Rough idle, low vacuum

[LateApex]

My A4, 1.8 2003 stock, has a rough idle problem when cold. Sometimes it sounds like it is going to die. I decided to put a vacuum gauge on it to see if I had a vacuum leak and observed an interested result. Upon cold start the vacuum read under 10 inches. It held this level for about 30 seconds then bumped up to about 14, then after a little longer up to 18 which it then held. The idle smoothed out and the engine quieted at each jump in vacumm. Is this normal?

[Cincy]

on a cold start, the sai (secondary air injection) kicks on to warm up the cat. YOu will have increased idle which will lower vac. However, you shouldnt have stumbling. You still should check over the vac lines coming off the intake manifold along with the sai lines

[LateApex]

OK, makes sense. Thanks.

[crazyeight]

how much for vacuum should we have at idle w/o leak ?

[Cincy]

depends on elevation and what else you have done. 20in hg is the normal +/- when the engine is warm

[crazyeight]

does it concern secondary air system? or evap ?

[Cincy]

when the sai is on, the revs will be higher and the vac will be lower. When it shuts off, the vac will go back up. If you remove both, it should go up a bit when the engine is warm

[l88m22vette]

My vac usually sits around 10in/hg when the SAI is on

[AWDrifter]

I dont remember where it is with sai on but once warm its at 20 and 25 on decel