Does anyone know if a automotive idle air control motor will work on a 500efi or do I have to buy one from mercury?

There are lots of standard auto choices for Merc part number 805224A1.

https://www.tnaautoparts.com/2002-ch...e-p-48444.html

http://www.rockauto.com/catalog/more...55&cc=10000741

http://www.mercruiserparts.com/Show_...And+Components

Here looks to be a complete list of IAC's. Just find your OEM part number and try to cross the number in here.
http://www.rockauto.com/dbphp/x,carc...IAC_Valve.html

I bought one for my 496 mag from Rockauto for $43.00. Alot better than $100+ form Mercury.

I bought some from Auto Zone for way cheaper and so far, knock on wood, they are lasting way longer.