1998 Ls Cold Start

[RCRFAN3]

I have a 98 LS400 with 130,000 miles on it. I started my car when it was 17 degrees(F) out and the car idled at 1500 RPM and continued to idle at this speed after car got to operating temp. I drove the car for 60 miles and the same thing occurred. I pulled my car into a heated garage and turned it off, and let it cool. I then restarted the car and since then the car has been operating normally, idling at 750 RPM. On a cold start, outdoor and motor temp, it idles at 1500 and has a very slight vibration to it. It feels like an oscillation of a motor or a possible flooder in the fuel system. It goes away after the engine warms up. I have been told that it is possibly the throttle control motor with magnetic clutch, or the same part called an IAC, idle air control valve. These are commonly replaced on American made cars, but I have found no such history for Lexus. Fuel mileage has not changed since I have experienced this. If you have had this problem or have any info for me, please respond.