Among the different Scooby sensors, the MAF and the rear O2 sensors are the more prone to ground or signal perturbations because a small amount of voltage creates a big difference in results. The O2 sensor, used during idle and closed-loop phases is definitively the more sensible as its transfer function around the stochļo point looks like a filtered step. Hundreds of millivolts are enough to change the reading from 12:1 to 16:1 !
I've measured hundredths of ohms between the rear O2 sensor's ground and ECU's ground, enough to create hundreds millivolts noise when ignition coils are fired !
The mod simply consists to connect the rear O2 sensor's ground wire to the ECU ground using normal small electronic wire. For example, on a 01-04WRX/STi, you just have to connect B134 pin 22 to B135 pin 19. Results are even better on a MY95-00 scooby.

 

Great info! Can you be any more specific about the classic cars? I have an MY99 and to the best of my knowledge, it only has one O2 sensor, on the downpipe. I have carried out an earth improvement mod already, but didn't pay any particular attention to the O2 sensor. Since making the mod, I have had a 4 wire O2 sensor fitted in lieu of the 3 wire OEM unit (purely due to availability at short notice). The earth wire is currently attached to the exhaust system, but I plan to connect it either directly to the battery earth, or to the nearest of my additional earth points.

Thoughts??!

Cheers,

Andy

 

Sounds very interesting. So how do you do it, in simple terms, on an early classic...please.

 

Sorry, I've been trying to find this ground O2 sensor signal on MY95-00 scoob, but it's not fitted to the ECU as opposed to 00-04WRX/STi. It seems that it is connected straight from the O2 sensor to the engine's body (good reason for the problem). I haven't been able to locate it.
Then the only mod consists in cabling that engine point to the battery ground, a big wire is not needed.

 

I thought it was the front O2 sensor on the Newage cars that was "used during idle and closed-loop phases" and the rear O2 just monitored the CAT(s)
efficiency.
Am I mistaken?

Mark.

 

You're right Mark. I made a mistake...but that's good as catless CE disapeared definitively :-)