Fuel cutoff- what triggers it?

[Orin Eman]

> 
> At 11:42 PM 10/23/00, you wrote:
> > > Now, the ecu must know what the pressure is in the manifold, for one main
> > > reason - timing. If there is a minute leak in the line to the ecu, there
> >
> >Boost control too.
> 
> yes, but really only to regulate max boost when WOT is pressed and to 
> cutoff fuel at 1.6 bar. The ecu uses the boost value constantly, however, 
> to determine the correct timing.

It would need the sensor for boost control, regardless of whether
it uses it for timing.  You can use an air mass sensor just
as well as a MAP sensor for timing.

> >So I don't think it explains the symptoms.  It sounds like a
> >simple overboost fuel cutoff.  Which usually occurs for one of two
> >reasons.  Torn diaphragm (common IME) or bad hose to the bottom
> >of the wastegate (less common IME).  A stuck wastegate is also
> >a remote possibility.
> >
> >The dash boost gauge cannot be trusted at the best of times
> >and should be ignored as a diagnostic tool.  You have
> >to plumb in a mechanical gauge to find out what is going on.
> >
> >Reality is, there is no reason for the ECU to cut fuel at 4400 RPM
> >with the pressure transducer indicating 1.2 bar.  It will happily
> >go over 2 bar if the transducer is broken/disabled at which point you will
> >_HEAR_ the knock and back off the gas before it gets around to
> >cutting fuel, if it ever does.  BTDT with an ECU programmed for
> >1.95 bar max boost and a BIG wastegate spring.
> >
> >Orin.
> 
> 
> Well actually, a torn wastegate will allow exhaust to escape thru the wg, 
> bypassing the turbo, so the boost will only reach 1.2 bar or whatever. But 
> this DOES NOT explain why the engine is '_much_' more powerful as stated. 

You don't understand how the wastegate works.  The spring holds it closed
ie. all exhaust going through the turbo.  The bottom chamber of
the wastegate is fed by boost pressure from the intake manifold.
This pushes against the diaphragm and a metal cup which are held
down by the spring.  If the diaphragm tears, the boost pressure
escapes through the diaphragm and cannot lift the cup and hence
open the wastegate.  You get overboost and it happens very quickly.
The symptom of a torn wastegate diaphragm is overboost.  You should
search the archives for wastegate diaphragm...

> Also I don't buy that the stock boost sensor is so inaccurate. I'm sure 
> Audi chose a decent sesnor. They didn't need a very fine resolution, but it 
> needed to be decently accurate. Most likely the sensor has a much finer 
> resolution, but the gauge just can't display it cuz it's only 2 digits. You 
> can't judge a sensor simply by the gauge it uses to display it's output. It 
> used to go to 1.4 bar, so it's doubtful that the sensor or gauge just 
> suddenly stopped going past 1.2, but might be remotely possible.
 
The stock MAP sensor is fine.  It's the buffer circuitry and/or
the dash gauge itself that are inaccurate... see Phil's posts on the
dash gauge.

> A leaky ecu line will allow the boost to increase w/o control, agreed? 

No wastegate control will only give boost defined by spring pressure
in the wastegate.  What would happen with a leaky ecu line is
that the ECU would control the wastegate to try and increase
boost.  With a stock spring and changing the desired boost
tables in the ECU, the system is hard pressed to even reach 1.8 bar.
A leaky line can't magically make the ECU control the wastegate
any differently than if I change the desired boost tables from
1.43 to 1.8 bar.

> Point is replacing the ecu line will cost under $5 for a new hose. Opening 
> the wg will be a pain the arse, and replacing it will be quite expensive, 
> even if it's used. I think replacing the hose is beneficial anyway, so that 
> way you know for sure it's ok. If it doesn't do anything, it's not like you 
> wasted lot's of money. When diagnosing a problem and you don't know the 
> cause, don't replace the most expensive part first, replace the cheapest. 
> Cuz it might have needed replacing anyway, and it'll be a lot cheaper when 
> you realize replacing a $75 diaphragm didn't change anything. So many 

The diaphragm is incredibly easy to test.  Does the upper wastegate
chamber hold vacuum.  No?  Diaphragm is torn.  I'd replace the lower
wastegate hose before the ECU hose.

Orin.