I am putting an O2 sensor in a 1997 Mercruiser 7.4 liter MPI equipped with corsa captains call exhaust, Could someone recommend a location for this sensor.

Port Side just behind the captains call...just remember take readings when the captains call is closed

Are you saying to put the sensor on the downstream side of the butterfly valve in the captains call exhaust, if so then how do I get a reading if the butterfly is in the closed position.

Thank you

Brian

No good if its a wet exhaust.

Yes it is a wet exhaust, however I have done some mods to the engine and I need to change the parameters in the ecm to reflect these mods so I have to obtain a fuel air ratio readout using the o2 sensor just for the period of time to make the parameter changes.

Won't work unless you read directly in the exhaust port w/ no water.

OK that is fine but where can I install the sensor in the exhaust with no water present?

You have to mount the sensor after the captains call to the tail that is going out thru the hull not through the captains call. The bung has to be welded inside and out to keep the water off of the o2 sensor.

Put the bung in the riser just after the manifold. You have to keep the sensor as far as way from the water as possible. Not only will the sensor give you a false lean reading by picking up the oxygen in the water, but it will kill the sensor. The O2 sensors are highly allergic to water.
It needs to be just after all of the pipes converge. You do not have to weld it inside in out. Drill a hole in the pipe. Then take the rounded end of a ball pein hammer and put it on the pipe in the hole. Hit the hammer with another one and it will cause the hole to dimple in just a little. This will help close up the gap between the two pipes. Then you can weld up the gap between the pipes and seal off the water. Once you have the water sealed up, then set the bung on top of the pipe and weld it in place. You do not have to weld the inside because you have sealed up the water already. It's hard to explain typing, so if you do not understand or if I didn't explain it good enough, give me a call and I will walk you through it.
Eddie

You would have to put it where there is no water. In your cast manifolds there is no place for a O2 sensor( at least not that I know). To weld a bung in would be hard. You would have to go thru the water jacket then into the ehaust stream. To get a accurate reading of just one side of the engine. It would have to be where the runners combine before where the water mixes in with the exhaust.