[Diy_efi] Momomotronic lambda terror

[Jens Knickmeyer]

Hi all,

i drive a '92 Volkswagen Polo. This car is equipped with a Bosch
Monomotronic, it is not OBD2 compliant. Since three weeks, I have trouble
with that car and can't find a solution. This is why I ask here for some
assistance.

Symptoms:
I use a lambda display (the LED knight rider solution). The engine is
running much to rich. When I drive with constant speed and then switch to
idle (e.g. when stopping in front of a traffic light), the engine
immediately runs rich (0.9 V) and stays there for at least 20 seconds,
sometimes even for one minute. This also happens when I use fuel cut-off
before running idle. I checked that to make sure that the EGO does not
measure some gases left in the exhaust pipe.

When driving with constant speed and revs, the lamda readout is quite
strange in my opinion. Former drivings with lamda display connected showed a
very well controlling engine, i.e. the light moved to rich, stayed there
(exactly one LED on), then moved to lean, stayed there (exactly one LED on),
and so on. Now, the changes between lean and rich are performed much faster,
and furthermore, the LEDs flicker like crazy. Even being at the rich or lean
point where the engine stays for approx. 1 second, 2 or 3 LEDs on the
lean/rich end of the scale are flickering.
This flicker happens even when performing the movement from rich to lean or
vice versa. It deteriorates when I accellerate (preventing the engine from
WOT enrichment, of course). I already saw the LEDs flicker so much that the
visual effect was as if 8 LEDs were on.
On the other hand, both fuel cut-off and WOT enrichment work correctly.

This drives me crazy, esp. because in few cases, the engine tends to stall
when running idle.

I've already
- cleaned throttle plate potentiometer
- changed coolant temperature sensor
- checked air temperature sensor
- changed EGO sensor
- changed ECU
- cleared ECU's adaption values
- read DTC via Volkswagen protocol (no DTC were found)
- performed actor tests via Volkswagen protocol (everything worked fine)
- checked injector

So far, so bad, because now I have no further ideas, and that is why I ask
on this list for any suggestions what to do. Has anyone had such a bl**dy
behaviour of a Monomotronic euqipped car?

Thanks for any help,

Jens Knickmeyer

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