what does vac tell one about there car?

[truwarrior]

Cold engine
Weather: 0 degrees F

When I turned on my car vac read 10 (rpms 1100).
After a few minutes of ideling it drasticly dropped to 13 (1100 rpms).
When I got home the vac was 18 (850 rpms).

What the deal with the variance and what does it mean? Thanks for any info!

 

[custom240sx]

I forgot how much you want but it is around 16-18 or so. I dont know it has been awhile since i have heard this question. If you dont have enough vacum you could have a leak somewhere.

 

[kpt4321]

That's strange that you had less vacuum at a higher rpm idle, my car does the opposite.

If it's way off, it usually means you have a leak, or aftermarket cams, or the cam timing is off.

 

[Defiant]

I think the ISC cracks the throttle plate open on warm-up. That'd dump your vacuum.

As for what vacuum can tell you? Everything.

http://www.iwemalpg.com/Vacuum_gauge.htm