91 5.7 runs good cold not hot

i have a1991 silverado 5.7 runs good cold but after 5 min volt gage drops and truck runs terrible.alt. output is 14.2 all of the time the guage reads 14.2 when cold but drops to 9-12 when warm and starts to run bad.need help please.

i have a1991 silverado 5.7 runs good cold but after 5 min volt gage drops and truck runs terrible.alt. output is 14.2 all of the time the guage reads 14.2 when cold but drops to 9-12 when warm and starts to run bad.need help please.


since this is a rust belt vehicle and very old you must inspect and clean all grounds/and positive connections.. then check out the fuseable links for a green color ..if green replace them ..have the battery load tested to be sure its good.. then if all this does not fix the problem it may be the alternator has bad windings. this can occur when the alternator gets hot windings can short out ..

I have checked all of the grounds they are good i will check the fusible link tonight and post results
Thanks

I used to have an 88 full size Jimmy with the 5.7 in it and it did the exact same thing to me once too. Turns out it was the throttle body base gasket, or rather lack there of. There was 1 piece left of it approximately 2" long on one side, the rest of it was just not there. Easy way to see if it is the gasket is to spray a little WD-40 at the base of the throttle body when the truck is warm and running crappy. If the truck instantly runs better when the WD-40 is sprayed there, then you know it is the gasket.