The problem with nearly all of USB to serial converters or problems with them falls into 2 catagories...

1) They cant acheive non standard baud rates (Such as the L8 IAW programs use)

2) The program software you want to use doesnt use the windows system correctly to make use of the converters.
I.E. The programmers of the software you want to use take short cuts to save time which leads to these kind of non working problems.

Any software I write using comms routines have never had issues with any USB-serial converters except for when Item 1 above is beyond the programmers control.

I have found a brand of USB-Serial lead that has never failed to work on anything I have tried although I have never tried it on Megaquirt as the name just makes me shivver

Here is the link...

http://apple.clickandbuild.com/cnb/s...+RS232+%2B+DB9

About twice the price of others but it is quality...