Multi misfire - weak idle T6

[Jrsimp78]

I have a weak idle and occasional misfire code (multi). The initial fault was the intake manifold gasket. I replaced it and this seemed to fix the problem........ but only for 3 days. I therefore suspect another leak but there is nothing obvious. - I've tried pressure testing by plugging the main boot pipe and introducing pressure into the brake servo port.... No clear sign of leakage.

Does any one know a good way to test the invisible PCV pipe that goes from the PcV valve on the underside of the manifold to the catch can.
- Positive pressure into the dipstick tube ??

- It's also a bit low on power.

Could the fuel filler cap seal cause such issues ?

also with most car's I've worked on , they don't like it if you remove the oil filler when running .. My T6 continues exactly the same with or without the oil filler cap- I've tried the glove test- at idle the is a mild vacuum. I you blip the throttle there is momentary positive pressure. -- Is this all normal for a T6 ?

[cuhfs]

I believe the fact that at idle the glove has mild vacuum and does not blow up is excellent. these kind of mystery symptoms are the toughest to figure out.