[Vwdiesel] Overboosting Problems

[Tad]

Curious, what caused the head gasket to fail?  I understand that's very
uncommon on TDIs.

Can't say for sure but it doesn't make sense for the valve to pass vacuum
at idle because more vacuum = more boost in this system.

Are you sure that you're getting the proper amount of movement?  Applying
vacuum is the command to *add* boost, but perhaps the vanes aren't moving
far enough in the opposite direction, causing overboost under certain
conditions (high rpm?)

Are there specific conditions that cause the overboost code consistently?
 high/low RPM or load?

Years ago when I installed a mechanical boost gauge (tapped into the
plastic pipe just upstream of the 90-degree rubber bend), after getting my
ECU chipped, I saw that I would hit spikes in excess of 25 PSI for a few
seconds before the system backed off the boost.  I wasn't happy with this
response time so I added a mechanical boostvalve tapped into the bottom of
the IC and spliced into the control line to the turbo.  This basically
blows pressure into the turbo control system when the boostvalve opens,
quickly opening the vanes.  This does a great job of clipping the peaks off
of the boost spikes at about 20 PSI and then keeping the system regulated
within 1-2 PSI of the target.

The takeaway from the paragraph above is this: I *never* saw an overboost
code when my car was hitting these excessive spikes, so the system pressure
has to be way too high for several seconds at least before a code will be
tripped.

A VagCom connected to the car can log the actual vs. requested boost, which
will tell you how the boost pressure sensor is reading as compared to what
the ECU is asking for.  This will give you an idea of how the sensor is
working.  I think you can also get absolute readings from the pressure
sensor using VagCom.  Remember that these numbers are *absolute* pressure,
so you have to subtract ~1000mB(14.7psi, dependent on your altitude of
course) to get the more common "relative" pressure.

I guess one test you could do would be to disconnect/cap/kink the vac line
to the turbo... no vacuum should equal no/little boost.  If you still
overboost with no vacuum to the turbo, you have a problem with the VNT or
otherwise isolated from the vacuum control system.

Another thought would be to temp mount a mechanical boost gauge, to see
what boost levels you are really getting.

Last thought is maybe the control rod from the vacuum pot to the VNT is
miscalibrated (too long/short)?

On Sun, Nov 27, 2011 at 6:50 AM, Dave Cook <davevw at yahoo.com> wrote:

> I forgot to add to things done:
>
> The intake was cleaned when I replaced the head gasket this past spring.
> That's not it.
>
> I also replaced all of the vacuum lines with proper sized, German, braided
> ones at the same time.
>
> Can anyone tell me if it is proper for the boost control valve to pass
> vacuum at idle or not?
>
>
> Also, I figured that since I could get the proper amount of movement down
> at the turbo, that meant the vanes weren't sticking.  Is this an incorrect
> assumption?
>
> We have been known to quickly turn it off and back on to reset it, but it
> is now more quickly going back into limp mode.
>
>
>
> Dave Cook
>
>
> ________________________________
>  From: "LBaird119 at aol.com" <LBaird119 at aol.com>
> To: vwdiesel at vwfans.com
> Sent: Sunday, November 27, 2011 1:26 AM
> Subject: Re: [Vwdiesel] Overboosting Problems
>
> Different setup but Dad's Passat was going into limp mode due to a  kink in
> the line
> to the boost sensor.  Boost sensed was too low for the throttle
> position/load so it'd
> limp.
>   I found that if you turned the key off and back on, very quickly,  while
> driving, it reset
> the limp mode and you were back in business.  At least until the next  limp
> mode, but
> it made it way less frustrating to drive until we found the problem.
>      Loren
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