It looks like the O2 sensor port, but isn't...1987 GL-10 Turbo

[charlieb71]

Help please.  There's a "hole" on the exhaust manifold near the head that looks like it should be for a sensor.  I noticed it on the freeway when it must have blown out whatever was mounted in the hole and the car sounded like it had an exhaust leak.  The O2 sensor is is downstream from there and attached.  What goes in this hole?  I apologize if my terms were incorrect or unclear.  Any help is appreciated.

[naru]

I think you have an EA-81T up pipe on your car.

They mount the O2 sensor there.

Plug the hole w/a dead sensor or WHY.

[john in KY]

Having owned several turbos, been more than a few years ago, I recall that hole is where the O2 sensor goes. Turbo models only used one sensor so can't explain the second one downstream. 

[charlieb71]

The previous owner did do some funny stuff.  I think you all are right.  I'll just plug it up.

Thanks!

[SiriusBlack]

That hole could have been installed by a previous owner for an air to fuel ratio gauge.

Edited April 7, 2020 by SiriusBlack

[carfreak85]

2 hours ago, SiriusBlack said:

That hole could have been installed by a previous owner for an air to fuel ratio gauge.

The EA81T had the o2 sensor in the uppipe, the EA82T has the sensor in the downpipe.  Besides those two differences, both the exhaust manifold and the downpipe can be swapped between generations, as long as you are aware of the sensor placement.  That is likely what happened here.