I am trying to fix a chronic low idle problem in a 2002 Lexus ES300. This is the 4th gen. ES (2002-2006 300/330), with the 1st generation Toyota drive-by-wire system. The low idle has been persistent for years and resisted most common diagnostic methods or simple maintenance. The car drives superbly and has no error codes. Other owners report the same on the make forums, but there's no common-wisdom fix. On a 3ES or earlier Toyota, the main culprit is a dirty Idle Air Control Valve, though usually accompanied with rough warm idle. These DBW models do not have that IACV under the throttle body.
Over the years I have maintained the car well, with much more than "by the book" maintenance. I have had the valve covers off for new seals, new intake gasket, new platinum plugs. The intake systems has been reviewed a number of times for leaks with no luck, most recently a month ago. I go over all the vacuum hoses once a year at least and replace anything suspect, and did that last fall most recently. The car regularly idles when warm at 600rpm, verified with Torque/OBDII system and not just the cluster gauge. In the past, I could sit at a stoplight and slip the car into neutral, and the idle would rise a bit to 675/700. No longer; it stays at 600 now regardless when it's warm, a/c or no load. While the idle is low, it is consistent and not rough, and the car runs absolutely perfectly otherwise, so it defies common symptoms.
Last week I removed the intake and also broke the throttle body into it's two parts. There is an upstream section with a single venturi and a heater warming circuit plumbed through it. I've cleaned that often and it was very clean. I scrubbed it thoroughly with a bottle brush soaked in B12 anyway. In the 2nd section (behind the metal intake screen), there are two venturis and a butterfly valve on one of them, I did the same thing. I have never done this area before, It was fairly clean, but what the hey. I also pulled out the MAF sensor from the resonator box intake section, which was very clean upstream but a little dirty on the downstream side. I cleaned it gently with dilute dish soap and water on a q-tip; I had last cleaned it maybe three years ago. The car now idles around 650/675. Which is a small and helpful improvement, but one I suspect is going to degrade again over the winter. Unfortunately, I did multiple things at once (cleaning the twin-venturi section which I had never done before, and also cleaning the MAF sensor a little bit, which has been done in the past), so I can't nail down what made the change.
I suspect I need to think about the solenoid that powers the throttle plate? Is there a way to check the voltage going to it with the car running? What else do I need to look at and how to test it? I don't intimately know this DBW system works other than reviewing some youtube videos/online diagrams, and haven't been able to find diagnostic procedures. It is probably something simple, but at this point people just recommend throwing parts at the problem (which gets pricey fast, and is something that just offends my mechanical/diagnostic sensibilities).
Does anyone have a lot of familiarity with this Toyota system and has seen older ones/high mileage ones act up like this?
