1997 F-150 4.2 4x4 255,000kms

This morning I got in the truck to go for a drive and it started and ran as normal. I turned off the truck to get gas and when I went to start it to leave it backfired and would not idle and kept stalling until I gave it gas as soon as it caught. It was surging quite badly but smoothed out if I gave it gas. I was able to make it home but noticed a hissing noise from under the hood and I am suspecting a vacuum leak. Is this a good assumption? I have been searching and remember reading about the possible locations to look but can't find or remember where they are. I would appreciate if anybody can help me out with this as I need my truck. No check engine light as of yet. Please let me know if I am on the right track and where all the trouble spots are. Thanks

 

Check the PCV hose on both ends. The elbow that connects to the rear of the intake manifold is known to crack.

 

I managed to troubleshoot it myself. I started the truck and listened for the hiss and found on the passenger side on the back of the upper intake or plenum, there is a large round grommet with a vacuum line to it. The grommet had fallen out of the plenum. Put it back in and it is good as new.