[Schulisco]

Originally Posted by 99boxster559

Still car wants to take a second to fire up at cold start but its better for some reason now. The popping under decel is what I am looking at currently.

Bad decel valve? my car is an 82 so it has the pancake diaphragm on the right side of the engine with the small vacuum line going to it. It is plugged in and going to manifold vacuum on the back of the throttle body.

Will a bad one act as an air leak as well causing starting problems that get hidden by the lamda system kicking in adjusting things once the car has some temperature in it?

It depends. Either it won't open and stay closed - then it's no vacuum leak. But exactly this will causes the popping in the exhaust under decel. The extra air from this valve helps burning the shorttime superfluous fuel in this manner which otherwise will pop in the exhaust. The valve opens up for just a second or so. You can also crosscheck this when releasing the throttle pedal and the rpms immediately falls to idle which it shouldn't. Instead the engine should stay above idle around 1500-1800rm for a second to make shifting a lot easier as this smoothes the gear change because of better fitting rpms transitions.

Otherwise - in case of a ripped diaphragm - then it's defintely a huge vacuum leak and supplies the engine in any situation with more air than required. You can work this partly by closing more the idle screw. But this won't solve the problem. Instead you move the whole system out of the working window with all negative outcomes: Less power, reduced efficiency, bad start etc.

How many turns do you currently need to fully close the idle screw? (reset it afterwards)

From what Ive read on here people plug them to get rid of rev hang which seems to add the pop and burble I am experiencing. Can this part be repaired with the squeeze it back together method? Is there a good way to confirm function with the part in or out of the car?

When the valve reacts too slow or is hanging it's probably glued with to fuel residues. Then I would flush it with some shots of silicon spray (no WD40 or similar oils as they will destroy the rubber diaphragm over time!)

You can take it out of the car and test it with a vacuum pump. JoeEngineer showed this on his page too:
https://j_o_e-e_n_g_i_n_e_e_r.com/porsche-911-cis-subcomponents-and-testing/ (without the underlines) or https://rb.gy/vave3m

Thomas