88 2.9 running rich

It used to run really well, but recently, all of the sudden it started acting up and runs rich. As soon I start it it will run okay for about 4-5 seconds, then it runs like crap. I checked the vacuum lines and they appear to be ok. Replaced all plugs, and both fuel filters. I am getting about 40 psi of fuel pressure. I pulled off IAC motor and cleaned it up. Still continues to run rich. Any ideas? I hooked up a scanner and it could not read the code

On that model you have to rin a key on key off engine test to check systems for codes.
But one thing that would cause something like this is the map sensor. It could still be working but incorrectly. You would have to back probe and check the megahertz. But I think I would just try a know good map sensor and see if it helps.

I forgot to add that I swapped out the map sensor and a computer off a good ranger. How do I test the key on key off?

Umm well glad to know you have try map. On the test, you neeb scan tool. On that model the tool will run a key on engine off test. Then you tell scanner to run slow test. The scanner will then run a test on all sensor and solenoids and do a voltage test. It then will tell you the codes. The scanner can also run a key on engine running test too. It takes this to pull codes on that model Ford.