The MSnS-Extra Hardware Manual contains the following statement in the Water Injection section:

"Please note: Whilst the water injection pump is on the O2 sensor is ignored (open loop mode), as the water would upset the readings."

How needed is this? It seems most others that do WI outside the squirt don't have issues. Kind of a bummer to abandon the wideband for open loop just when you need it most (in boost).

 

You definitely do not use the wideband in closed loop mode under boost. Ever.

 

We don't run closed loop under boost on our cars. Our view is that it would be kind of a bummer to have an O2 sensor failure cause your fueling to go awry when you need it most.

Water is going to upset the readings on any conventional wideband. The reason is that water is a product of combustion, so dumping water into the intake is very nearly equivalent to uncorking a bottle of exhaust gas from a different engine and injecting it into your exhaust. Unless the wideband controller can talk to the water injection controller and know to anticipate the extra water in the exhaust, its reading is going to be off.

Anybody whose ECU is designed to run closed loop while water injection is active has apparently decided, "Screw it, we'll just ignore that our wideband reading isn't as accurate with the water in the exhaust."

 

Thanks guys. That answers that!!