Vacuum Reading question, what should it be?

[S4Denver]

I have a 94 S4, recently found I had a blown intake gasket ( had high RPM's), replaced , upgraded injectors, now runs fine all parameters are within spec's on freescan.

My question is the vacuum at idle is only 6-8 Hg it also flutters very badly? I know in a V8 that would reference a bad intake valve, but no noise at all from intake and car runs fine? is this normal for this 4 cyl? also since repair my BOV is not operating, have checked it is not a mechanical issue as it works with a vacuum pump I do not know what the readings were before repair it ran fine so I never scaned it or bothered to look. I know there are no vaccuum leaks so why the low vacuum that flutters like a bad valve?

[sailorbob]

I see about 0.7 bar MAP at warm idle.

Don't trust the Freescan values, they aren't accurate.

[S4Denver]

Reading was with mechanical gauge, taken both from upper and lower passages on intake, thought it might be caused by IAC but bottom passage read the same, reference to freescan was just to verify that it was running well, nothing outside of limits, so why the low reading and fluttering as if there was a bad valve? is this normal for this engine?

[sailorbob]

Fluctuating isn't normal, this is a screen shot of a datalog from a different car. The MAP varies by no more than 0.01 bar (again, at warm idle).

[sanj]

Follow the small plastic hardline going to the MAP sensor and measure it there, you will be amazed how much that small diameter tube dampens the fluttering.

[Andjons]

I also have high RPMs, and I don't think it's the IAC valve. You mention a blown intake gasket. What are you talking about, and where is it?

[ramjet]

Split hose?

[Andjons]

Which hose? I have no EBPV and have done the necessary mods to the vacuum lines. I have reset the IAC and followed the suggested procedure in the Freescan manual.

When I start up, the idle is regular, but about 1400. When I stop at lights, for example, the idle doesn't go below about 1300-1400. If I switch off and start up again, the idle is a comfortable 1100 or so. Aaaargh!

[Andjons]

To add to my previous post - IAC value is about 38 at idle, and MAP is around 38-40.

[Vulcan Grey]

You IAC sounds good (engine at 80C andidle, correct?) My MAP at 5300ft altitude is .38bar (in freescan).

 

A high idle is also controlled by the Throttle jack capsule.  Have you removed that and the linkage on the butterflies?  On my engine that linkage was sticking, causing a high idle after driving on the highway, so I removed the capsule and the linkage, and then lubed the throttle linkage.

 

Even with the throttle jack removed, the IAC will still be able to demand a higher idle for cold starts.  That is normal (and desired).

[Andjons]

In his post at the top, S4Denver refers to a "blown intake gasket".  I am trying to work out which gasket he is talking about.  Anyone offer any suggestions?

Edited March 30, 2013 by Andjons

[andydclements]

Inlet manifold gasket?

Turbo plenum gasket?

[Techspy]

I am interested in knowing which gasket as well. I am having random high idle (1400rpm or so) issues after driving for awhile when stopping at lights etc. I can blip the throttle and it goes back down where it should be.