The FT232RL will only work if the TXD and RXD pins are inverted, since standard RS232 is already inverted from TTL. Fortunately FTDI included that fix in their programming utility. It's a no-brainer to download the free "FT_Prog" utility and set the TXD and RXD pins to "Inverted" status. http://www.ftdichip.com/Support/Utilities.htm#FT_Prog
That brings the polarity back in sync with the TTL versions. You can't directly tie the two pins together however. See this link for the wiring: http://www.firebirdnation.com/forums/topic/457761-make-an-obd1-cable-for-under-20/page__pid__866681#entry866681