Date:         Fri, 11 May 2001 10:59:38 -0400
Reply-To:     David Beierl <dbeierl@attglobal.net>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@attglobal.net>
Subject:      Re: Gas Milage and the Fuel Pressure Regulator
Comments: To: Mark Keller <kelphoto@islandnet.com>
Comments: cc: Darrell Boehler <midwesty@MIDWEST.NET>
In-Reply-To:  <3AFB5190.74D36AA6@islandnet.com>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 10:44 PM 5/10/2001, Mark Keller wrote:
>The information I received from Darrel Boehler was that that extra fuel
>enrichment is added by the ECU when the TEMP I sensor indicates near
>freezing air. The ECU's map may then wait until either an indication
>from TEMP II, i.e. engine beginning to warm, or just wait until close
>loop before the TEMP I enrichment value is canceled out.
>
>   Either way, Darrel "guestimated" the value to be 20% extra to the
>regular cold start mixture map. It seems Darrel stated that this is the
>only purpose for the TEMP I sensor.

I defer to Darrel's specific knowledge of the Digi* -- but that ain't how
it 'sposed to work in the L-Jetronic, or any other system using a vane
sensor/Air FLOW Meter rather than a hot-wire / Air MASS Sensor.  Quoting
the Bosch FI book: "Air Temperature sensor...Known as NTC I, this sensor
also has a negative temp coefficient.  Remember, the control unit [the ECU]
sends a signal [to the injectors] which is changed by the air temperature
sensor.  Cold air, weighing more [per unit volume], needs more fuel, so a
cold air signal also adds fuel to the basic-pulse time.  The air-fuel ratio
relates weight of air to weight of fuel.  The combination input signals of
volume (from the air flow sensor), corrected for temperature, are the same
as inputs of weight or mass of air passing into the engine [from an air
mass sensor]."

The change in air density is about seven per cent over a 40F span, so this
is a first-order effect we're talking about...

david


David Beierl - Providence, RI
  http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"