Should thread sealant be used on vacuum fitting the screw into the intake manifold? Note: i'm talking about the part that screws into the intake manifold, not the hoses that connect to the fitting.
Yes, can become a small un metered leak. I like to use thread sealant over t tape (and not the plumbing type).
Teflon tape...Booo. It gets places where no one wants it to be. Fuel pump filters and screens, oil pump screens, carburetor needle/seats. Use teflon tape nowhere ! Most plumbers...don't even use it. I just use oil. This helps make sure that the fitting (both vacuum & pressure) go together with no galing. I've never had a leak, using a little oil in any of my automotive assemblies, and I'm old..! Though I did have a leak in a high pressure air gun (4350psi !). I had to use a special "paste" (NOT tape) to help seal that one. Ok, all you tape experts...have at it.., you'll not change my mind. Mike
I've had wonderful luck with Blue Monster on sealing everything and you don't have to over torque anything to get it to seal, great stuff, it's a paste.
The amount of air that could possibly leak around unsealed pipe threads into the intake manifold is insignificant and will not affect the mixture. But yes, I'd use a bit of paste or neverseize on them. I do use Teflon tape on occasion, and put it on the first threads even, because that's where it's needed. It's unlikely that any loose shreds will cause problems in an intake port. Often times you can run a small "crack torch" type burner into the bores of the threaded fittings and burn off stray bits of Teflon tape. By the way, any tape that can be seen in an assembled joint is outside of the joint and does no sealing. It's only the 3-5 threads that are actually engaged that have the opportunity to make a tight joint.
I use the crap out of teflon tape myself with no problems. The only trick to teflon tape is not starting it hanging over the edges of the fittings.
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