I bought an 90 GT today 1st one in 10 years, first one I have worked on in about eight. So I need a little help, I am a little rusty. I have an erratic idle, I am also dumping excesive fuel. If memory serves this is an maf sensor problem. I have not checked the codes on it currently, can not remember if you have to have a scan tool or if you can jump the proms to get the check engine light to flash the code at you. Any help with these would be appreciated, I am an ASE tech but I have been used to working on dailey driver cars not mustang gt's for the past 13 years. As far as I know the upgrades go as follows, E303 Cam Shaft, Headers, H Pipe, and Flow Masters, other than that it is stock to my knowledge.

 

I believe up to 95-96 you can get the codes using the CE lite. Did you get a chance to check the fuel pressure regulator vacuum line for gas in it?

 

This is a case where a really good visual diag can save a lot of time. Check all the vacuum caps/fittings/lines. Check the throttle body is clean and the IAC isn't sticking. The throttle bodies on these had a long history of gumming up, getting cleaned, then not idling right due to wear. If there is a K&N air filter, that's the best place to start. There always were a lot of mods out there, so check fuel pressure and the vac. line from the FPR. Good luck!