Open a propane torch (but don't light it). Move it around the engine while it's running. When/if the engine changes the idle speed, you've found a leak.

 

Is it possible that the vacuum leak could be coming from the spark plugs? I'm out of ideas on where the leak could be coming from due to the locations I messed with.

I used a carb/choke cleaner method rather than the propane method and it didn't seem to have any leaks around the upper intake manifold, on the PCV valve and line, or around the two vacuum lines on the back. Are there any other spots I can check?

 

Check your MAP sensor and IAC valve. Those can both have an effect on engine idle speed. Make sure the IAC passage is clean and unobstructed. Make sure the MAP sensor is putting out the proper voltage under the proper vacuum. Make sure the IAC pintle is extending and retracting properly. Check that the grounds and source voltages to both are proper. Those are both important control points for the 4.3 Vortec.

 

WOOHOO! Just wanted to let everyone here know that I have finally fixed the problems! After my last post I gave this project a break to enjoy the last bit of my summer. My friend and I decided to start working on the gas tank to finally have it stop leaking, but wanted to try a new strategy... so we cut a big hole in the floor to access the gas tank! We shielded the gas tank well and used a jigsaw to make the hole, so no sparks to worry about blowing us sky high. We then had easy access to those leaking lines and good thing we made the hole, every connection was leaking and one of the lines had even corroded and needed to be epoxied. We fixed that all up and got the gas tank to stop leaking.

I decided to tackle the idle issue today and I decided to re-investigate the throttle issue because now the accelerator was stuck at the floor. After some fiddling, I have now determined the air flap had seized up and just needed a generous dose of WD40. The Blazer starts up great now!

Thanks again for all the help, I'm sure I couldn't have done it without y'alls insight!

Here are some pictures of the hole we cut, we'll be turning the cut out into an access door soon.