How do you check the vacuum?

[Vettefan87]

I have a vacuum gauge, but what do you connect it to to pull vacuum? What kind of vacuum should I be reading at idle? The car is old, and the motor has been in and out several times, so it is possible a little vacuum line is cracked.

 

[Hrtbeat1]

Hook up to one of the vaccuum ports on the passenger side of the Plenum. There should be two or three there. You can pull the one going to the cruise control. You should have around 12 psi at idle. It should be steady also ... if it flucuates a lot it might be a sign of a vaccuum leak.

 

[Vettefan87]

Thanks, I will check that when I get home, along with shimming up that front 3 inch Y pipe.

 

[Lucky 3]

I don't want to be technical but vacuum is measured in "inches", not PSI. You should have a steady 12 to 15 inches of vacuum at idle, it will go down as rpm increases. Start checking all lines for signs of cracks or rubbing on something. good luck. Bill

 

[Hrtbeat1]

The actual measurement is "inches of Mercury" its a measurement of pressure.


in Hg, bar, atmosphere, kilopascal, or psi are all common units of measurement.

But I was wrong in saying 12 psi .... It should be 12 in Hg(inches of Mercury)Just to be technically correct.

 

[Ken]

How is vacuum measured?

Vacuum level is usually measured in terms of inches of mercury ("Hg). This is the same term used when the weatherman reports the barometer reading "the barometer is 29.21 "Hg and falling". The maximum vacuum that can be achieved on a standard day is 29.91999.

Inches of water is another term used to measure vacuum, where 1"Hg = 13.6"H20. Inches of water is normally used when measuring low vacuum or air pressure.

Vacuum can also be measured in torr, microns, mbar, atmospheres.

Elevation from sea level affects the amount of vacuum that can be achieved. A rule of thumb is to subtract 1"Hg for each 1000' above sea level. Thus, if a pump normally reaches 27"Hg at sea level, then in Denver (5000' above sea level) it will only get 22 - 23"Hg.

 

[Hrtbeat1]

Thanks Ken

 

[Vettefan87]

If I check the vacuum off the plenum, wont that just give me what the manifold is? If I have a vacuum leak will it show?

 

[Ken]

Apply vacuum to each device via its associated line.

 

[G Winter]

yes it will show a leak anywhere in the system . A strong pulse in the guage may indicate a leaky intake valve. Or long duration intake lobes on the camshaft.