This is a new one for me. Sam said he heard hissing. Passed a smoke test. But, my son wanted to apply smoke to the area where he heard hissing.

We found this:



Can't wait to find out what happened.

 

The engine is at idle and it will be pulling maximum vacuum that will be close to 20 inches Hg so you have a powerful "vacuum cleaner" sucking in the smoke.

Clearly the lower O ring on the injector is not sealing [paint missing?], the ring has failed or is simply not present. If it has the more common [smaller size] O ring typically found on injectors that will also leak profoundly