prevent backfeed with diodes ?

I want to use arduino as a serial adapter by disabling the micro controller and hooking up the tx and rx to a bricked router, I've done this 2 times. The 1st everything went well and the 2nd my arduino broke. So I can I be sure this wont happen? Should I add resistors or diodes or something?

Did you forget to connect the GND ?
There are already resistors between the usb-serial chip and the ATmega328P microcontroller (assuming you have a Arduino Uno).

Why not just get a USB<->serial adapter for a buck shipped??? Much cheaper than playing around with a $8-25 adapter with an Arduino dangling off it.

http://www.ebay.com/itm/USB2-0-To-TTL-6Pin-Converter-Arduino-CH340G-for-STC-PRO-Serial-Replace-PL2303-/321754572767?hash=item4aea1127df:g:12MAAOSwymxVPddi

It's hard to say why your arduino broke without more information.

Most routers are not tll level there rs232. I don't no of any that are at 0 to 5 volt levels you need a usb to RS 232

be80be, good point, thank you. I should have mentioned that.
If it is a (custom) JTAG serial port inside the router, it might be a TTL serial port, but then it could be 3.3V or 5V. If the serial port is on the outside it is probably a RS-232 and could damage the Arduino board.

Why not just spend your money on replacing the faulty router?