Fuel cutoff- what triggers it?

[Orin Eman]

> 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.

> 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.

> 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.