Fuel cutoff- what triggers it?

[Ameer Antar]

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.

> > will be reduced pressure going to it under boost, ie. 1.2 bar instead of
> > 1.4 bar or whatever. With the throttle in the idle position there may be a
> > higher minimum pressure reading, b/c air is getting in through the leak,
> > when it's under vacuum, ie. 0.3 bar instead of 0.2 bar. So that explains
> > the low boost reading. Now the increase in power should be obvious. The 
> ecu
> > thinks the engine is under less boost than it really is, so it dials in
> > more pressure on the wastegate, until the wg duty cycle approaches 100%,
> > and no more boost can be obtained. This could cause actual boost to get to
> > 1.6+ bar. Bond that, the normal ecu, thinking the boost is only 1.2 bar,
> > the engine will run out of fuel and can detonate b/c of advanced timing.
>
>The fuel quantity is mechanically determined with no
>difference whether the ECU thinks it's 1.2 or 2.0 bar.
>The knock sensor can handle up to 1.8 bar - all the resistor
>and zener mods rely on it at 1.8bar.
>
>The addition of boost on top of the diaphragm can take it
>up to 1.8 bar BTW.  I programmed the ECU to do that once.
>Takes a while to get there with the stock spring and a
>working diaphragm though.

true.

> > This could be the cause of the 4400 rpm limit and increased power
> > underneath that limit. So this explains all of your symptoms. This can 
> be a
> > real dangerous situation if driven hard in the wrong circumstance, so I
> > hope you think about this...good luck.
>
>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. 
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.

A leaky ecu line will allow the boost to increase w/o control, agreed? 
Actually there is a gas filter w/ a desecant in the ecu line that might 
also have leaky connections or the filter itself might have a leak. Well, 
when the throttle is open 55 deg. or more, which is the when the WOT switch 
is closed. Throttles of 55-90 deg is very common, esp. when bringing it 
past 4k rpm. So if boost is actually say, 1.8 or 1.9 bar [which is easily 
achievable w/o wg control], and the engine's timing is set for 1.2 bar, the 
engine will have a real tough time running. B/c ignition will be far too 
advanced, and fuel begins running out, esp. if the CIS system is not up to 
factory specs and fuel filter is a couple seasons old. At 4400 rpm you 
should be able to 1.9+ bar, the point where the CIS system needs help, b/c 
the turbo can even reach 2.2+ bar. I really do think this is a possible 
cause. It does satisfy the low boost value, the increase in power, and the 
4.4k limit.

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 
mechanics who are clueless as to a cause just start replacing things left 
and right until they find the answer, making you pay for their wrong 
guesses. That's why I do my own work. I choose to start from the bottom, 
not at the top. It's like saying if you don't have enough boost, replace 
the turbo...who knows, maybe yer turbo is malfunctioning. But why start w/ 
that when you can start w/ a $5 item? OK, I think that should be enough 
explanation. The only thing to do now is to start trying the different 
fixes and seeing if it changes things. good luck...