Too much vacuum?

[Spyderwoman]

I recently installed a VDO boost gauge (BCS restrictor mod & air filter at the same time) and I've noticed that the gauge seems to show a lot of vacuum pressure. When it idles, it shows 20-22 in Hg but when I take my foot off the accelerator (without braking) the gauge reads 28 in Hg. I was wondering if this seems normal or is this too much vacuum? Does this indicate some problem? Also, does 14 psi of boost seem correct for a nearly stock 2g?

 

[jim97gst]

The spec for vacuum at idle from the factory service manual is 18 in HG minimum. There is no max spec. One of my general automotive books section on normal vacuum reads: "Needle will be steady between 15 and 20 in HG while idling. When the throttle is suddenly opened and closed, the needle will drop to below 5, then bounce up to around 25." None of the vacuum problem descriptions say anything about too much vacuum. They are all either too little vacuum or some sort of fluctuation problem. It does sound like yours is reading a little higher than my 97 GST, but it doesn't sound like it's a problem.

The standard value boost spec from the manual is 6.4-14.7 psi (does your gauge read to tenths?). Mine reads between 14 and 15.

 

[johnfsmith]

Originally posted by Spyderwoman
I recently installed a VDO boost gauge (BCS restrictor mod & air filter at the same time) and I've noticed that the gauge seems to show a lot of vacuum pressure. When it idles, it shows 20-22 in Hg but when I take my foot off the accelerator (without braking) the gauge reads 28 in Hg. I was wondering if this seems normal or is this too much vacuum? Does this indicate some problem? Also, does 14 psi of boost seem correct for a nearly stock 2g?

Mine does the same thing.... even gets up to 30 sometimes....

 

[candela]

makes me feel better....mine reads about 18-19

 

[Turbo Guru]

The vacuum reading will be higher under deceleration (lifting foot off gas.) This is because of the suction caused by the pistons at higher speeds having no air in the intake manifold to ingest when the throttle plate is closed. In short... its normal.

Joe

 

[Spyderwoman]

Thanks for the information. I guess it's normal and I won't worry about it.