Not sure if my question is why, but maybe more where. I read a rough idle at start can be a vac leak, but where would that leak be? The TB, manifold etc will all be at atmospheric pressure after shutdown, so where is the vac that is leaking down. What part besides the brake booster holds a vacuum after shutdown? Is there another check valve somewhere?

My car just started to do the rough idle at start also, so my hope is to identify this other area that holds a vacuum after shutdown and I will find my issue, I hope I hope I hope.

PS. I am miles past the normal issues, not imt, not intake gaskets, not the 'hose', not the booster cv

 

The vacuum leak is while the car is running; only the brake booster should be holding any vacuum after shutdown.

Three most common leaks on or cars are the PCV valve, IMT valves, or the brake booster line. On the booster, the center check valve dries out and leaks.

Any of those three issues should throw a code given enough of a leak, though.

 

Guess I can eliminate a vacuum leak then, time to spend money, whaaaaa!

Once school is out guess I will be heading to southern MD.