What should the inches of vacum of be at idle?
I am running at 12 inches and some of the mechanics say i have a leak somewhere is this true?

 

I'm usually around 15 with the a/c on, close to 20 with it off.

 

**** I"m around 9-10

 

I'm at about 15-20.

 

I have always been at 20.

 

thats from my aeroforce

 

At idle I'm at 20. Cruising I'm at 15.

 

Standard atmospheric pressure is 29.92" of mercury, 30" to keep it simple. Do not confuse that with PSI. Standard pressure in PSI is 14.7. Even further, its also called one "BAR", short for barometric unit.

A boost/vac gauge references "0" psi/inches to the current ambient atmosphere. Then measures the pressure difference between the manifold and ambient. The negative range is measured in inches while the positive range is in psi. The negative range is really kindda labled backwards, but it is more logical that way, so thats how they do it.

At idle, with the throttle closed, the engine is trying to pull all the pressure it can out of the manifold. Typically, a piston engine running at a normal idle of about 700 RPM will create 10" of absolute pressure, or 20" vacuum below 0. More load, like air conditioning will require the engine to produce more power, so the throttle opens a bit bringing that number closer to zero.

You may already know most of that, but I thought I would be thorough.

Your 12" at idle is too high (relative term remember), and most likely does mean you have an intake manifold leak. You would also see low boost pressures at WOT with this problem.

Look where the light shines best. Have you ever taken anything off of your manifold? If not, start with checking the MAP sensor, then move on to the gasket surfaces.

Or better yet, It's under warrenty, bring it in.

 

I have tried to take it in they said as long as it is not throwing a code nothing is wrong with it.
I also have had boost issues since day one.
30,000 miles and still 10 psi max.

 

at idle i'm show 20 for vac

 

what is you elevation, the higher in elevation you are the less boost you will get and the lower the idle, i idle around 15 psi but i am at like 2000-4000 feet. also if you have your ac on or more load on the electrical system (blower motor) then you will have less vacuum.

 

You idle at around 15", not PSI. Like I said, it can get confusing.

 

hey HALFCENT, thanx for the pm about my header/downpipe delimma.

 

You're welcome. And now, not to be an ass, but fix your signature file. It's too big.

 

Aeroforce shows 8.9- 9.4 at idle with no A/C on for me.

 

15-20 on the stock guage here

crusin down the highway @80 MPH and 3000 RPM's i'm at 10 psi

 

Good lord I hope not! Again, I think you mean 10" vacuum, not 10 psi boost.