Good morning Stanley,
I'm not sure if you noticed the date of the last post, but this is an old thread. But I'm glad to see someone posting in it ! I bought my RX with 50K on it, back in 08. Two years after I got it, it started having the dreaded vibrating idle issue. I put so much time, effort, and money into trying to eliminate it- but sadly without success. I even wrote a letter to Lexus at some point. I don't remember exactly what their reply was. But they weren't willing to do anything for me. And really, I bought the car already 7 years old. So what could I expect. But the head mechanic at Lexus in Cherry Hill NJ admitted to me that this was a known issue, and there was no known fix for it at the time. This was years ago now, so I'm doubtful anyone has put in the time to solve the issue. But, I'll share with you quickly my thoughts on it. It's still something that annoys me a bit to this day...
I bought an bluetooth car scanner that would send data to my phone. And then set it up so that I could watch live data coming from the ECU while I was driving. I was mainly watching the 02 sensor feeds to see if there was some issue with the fuel mix. Like if the car was periodically running rich or lean. I never saw correspondence with that and when the vehicle begin to vibrate at idle. I also changed the motor mounts, and IACV. And numerous other parts in hopes to get rid of the vibration.
But here's something I've noticed. The vibration is only an idle issue for me. Sitting still. Typically in Drive. I believe the vibration always goes away if I put the vehicle in park. But I'm not 100% on that. The engine seems to idle properly and smooth when the tach is around 900rpm. But when it drops in rpms to about 600-750 then the vibration begins. Its as if something suddenly loads up on the engine, bringing the speed down and not providing enough power to idle smoothly. Now many systems in the car send a signal to the IACV to up the throttle, when those systems power on. IE - turn your steering wheel a bit, and you'll bring the idle up. The PS pump as a sensor to detect use, and will kick up the idle to compensate. The AC compressor, and headlights do this as well. ... So, question is what is that loads up on the engine, but does not send a signal to up power ? My guess is that it's a tranmission issue. But I don't know enough about transmission function to say that's it for sure. If I ever get to it, I'll figure a way to hardwire one of those systems so that they're always sending a signal to IACV to up idle speed. From my understanding there is no proper way to adjust your idle speed on our vehicles.
Anyhow, I need to head in to work. Let me know what you think ?
And did I understand this correct ? you're running natural gas in your car ?