some of you might remember the air sucking issue i experienced last fall, i went thru 3 intake gasket sets and even with the thick ones from Best Gaskets (14046 E), it still sucked air. not as much as with the thin metal ones, but enough that the engine would stall when brake cleaner sprayed on between manifold and cyl.head.
now, i took the manifold down again, cleaned the surfaces and mounted it without gaskets but with marking paint on the head side.you can now clearly see that the 2 surfaces are uneven and only seats correctly on the 8 spots where the bolts are. looks like the manifold was tightened too much.
i have to find a machine shop that can make these surfaces even.
How are the surfaces in heads? That could be checked with metal ruler.
did not check that yet, but i guess the manifold side is more prone to be uneven.
I would double the gasket thickness ( two thin or two thick gaskets both sides ) and see if that would stop the vacuum leak — as you know you manifold or head matching surfaces are somewhat irregular from your matting paint test
A good trouble shooting matting test—- for sure
Should be the an easier fix than other options if it works — worth a try !
A machine shop can plane the manifold and the head but much more involved and some expense as well
Not sure if you need to be doing both heads and the intake surfaces
Machine shop will advise on that
Many have issues with our intake manifold seal ( to many ) leaking air ( vacuum leak ) which cause engine performance issue ( cylinders running lean and hotter than normal )