I have a 92 GT with 115k. The car ran fine, but then when I was stopped at a red light I started to notice that the idle was rough. I kept driving and it smoothed out. The next day it ran fine. I didn't drive it for a few days, however the next time I started it up -it ran fine for about 5-10 sec ;then it started to run rough.The rpm's at one point go down below 500. It doesn't stall, it doesn't surge, but when I drive it runs fine. I gave it a tune up. No Joy, I changed the IAC ,No joy. I'm going to clean the EGR and TB tomorrow. Does anyone have any ideas, input,something please help tx

 

Pull codes first, that may help pinpoint it.

Otherwise, a vacuum leak would be what I'd look for next of no codes are thrown. These cars are getting up in the age, and the vacuum lines eventually go bad, causing all sorts of issues with what was a smooth running motor the day before.

Good luck, post back with what you find.

Welcome to the forum by the way.

 

Agree with the code pulling, and vacuum checks.

I have never looked into what that year 5.0 should pull, but my 351 pulls 15lbs. (hopefully this helps as a reference - I dont know a whole lot about vac)

Make sure to clean/check the mass air while youre doing the rest. If you can, might as well check fuel pressure too...