a proper diagnosis before purchasing a new one. If you’d like to find a trusted, certified repair shop in your area, you can check here: https://repairpal.com/certified If you need more advice, or have any further questions about your vehicle, we’re here to help. Just give us a call at 877-913-1554