problems with EFI car idle and pressure sensors

[Bruce Bowling]

Hello David,

The setup you describe should work fine at idle, and I agree with others that it may just be reproducibility with the throttle position sensor setup. Are you using a linear pot for the
TPS sensor - I know that many alpha-N systems out there use non-linear potentiometers, or drive the pot with a constant current setup, all to combat the "sine" effect of the airflow vs.
throttle angle.

My first try may be to use a "fixed" value for the idle pulsewidth whenever the ADC value of the pot is below a certain "threshold" , i.e. if the TPS reading is less than 0.6 volts then
use a fixed number (also check RPM). This is aken to the TPS switches that some early systems had indicating closed and open throttle position. For a non-modified engine, the MAP should
be pretty much constant at idle without load. This is only a temporary fix (it will not work when you put a load on the engine), but you should be able to get a decent idle out of it,
verifying to you that there is not something else wrong (ignition, etc).

Another thing - what is your injector pulsewidth at idle? If it is real small, like less than 2 milliseconds, then you may be in the injector "open" time region where things are not that
linear. Also, is the fuel rail pressure stable - this will screw you up. Hook up a pressure gauge and a manifold vacuum gauge and determine if the fuel rail pressure is correct.

- Bruce

> > > >
> > > > From: David Sackett
> > > > Subject: problems with EFI car idle and pressure
> > sensors
> > > > hi, i'm developing a simple EFI computer based
> > on a Atmel AVR
> > > > microcontroller running at 8Mhz for a Uni
> > project. I have installed it
> > on
> > > my
> > > > old 1977 Saab 99 and merged it with a Bosch
> > D-Jetronics fuel injection
> > > > system. the car originally had carbies and i
> > have only used the inlet
> > > > manifold, injectors and fuel system from the
> > bosch system.
> > > >
> > > > just to get started I decided to only use
> > throttle position and rpm as
> > > > inputs to the computer. a lookup table is used
> > to determine the correct
> > > > injector open time from these two inputs. we
> > have successfully performed
> > > > test runs in the car and it accellerates fairly
> > well.
> > > >
> > > > the problem we are having is that we cannot get
> > the car to idle
> > smoothly.
> > > > the car refuses to run on all cylinders until it
> > is above about 2000rpm.
> > > the
> > > > car runs VERY rich and the plugs have to be
> > cleaned every 1/2 hour or so
> > > of
> > > > running. if we reduce the mixture at idle it
> > just stalls. also the
> > system
> > > > fires all injectors at once every revolution of
> > the crankshaft. i have
> > > > replaced much of the ignition system and am
> > fairly sure it is working
> > > > correctly.
> > > >
> > > > does anyone have any ideas as to how to fix the
> > idle?
> > > >
> > > > also i've read about the Megasquirt system that
> > uses an MPX4100 MAP
> > > sensor,
> > > > and as a general improvement to my system i wish
> > to add a similar
> > circuit
> > > > board mounted sensor. I live in Melbourne
> > Australia and cannot find any
> > > > distributors who will sell me less than 20 with
> > a lead time of 12 weeks.
> > > > does anyone know where i may be able to get just
> > one sensor or if there
> > > are
> > > > any other suitable alternatives?
> > > >
> > > > thanks
> > > >
> > > > David
> >

- Bruce

---------------------------------------------
             Bruce A. Bowling
         bbowling at earthlink.net
   http://home.earthlink.net/~bbowling
---------------------------------------------


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