While searching for causes of very low vacuum (8 in hg) I read a post that mentioned that a vacuum leak might be internal. What would cause an internal vacuum leak? What would cause my street ported, S5T2 to show only 5-8 in hg using an hand held gauge? The TPS is set to 1.0 V and yet the idle continually varies between 900 and 1500. It will idle at 900, slowly climb until it reaches 1500, drop to 900 and repeats.

thanks
ash

 

are you running a BAC?

A big street port will cause low vacuum reading. Its typical to see a large street port pull in 8-13inhg. A complete stock port healthy engine will pull 16inhg+

 

Yes, it's emissions free but it still has the BAC. I don't think the BAC is manually adjustable on the S5, at least there is not an adjustment on mine.

 

it is possible to have vacuum leaks that go from the inside to the inside. the most common is the ACV, it can leak from the air pump area directly to the intake runners. you can look all you want to from outside, but you won't find it.

it might be the throttle body spacer thing too

 

any vacuum leaks past the throttle butterflies in the intake tract will show up as low vacuum on an engine gauge.

carb cleaner works on a running engine to find external leaks as well as a PVC cap with a tire stem on it to pressurize the intake, use soapy water in a spray bottle to find the leaks(difficult on a stock car assembled).

i would suspect it is a larger vacuum leak in the intake tract, use the methods above to try and find it. i say this because the engine itself would have to be in very poor shape to even attempt to fire up if it was pulling that low of vacuum.

 

I don't have an air pump but thanks for the explanation about the internal leak.

 

I had a vacuum leak last spring that was under the upper intake, maybe the same hose came off.

Thanks Ben