coupelex

The idle on my 1993 SC400 won't hold, so I have to keep a little bit of pressure on the gas pedal to keep it running at a stop or backing up. I know this is a common problem, but where should I start first? It has about 180k on it and the timing belt was done within the past 20k and the plugs and coils are all recent.

Would this be an idle air control valve issue, a mass airflow sensor issue, an ECU issue, or that fuel pump issue?

Thanks for your help!