Vacuum Pressure questions

[97sprt4x4]

I have a Vacuum pressure gauge installed on my truck, and i'm trying to figure out what i'm actually reading when i'm driving. With the truck in park when i increase the RPMs slowly from idle to 4 grand the pressure drops out ending at 8 at 4-5grand, When i'm at idle it reads 19 pounds. I seem to remember reading somewhere that if it drops out like that then it's an indication of a vacuum leak somewhere, and that if the motor is in check the vacuum will stay the same if the rpms are increased slowly.

I'm trying to figure out if i need to replace my intake gaskets, as my gas mileage is around 11 or 12 mpg and my idle has a studder when warm.

Thanks For the help

Justin

 

[ezrider440]

Hi There
Well here's the deal typically and it depends on the effecientcy of the motor's seal, a vacuum gauge should read at motor idle- around 25-20psi at the intake, it also depends where's your test point and the reading should be steady, when you rev the motor past it's idle point, the vacuum gauge should drop drastically since you're sucking more air into the intake and your opening your throttle, so what you're seeing is typical

what you use a vacuum gauge for is the following : you look for smooth drops when you rev the motor, you look for a steady indication at idle and when you're at working load, meaning cruising down the highway at 65mph you look for an even reading at you will be most effective when you reading is closest to it's idle reading so if you idle at 20 psi you should be cruising at 10-15psi, when the more vacuum your intake produces the motor air/fuel is sucked in effeciently

thats your vacuum 101 course

 

[Gadget X]

Yeah, what he said!

 

[2000StreetRod]

vacuum gauge units of measure

A vacuum gauge is normally calibrated to read vacuum in inches of mercury (in-Hg) and not in pounds per square inch (psi). For accurate engine diagnosis the vacuum gauge hose should be connected to the intake manifold.

An intake manifold gasket leak is most significant at engine idle when the airflow thru the engine is minimum. As the engine speed increases the intake manifold leak becomes less significant. According to my Haynes manual to test for leaking intake manifold gaskets, look for a low, fluctuating reading (three to eight inches below normal and it fluctuates at that low reading).

 

[gijoecam]

Ditto on the gauge markings... It's impossible to get more than 14psi of vacuum anywhere on the face of the earth (with the exception of maybe death valley or any other place located below sea-level).