Vacuum fluctuation at idle.

[Jordan_J]

It moved up and down a couple of pounds really once the car has warmed up but until then it runs fine. Also when I let of the gas and am just rolling it does it. What could cause this?

Cam?
Head gasket?
Vacuum leak?

[Steve Wood]

Inches of mercury  

It should read fairly steadily at idle, cruise or any steady throttle position as long as there is no load change.

Typical might be 15" at cold start up  increasing to maybe 17" as it warms up...all at idle.

going down the road at 70 mph, it maybe 12" and drop to 6-8 going up a hill.  

If you step on the gas, it may go to zero, or onward into boost.

If it is jumping around at idle without the gas being touched, then it could be a leak, bad valve, poorly damped gauge, or something else.

My numbers are ball park guesses as the engine combination, altitude, engine condition, etc. can all affect the vacuum being pulled.

Most gauges probably don't move much more than an inch on a healthy engine if a quality gauge is used...that is my experience, anyway

[Jordan_J]

Awesome thanks for the info Steve!

Would there be any other key symptoms to a bad valve?

One thing I noticed is the RPM occasionally will jump up and down like 200 rpm which leads me to think it's a vacuum leak somewhere.

[Steve Wood]

you can do a compression check to see if you have a leaky valve...someti mes if it is an exhaust valve, you can hear/feel it in the exhaust