lower vacuum than normal

[Module Unknown]

So ever since I installed my boost/vac guage it has always read 18in hg (+/- 1) when idleing. However, lately when I go out and start up the car in the morning it reads 15in hg while idleing, then after a 30min freeway cruise to work it's back at idleing around 17-18in hg, occasionally dropping to 15. Still reads what i'm used to seing during deceleration though. But anyway, just looking to get some ideas on causes, reasons, or what to look for to try and correct this. Thanks for any suggestions or help you guys can throw at me.

And before anyone throws up a link to the vfaq for me to go build an intake leak tester, I do have one and will play with it this weekend, as well as checking into anything else you guys tell me to look for now.

 

[Megags]

I have the same problems, exact same readings too, My vacuum gauge wiggles when I first start it as well, probably 1/2inch travel moving very quickly.

I have suspected my isc to be having trouble as my cold start is non-existant as well as my ac stepping up.

 

[92redman]

depending how cold it is, your IAC might be open, how high is your idle when you are getting these vacuum reading, it should be judged at curb idle ie 750 rpm

 

[Doug99RS]

<--- agrees. What RPM are you taking your readings at?

Also, during cold weather the engine will normally operate at a higher rpm, at idle, in order to bring the engine up to operating temperature. Before the weather dropped off here in NC first thing in the morning my RPM's would flare up at start up and work it's way back to about 800 rpm within 30-60 seconds. Now that it's around 30-40 degrees in the morning I find it only dropping down to about 1200 rpm until I put it in gear which will then drop it down about 3-400 RPM (it's an auto).

Expect the same from most any car. Remember this as well. When performing a test for the purpose of diagnosing a problem or taking measurements the engine should be at full operating temperature.
Doug

 

[Module Unknown]

Good point. I would like to say that it still happens after the car has been driven a while and is up to operating temperature, however after thinking about the last couple replies I'm now second guessing myself and can't say I'm positive on that, soooo i'll will watch it again tomorrow and pay closer attention.