Vacuum leak?

I replaced the upper intake on my 96 Blazer. It started right up, but I can hear a hissing sound like there is a vacuum leak. The hissing sound is near the firewall. The engine is not running very smooth. Engine does not die when I take my foot off the gas, but service engine light is on now. Any help with this would be greatly appreciated.

There are a few vacuum fittings and lines near the back of the plenum. Make sure they're all connected.

A smoke test works good for pinpointing vacuum leaks.