Fuel cutoff- what triggers it?

[Ameer Antar]

actually if you think about it will make sense. Depending on what type of 
Audi we're talking about the boost is controlled simply by the wastegate 
spring which is rated for 1.2-1.3 bar. The wg spring opposes a a diaphragm 
which allows exhaust to bypass the turbo, thereby regulating boost. Only 
when the WOT switch is pressed will the ecu begin to control the boost via 
the wg solenoid. The ECU brings up the boost from 1.2-1.3 bar to 1.4 bar by 
using some of the intake air which already under pressure and pushes 
against the diaphragm, to hold the wg open a little more, thus increasing 
boost. So if there was a leak in yer wg hoses, you would not be able to get 
past 1.2 bar. However, this would also be accompanied by a slight loss in 
power, not gain, as the engine would not be receiving as much boost. Also 
w/ leaky wastegate lines, you still should have no problem getting to 4400 
rpm. In fact when I had a dead turbo, my car could still go up to 5k, easy, 
but not past .9 bar. So that's why I doubt the wastegate lines are the cause.

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 
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. 
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.

-ameer

At 07:12 PM 10/23/00, you wrote:
>Thanks, but none of those suggestions would explain why the car has
>more boost and pulls very strongly to 4400 rpm.
>
>Taka
>
>-----Original Message-----
>From: quattro-admin at audifans.com [mailto:quattro-admin at audifans.com]On
>Behalf Of Ameer Antar
>Sent: Monday, October 23, 2000 12:09 AM
>To: quattro at audifans.com
>Subject: Re: Fuel cutoff- what triggers it?
>
>
>sounds like a leaky ecu vacc. line. Check the connections at the ecu and
>the back of the manifold for a loose connection. If the ends of the hose
>seem fine, I'd replace the whole thing; it might have gotten a tear in the
>middle. there's also been some mention of having the wrong fuel pump relay
>causing strange rpm limits. But b/c the engine feels more powerful, the
>gauge only goes to 1.2 bar it's most likely some leak between manifold and
>ecu. You could possibly have a plugged fuel filter, which doesn't allow
>lots of flow when you need it at 4000+ rpm. But w/ a bad filter, there
>should be some loss in power, not gain....good luck.
>
>-ameer
>
>
>At 11:33 PM 10/22/00, you wrote:
> >Hi listers,
> >I've encountered a rather interesting problem.
> >
> >The car in question is an '86 5000CS quattro, MC engine.
> >
> >My car won't rev past 4400 rpm, at which point the fuel
> >cuts off and the car jerks rather violently.
> >
> >The car feels _much_ more powerful recently, all the way
> >up to 4400 rpm.
> >
> >Boost gauge in the trip computer won't go past 1.2 bar,
> >although it would do 1.4 bar previously. Throttle body
> >hose is in good condition and less than a year old.
> >
> >Any ideas?
> >
> >Thanks,
> >Taka