hey I got a 5.8l efi that's running very rough, when idling will change idle speed constantly, it also has black smoke coming out of the exhaust, what should the first thing I check? its been sitting since October and the guy said it ran fine when he parked it.

 

Due to the excessive black smoke, I would say its overfueling badly. Try throwing some fresh fuel with a double dose of injector cleaner and floor the old girl. Also check for a rats nest or similar in the intake. Had that problem a year back with my old black f350. Cleaned out the intake box and changed the filter and it cleaned up the rich condition immediately. If not cold be a stuck injector. Best of luck

 

Welcome to FTE!

Sure sounds like its pig rich. Need to check fuel psi. Sounds like it's the Fuel Pressure Regulator.
Also, could be a leaking injector. Try removing the vacuum hose from the nipple on the FPR and see if fuel is evident or running out of it.

And last but not least, Check for Codes!

Report back with results.

 

thank you will try that out!

 

I would seafoam it. Go get two cans of Seafoam. Read the instructions. I can to mainline thru the brake booster vac. line and one can to put in the tank start with that then start replacing parts.