Working on replacing my clutch and tracking down a oil leak up high on the rear of the block, started taking off the "Cylinder head Rear plate" on the back of the head and coolant started weeping out so I tightened it back down, what goes through here? I thought it was oil since I have a spare one and the passage is all gunked up. Does it also act as a coolant block off? Trying to get everything cleaned up before my Marlin HD clutch arrives later this week, much nicer sitting under the truck with the tranny out to work on the back of the engine vice contorting yourself over the top!

 

It definetly covers a coolant passage. I believe it is part of the EGR system, but I need to see it to confirm. The reason there is oil all over the place, is the back of the valve cover leaks right at the 1/2 moon, which is right above that plate.

 

I believe the plate is an EGR cooler. It is gunked up from exhaust. I agree with DeathCougar about the half moon. Put some black RTV around it, and then put two small drops of black RTV on the corners of it before you put the valve cover back on.

If you loosened the EGR cooler enough to leak coolant, the seal is probably broken. Tightening the bolts is enough to stop the leak cold, but it might start leaking again when the engine is warm and the coolant is under pressure. Changing the gasket is probably a good idea while you can get to it easily.