My 90' 5.0 is running super rich the fumes just burn your eyes at idle. It idels and runs fine and my milage is terable. Ive got headers, msd 6a, flowmaster single into dual exauhst, I thought meaby it was a cat prob so i punched it out but no change. Any thoughts as to any certain sensors or blown gasket possibiltys???

 

my first clue is that your O2 sensor is going out.I don't know much about sensors but my girlfriends vw bug did the same thing and both O2 sensors were out and the mass air flow sensor was out also. changed both of them and now its runing fine.

 

Good call on the O2 sensor. Grad a $30 code scanner at the parts house and do an engine running scan. It'll tell you for sure. An O2 fault won't always trip the Check Engine light. I know, I know. 30 bucks is 30 bucks, but it's a whole lot cheaper than throwing parts at it until you pick the right one.

 

Thanks a-lot fellas I was myself suspecting the o2 sens with some positive second opinions I feel pretty good about it especialy with a 130000 miles on what I can almost garaunty is all original ford installed equip. I've only done bolt ons and general maint.

 

You don't need a code scanner for a '90. Use a jumper wire and a test light, or an analog volt meter. This is the site Steve83 always sends people to:
http://www.broncodata.com/tech/codes.htm
It tells how to pull the codes there.