Hello , I have a 99 s10 blazer and the 4x4 won't engage. All buttons stay lit like it should be working. I can a noise from underneath the vehicle like it's engaging and a click noise from behind the glove box. I recently had 3 days where the vehicle would not start , it turned over and wanted to start but it didn't. Then on the fourth day it started with a bump of the key as usual. I also noticed that over the summer the AC air blowing from the dash vents would dissapear when I pressed down on the accelerator and come back when released a little and now just since last week the air from the heater doesn't matter where selected wants to come from the dash only. I'm thinking I have some sort of vaccum leak but from where ? Maybe the EGR valve was stuck open making it not start ? It does idle a little rough but the engine runs very strong any other time.

 

Definitely a vacuum leak. And it's affecting anything vacuum related. I'd first check the vacuum ball - not sure where it's located on a '99 - then all the lines and I think there's a three way check valve that often craps out as well.