RE: TPS

["Eric Renneisen" <racingiron@xxxxxxxxxxx>]

I had an air-fuel gauge hooked up to my O2 a while back to try to glean
some info about the O2 signal when it surged.  As the LEDs cycled back
and forth around stoich, when the bike stumbled, you could see the meter
hesitate very briefly on the lean side.

If what you say about the bike dropping into open loop mode is true, it
must be switching back and forth between open and closed loop very fast,
because the meter continues it's dance around stoich during the surge,
with just the occasional stumble on the lean side.  I'd also think that
switching to open loop repeatedly would throw a fault code, in which
case, you'd have a strong reason to complain to the dealer and BMW
corporate that something needs to be done.  I've not heard of anyone
mention ECU fault codes due to the surge.

A resistor on the TPS seems to me like it would accomplish nothing more
than tweaking the TPS physically.  I had always wondered if a resistor
on the air temp sensor in the airbox would make a difference.  Since
these bikes don't have MAF sensors, I presume they rely heavily on the
air temp sensor to determine the density of the air.  If we alter the
signal to make the bike think the air's a bit colder outside than it
really is, it might richen up the mixture.  Then again, the air temp
might just be used for open loop running, because the O2 is obviously
paramount in closed loop.

Eric Renneisen
'94 RS
'88 RT
'71 R75
Chattanooga, TN

- -----Original Message-----
From: owner-oilheads@xxxxxxxxx [mailto:owner-oilheads@xxxxxxxxx] On
Behalf Of Tom Brown
Sent: Saturday, January 29, 2005 12:45 PM
To: BMW Oilhead List
Subject: TPS


Ben, Ben, Ben....

You can't look at this FI system so simply.    The reason the surge
happens is
because unburned fuel in the exhaust causes the 02 sensor to deliver a
high
value to the FI computer.   When it gets this high value it runs a
default
lean fuel curve essentially ignoring the value of the TPS.  The flipping
from
the rich curve to the default lean curve makes the bike surge.   If you
ride
it purposely in that "surge zone" you can almost sense this happening.
It's not just a matter of the bike running lean and needing more fuel
poured in.

The trick is to get the TPS set to a value just lean enough to avoid
tripping
the O2 sensor.   That's essentially what the 0=0 procedure does for you
if
it's done properly.

The reason the 83i Powerbox works is because you unplug the 02 sensor,
then place a jumper in the Cat Code Plug box that makes the bike run
this default
lean curve all the time.   You then manually add the amount of fuel
necessary
for good running by adjusting the box which delays shutoff of the
injection nozzles by minute amounts.

If you want to put a resistor somewhere, a better place would be in the
wire
between the 02 sensor and the computer.   I'm not sure about the voltage
value
of the 02 sensor or what value resistor would be effective here, but
it's the 02 sensor that causes all the trouble.  There is variability
between 02 sensors and sometimes the difference between a "good running"
oilhead and a "bad running oilhead is just the electrical value being
put out by the sensor.

BMW solved this with the extra spark plug.  It burns all the fuel in the
cylinders at these low speeds. The San Jose twin spark mod does the
same.  The high 02 sensor value isn't reached anymore.  Result is no
surge and better performance.

- -TB




Subject: Re: Add a resistor to TPS circuit?

On Fri, 28 Jan 2005 11:33:21 -0500, you wrote:

>I'm a newbie, but...
>
>The surging seems to be associated with small throttle openings (but 
>not idle... I'm not sure what "idle surging" would feel like) and 
>closed-loop operation with the O2 sensor contributing its 2 cents 
>worth.
>
>One fix is to twist the TPS, thereby fooling the ECU into thinking the 
>butterfly is more open than it really is (and thus fooling the ECU into

>spritzing more gas into the mix). But that's a pretty blunt instrument 
>and enrichens the mix all through the range.
>
>Folks at ease the basic math and a soldering iron, might suggest that 
>adding a resistor in the circuit with the TPS potentiometer could 
>influence the small-opening voltage but have rather less effect on the 
>larger openings.
>
>Anybody tried that? Anybody know the resistance value of the TPS pot?
>
>Cheers.
>
>Ben

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