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Discussion Starter #1
I have been doing some research on the 'fuel quality plug' that tails off the Motronic ECU under the dash. Based on how it is wired, the ECU may run in open loop or closed loop mode depending on specified fuel octane. Some info / photos here: Craig's Place, Fuel Quality Plug (S30)

Assuming i run the plug with no pins connected, does this mean the ECU does not rely on any input from the lambda/ 02 sensor?

In other words, should i not bother welding an 02 sensor bung into the exhaust system if running it in open loop mode?

Cheers!
 

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From my experience, yes. No "plugs" is open loop.

My memory is that 02 sensor signal doesn't do a lot for much of the time. The maps are that good.

There is a super-long thread on this forum where the 4.1 Motronic ECU is "electrically dissected". A good read.
 

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A closed-loop modern ECU system relies heavily on the 02 signal to keep the emissions in check, the catalytic converter healthy and to monitor air/fuel ratio's.

In contrast the 4.1 motronic uses the 02 sensor to slightly steer the mixture for emissions. But I'd hardly call it open loop.

With no pins shorted, the system also runs on a (EU) 98 octane map, without catalytic converter. However 1990's RON 98 is similar to current RON 95 so it works just fine on normal fuel.

I simply put on an exhaust for a catless 75 twinspark, which has no provisions for an 02 sensor.
Using a catalytic converter without an 02 sensor is a bad idea by the way, they can only last a good while with a working 02 sensor
 

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Discussion Starter #4
Thanks for the confirmations guys. It's going into a 105 so i won't be running a cat of course. I'll leave to 02 sensor unplugged, i always run 98 anyway.

Cheers
 
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