Vac leak??

[Alerosaint]

Ok guys I need ideas....my cars running real rough at idle sometime cutting out as i apply gas... sounds like its camming off and on. I check the plugs they are good. I have a steady SES light and a trac off light that comes on off randomly.
I dont know what the codes are...I have hptuners but im waiting to get it back to read the codes. My vac guage normally ready -10 thru -9 at idle but im reading between -7 thru -5. Any ideas? Im thinking vac leak but i check every where i know.

[mbeals]

Check the vac line between the MAP and the TB. It's a tiny little tube.

Mine was almost rotted in half and you couldn't tell without really looking at it. I was having about the same symptoms as you are.

[Vic28]

Thats a pretty low vaccum tho.. -20 on mine. Plus a vaccuum leak would make the car rev higher even at idle. Maybe something with the fuel?????

[Alerosaint]

not really...my buddy had a pretty big hole in one of his vaccum hoses in his a4 and it was idling real rough and stalling...he patched it And all was good..but i also was wondering about if it was a fuel issue as well...under throttle the car runs great

[mbeals]

When you open up the throttle, you drop the manifold vacuum. At lower vac levels, the hole won't leak as bad, and if it is the MAP line, the manifold vac will start to drop closer to the (incorrect) vac read by the MAP.

The reason I suspect the MAP sensor is that speed density systems typically react well to vac leaks...its just a bigger throttle opening to them. That is unless the MAP is getting the wrong signal, then they drive like crap.

You should notice the problem the most at high vac situations, like accelerating off idle and accelerating after coast down.

Out of curiosity, where are you measuring vacuum?

[[ion] C2]

@mbeals: Big leaks don't help produce positive pressure from the supercharger though if the leak is post-rotors.

Sounds like a vacuum leak for sure. Check the tightness on the intake manifold bolts.

[[ion] C2]

Thats a pretty low vaccum tho.. -20 on mine. Plus a vaccuum leak would make the car rev higher even at idle. Maybe something with the fuel?????

He's measuring in PSI from the GlowShift boost/vacuum gauge. It reads "negative" PSI, so it's basically half what inHg would be.

[Alerosaint]

Yeah im reading from a boost gauge....i dont think its the map....

[Vic28]

C2;514944']He's measuring in PSI from the GlowShift boost/vacuum gauge. It reads "negative" PSI, so it's basically half what inHg would be.
I figured thats where hes reading it from, In GlowShift(like most of them) once is on the vacuum its readings on Hg. Unless when he typed here he converted the numbers to PSI himself. I guess every engine is different obviously. And like you said, if the numbers changed than you leaking some where, just hope its not the motor itself.

[mbeals]

Must have missed the part about it being boosted.

you could always try the starting fluid trick and see if you can track it down.

[[ion] C2]

The easiest way to locate vacuum/boost leaks is to do a boost leak test... hook up pressure to the throttle body. Any performance shop can do it. Listen for hissing.