1991 Chevy S-10 ROUGH IDLE, HESITATION ,

Welcome to the forum. How long since a tune-up? Use a gage and check fuel pressure. Any codes? Also check for a vacuum leak.

TUNE-UP DONE 7/2009 ALL NEW HOSES BELTS FILTERS AND RADIATOR. PRESSURE GOOD, NO VACUUM LEAKS OVER THE WEEK-END PUT GAS TREATMENT IN JUST INCASE RAN GREAT , THEN STARTED TO ACT UP . LOOKED IN BOOK WOULD CHECK ENGINE LIGHT COME ON IF PUMP GOING? AND DOES TEMP MATTER BECAUSE RAN GREAT EVENING BUT IN MORNING RAN ROUGH AGAIN? THANKS I TRY TO WORK ON MY OWN BECAUSE I'M A SINGLE PERSON AND PEOPLE SOMETIMES TRY TO SELL YOU SOMETHING YOU DON'T NEED OR MAKE IT WORSE TO CHARGE MORE, I JUST WANT TO KNOW WHAT IT COULD BE INCASE I DO HAVE TO TAKE HER IN

Intermittent problems can be hard to track-down. The pcm may monitor the voltage on the fuel pump circuit, so if there is no voltage when there should be, it may set a code. I don't recall if the cel comes on,but, I have seen the the code on other models. You have an oil pressure switch in the fuel pump circuit, once you crank it enough to bring up oil pressure, the switch closes and the pump gets voltage.

Vacuum leaks can open or close with temp changes.

OK I'LL RECHECK VACUUM AND GET A CODE TEST TO SEE WHAT IT SAYS THANKS ALOT I'LL LET YOU KNOW WHAT I FIND OUT THANKS AGAIN

found out that my rotor had a crack in it and a tooth was missing no fuel pump problem even though had a tune -up they never checked rotor thanks for the help