I seem to have a water leak on top of the rear rotor housing right where the knock sensor is located. I read on the forum that the knock sensor epoxy can melt, but the leak looks like it's coolant/water. Anyone have an idea on what this could be? I need to get a better pic, but the source seems to come from right above the trailing plug.

 

The knock sensor is normally located just above the trailing plug of the front rotor in a threaded blind hole. When the sensor leaks, a black or dark brown goo can be seen running down onto the engine and sparkplug wire cap. I'm not sure what your picture shows, but it isn't a leak from the knock sensor. If it is stain from a coolant leak, the nearest source would be the coolant return line at the bottom of the throttle body.


Knock sensor is above the front top plug.



Here's mine, beginning to leak black goo.

Coolant diagram:

 

I think I found it ... it's the hose in red? Can't see if it's leaking, but most likely from here.


Knock sensor is definitely melting (orange). I also found another coolant line (red) on the rear iron near the oil pressure sender. My heater core is looped/disabled and the rebuilder connected this to the heater core line that runs the firewall. Can this be plugged as well?

 

I suppose it's possible that the cabin heater hose fitting near the oil pressure sender was the likely source of your leak, but it's a few inches below and to the rear of the rear housing's trailing plug where your leak stains begin. On the other hand, the coolant hose on the bottom of the throttle body is more or less directly above that sparkplug.

Any chance that those stains are from a fuel leak rather than coolant?

 

I would hope not a fuel leak as the engine was recently rebuilt, but anything is possible. The hard line from the TB is a little crusty though. I'll have to remove the hose and see if there are any cracks.

 

"the coolant hose on the bottom of the throttle body is more or less directly above that sparkplug."

most likely. also, your the coolant residue trace indicates lots of minerals. time for new coolant.

 

Check the coolant line going from the passenger rear side of the block going to the TB as well. It connects to the back of the TB on the drivers side near the firewall, at the top of the engine. It's popular to pop that open while bleeding coolant, and when it spills out it pools on the rear housing behind the "13B" stamped in metal and oozes down the side of the engine, I've seen it cause similar stains to this.
That hose is N3A1-13-691B.

The hose you snapped a picture of is N3A1-13-691A and is pretty inexpensive if it is leaking but can be a bit of a pain to replace with the UIM on.

 

During this teardown I'll be switching over to single turbo. If I block both ports on the rear iron, that should be sufficient since I'll use the ports on the water neck housing for the turbo? Essentially I can remove the hardline on the firewall and the one under the UIM.