I have a b18a1 and there's this open port with threads on it. My b18b1 doesn't have it so idk what it is, when I put water in the long block, it comes out of it.
This is it
Has threads, don't think it's a freeze plug
Sits behind the block, under the water tube. Tried to search in Acura catalog but only shows the front of block.
Don't think it's for knock sensor.

 

Left is fan switch oil temp operated
Right is fan switch coolant operated

 

fan switch.

 

i dont think the b18a1 came with knock sensors...?

And I think it is the fan switch now that you mention it kyden. Because there is no fan switch on the thermo housing like there is on the b18b1

So I am gonna go with fan switch..?

 

Just googled it. It is the fan switch port.
But I also noticed the sensors are different from the a1 and b1.
I can still use the b1 sensor right?
The engine is going into a 95 Ls which had b18b1