I just changes both upper and lower manifold gaskets along with the new green grommets on the upper intake. I have a 2003 4.2L. everything went well. It started great. I took it for a spin and there was a strange problem. When I accelerate when running the ac the fan stops blowing in the face and blows up the defrost. I am guessing vacuum issue, anyone ever come across this?

 

You missed a vacuum line some where. Or just got unlucky and the one way check valve went bad.

 

The line is on the passenger side by the firewall and runs under the battery. I broke mine and just haven't fixed it yet. Some other vacuum lines become very brittle over time.