I have a 1990 silverado 5.7 that runs fine until it warms up then it runs rough in gear hard to start when its warmed up

SOURCE: 1995 Chevy silverado with a

I have experienced this problem on similar vehicles. You should try and replace the coolant temp sensor. If it is not reading the correct temp to the ecm, it will not get the right fuel map. Also, the cts will not always throw a code.

SOURCE: when cold, it idles rough, revs high then low! After warmed up it runs fine

if it runs high when you first start it then mellows out,, theirs no problem , all of our chevys do that.... but if its sputtering or you hear a quiet popping noise check spark plugs

SOURCE: rough idle then stalls

Vacume leak or bad idle motor. With the car in park and running with the hood open listen for leaks moving your hand around the different hoses looking for a change in the sounds that you hear. The idel motor also makes the car idel up to 1500 RPM or so right at start-up then idel down, If it doesn;t the idel motor is likly. Check engine light is on when running? Test for codes, Follow code test charts don't skip any steps!.

Testimonial: "Found the problem vacume leak like you said."

SOURCE: 93 chevy lumina euro won't stay running

sounds like it could be a fuel injector shorting out. have to get it hot, pull off the plenum and check the resistance in each injector. if any injector has bad resistance, replace it. when an injector shorts out on these particular vehicles, it shorts out the whole injection circuit. and when it cools off the resistance goes back up and it will run ok. i think the correct resistance should be about 13 ohms.

SOURCE: 88 CHEVY PICKUP SILVERADO STOPS RUNNING IS JUST WARM

Sounds like a bad ignition coil. Check for spark right after it quits Coils can and do quit working after they get hot. Also check magnetic pick up coil in distributor. After reaching operating temp computer will move the coil and a broke corroded wire loses connection. Then when engine back to cold wire is back in contact and working. Hope this helped.