i apologize it is the MAF sensor. to check the vacumn issue follow all the lines from the sensor to the throttle body and below it down by the firewall. it also runs into the intake manifold below the throttle body. there should also be another sensor clipped into the throttle body,make sure its tight. check the egr valve. but the jist of it is the small black hoses that GM used on the tbi engines become brittle and crack i think theres about 4-5 all in all. all of mine snapped and i thought it was the MAF sensor but when i replaced all the lines and reset the comp it ran like a champ. i do think it damaged my idle control sensor because my engine will rev up on its own at a dead stop. try replacing the vac lines first and checking the sensors and grounds theres a million possible things that could cause this issue but this is the most common.

 

So i followed the vacuum lines and no leaks found. Ive narrowed it to the sensor. There is discoloration on the screens of the sensor that looks like it got too hot, so that might be why its bad. All i have to do now is find a sensor that isnt 1400 dollars from GM.

 

Well, i replaced the maf sensor and problem solved. It still runs rough when i first start it but goes away in about 8-10 seconds while the accelerator is feathered.

 

Hey got almost the same problem. Driving me crazy!!! my 91z28 350 tpi is skipping and loosing power going up hill whenever it gets warm. At idle runs fine untill the engine gets warm uped. Didn't start doing this untill I cleaned the injectors with chem 12 and put in a can of sea foam. changed plugs, cap, rotor button, wires, checked tps sensor {voltage in specs}, checked map sensor, only number 2 and number 4 plugs are black and sotty. All the rest are clean, also looses power going up steep hills, but regains power going down hill. I'm lost for answers, any help would be greately appreciated!!!

 

Have you tried looking at your idle air control valve? My 305 tpi wouldn't even idle until it warmed up, so I replaced the IAC and it idles great.

 

the bad idle and stalling out is most likely caused by the combination of the IAC (idle air control) and EGR (exhaust gas recirculation) valves, you'll notice the need to feather the gas upon start it up and once the car warms up these valves wont be doing there job...relativly cheap fix around 150$ and you may only need the IAC