cutting off injection going downhill was [Vwdiesel] Why gasss cars mpg

Sun May 15 21:34:17 EDT 2005

Yeah I have a VAG-Com, I just cannot mechanically justify the concept  
of 0 fuel to the engine.  I guess the way to test this is simple, go  
to the top of a hill, put the car in first gear and let it roll down  
the hill without using the accelerator pedal.  If the fuel is really  
turned off then the car should reach the bottom of the hill and stop  
because it is getting no fuel.  However, I suspect you will find that  
as the car slows it will begin to buck and sputter because of the  
slow running engine trying to move the heavy car at idle speed.  I  
understand what you are saying about the VAG-Com showing 0 injection  
quantity, however I don't think that is what is really happening.  I  
still think that there is some minimal quantity of fuel always  
reaching the injectors.  In the old IDI engines, towards the end of  
their run, they added a device to the top of the injector pump that  
looked like a turbo enrichment unit.  In fact it was but the pressure  
was applied in an opposite manner.  When the car would decelerate  or  
the engine would overdrive, as they say, a vaccum was applied to the  
diaphram which would pull up on a needle valve allowing most of the  
fuel to by pass the distributor for the injectors. (In a turbo pump,  
pressure is applied on the opposite side of the diaphram and pushes  
the needle in the opposite direction directing more fuel into the  
distributor part of the pump.)   In this way they decreased, but did  
not eliminate the fuel to the injectors.  A small amount still made  
it to the injectors and the ignition process in each cylinder  
continued but at a much decreased rate.  In this way they lowered the  
smoke and pollution produced.  I suspect that the process is similar  
on the TDI.  On deceleration, the total amount of fuel is decreased  
but is never really 0.  Injection simply takes place at some default  
rate, just enough to continue combustion but not really enough to run  
the engine.  If the engine shut off, you would be able to tell, the  
clattering of the diesel combustion process would cease and it does  
not.  Next time you free wheel your engine listen closely, although  
it is quieter when doing this, the unmistakable sound of diesel  
combustion is there.  hayden

On May 15, 2005, at 7:20 PM, Val Christian wrote:

> To "Start" the engine, you need fuel, oxygen, and ignition.
> Since the engine keeps "pumping air", it has oxygen, and it
> also has ignition temperatures.  When the injection of fuel is
> resumed, the engine "starts".  VAG-COM displays the total
> injection volume.  Idle speed in controlled by the injection
> volume.  Try turning on the AC and/or drawing a large electrical
> load (defroster, lights, etc.) and the injection volume goes up
> to maintain the same RPM band.
>
> Do you have VAG-COM so that you can watch the injection volume?
>
> Val
>
>
>
>
>
>
>>
>> The injection amount commanded by the ECU may drop to 0 but that
>> doesn't mean injection stops.  If the TDI actually cut off the fuel
>> to the engine as you went down hill, it would have to know when to
>> "restart" the engine.  How would it know when to do that?  I spent 6
>> hours in my car yesterday going up and down hills in East Texas and
>> Louisiana.  The engine never stops running even if I let off the
>> pedal.  It simply returns to idle status, meaning only enough fuel
>> being injected to keep the car idling.  The ECU injection quantity as
>> I see it is a demand ABOVE AND BEYOND what is needed to keep the
>> engine running, not the total amount of the fuel injected.  In other
>> words, there is some base amount - X that is injected all the time,
>> but when the ECU senses a signal from the pedal, injection quantity
>> goes up.  Let up on the pedal, injection quantity goes down.  There
>> is simply no practical way for these engines to literally shut off
>> when going down hill.  Just my two sense worth but as I said, I spent
>> 6 hours in my car yesterday thinking about this and trying to
>> logically justify the concept.  It simply cannot work.  Hayden
>>
>> On May 15, 2005, at 6:22 AM, Val Christian wrote:
>>
>>
>>>>> A couple of things...first, the TDI DOES shut off all injection
>>>>> in a coast operation.  I've observed that, and verified it as
>>>>> correct behavior for the TDI.
>>>>>
>>>>>
>>>>>
>>>>>
>>>>
>>>>   So when you top over a hill, (and keep going of course)  the TDI
>>>> engine actually shuts off?  Seems like something DOT wouldn't
>>>> approve of regardless of ramifications or not.  Seems like a
>>>> freewheeling (ala E-mode) would save even more fuel.  If that's
>>>> the intent.
>>>>     Loren
>>>>
>>>>
>>>
>>> When you top a hill, and start down, coasting, the injection level
>>> commanded by the ECU can drop to zero.  It's a function of power
>>> commanded.
>>> Taking your foot off the pedal, it will drop to zero.  Depending  
>>> upon
>>> the hill, not moving your foot may cause some fuel to be commanded
>>> to be injected.  But simply stated, yes, fuel can (and often does)
>>> drop to zero.  It's not freewheeling, per say, because the entire
>>> engine (altenator, compressor, power steering, pistons and  
>>> valvetrain)
>>> keep turning.  So when the available power exceeds those, you get a
>>> free ride.
>>>
>>> Val
>>>
>>>
>>>
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>>>
>>>
>>
>> TX AND US DISTRIBUTOR FOR DIESELPOWER TUNING BOXES.  GO TO
>> www.dieseltuning.com, LOOK AT THE INFORMATION AND THEN EMAIL ME TO
>> MAKE YOUR PURCHASE.  MORE POWER, MORE TORQUE, GUARANTEED!!!!
>>
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>> mydieselpages/
>> Proud owner of:
>> 2004 Passat TDI GLS, 2.0l, 8 valve, 134 HP, 247 lb/ft of torque, Blue
>> Graphite, Anthracite leather interior, ABS, ESP.
>> 1998 Jetta TDI (Wetterauer Chipset, 2 1/2" exhaust, K&N Filter, 119K+
>> miles, and running biodiesel)
>> 1981 Rabbit Pickup ( with a freshly rebuilt turbo engine, 10K+ on
>> engine now), burns biodiesel and a WVO system is in the works.
>> One, yet to be finished, Caddy trailer (the back end of the truck
>> that gave its life to my Rabbit Pickup).
>> And many, many, many VWs; from a 1946 Beetle (11 Beetles total), to
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>> Jettas (5), Passat (1), Dasher (1), New Beetle (1), and Rabbit
>> Pickups (3) most now gone but not forgotten.
>>
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>>
>>
>
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>

TX AND US DISTRIBUTOR FOR DIESELPOWER TUNING BOXES.  GO TO  
www.dieseltuning.com, LOOK AT THE INFORMATION AND THEN EMAIL ME TO  
MAKE YOUR PURCHASE.  MORE POWER, MORE TORQUE, GUARANTEED!!!!

Visit my website at:  http://mysite.verizon.net/resosxp8/mydieselpages/
Proud owner of:
2004 Passat TDI GLS, 2.0l, 8 valve, 134 HP, 247 lb/ft of torque, Blue  
Graphite, Anthracite leather interior, ABS, ESP.
1998 Jetta TDI (Wetterauer Chipset, 2 1/2" exhaust, K&N Filter, 119K+  
miles, and running biodiesel)
1981 Rabbit Pickup ( with a freshly rebuilt turbo engine, 10K+ on  
engine now), burns biodiesel and a WVO system is in the works.
One, yet to be finished, Caddy trailer (the back end of the truck  
that gave its life to my Rabbit Pickup).
And many, many, many VWs; from a 1946 Beetle (11 Beetles total), to  
Vans (5 - 3 splitties, 2 bay window), Rabbits (3), Karmen Ghia (1),  
Jettas (5), Passat (1), Dasher (1), New Beetle (1), and Rabbit  
Pickups (3) most now gone but not forgotten.