So I just replaced the majority of my cooling system. Radiator, water pump, upper T hose, thermostat, expansion tank cap, the hose from the engine block to the expansion tank, and serpentine belt. I'm not having any more leaks, but I am having an issue with the rover peeing from the expansion tank after I turn the engine off from a drive. I'm hoping it's just as simple as a new expansion tank, but what else could be the underlying cause? Why am I getting that much pressure in the first place?

Here's a video of it doing the stuff.

Also, background history: I had the head gaskets replaced a couple years ago by a L.R. specialist shop and it hasn't overheated since. I did these changes due to water pump bearings out and leaks. So I don't think that should be the issue.

 

First your dash temp gauge being in middle doesn't mean anything, it will stay there from 170* to 230*f, so you need to read the temp with an OBDII scanner to know real temp,
It could be as simple as bad coolant cap/reservoir , or engine running too hot due to a bad T-stat,,
Reservoir cap will hold up to 15 to 20 psi (depending what pressure it has written on it) and more than that it will let coolant out from overflow hose,,
It wasn't leaking b4 , because it was leaking from water pump as you said yourself, so wasn't building enough pressure,,
Check the temp with an OBDII scanner first & go from there,,

 

Just replace the cap and go from there. Cap loose not holding enough pressure.

 

Try servicing coolant to proper level as it looks low and likely causing overheat.
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I had the exact same problem, little coolant leaking out of the overflow after running, not a lot just a little bit. New coolant cap from the dealer, and it never did it again, that was 3000 miles ago

 

Here is what happen to me in a similar situation...

My heater core ended up getting clogged when my pump when out. Because the flow of the coolant was now blocked, it was backing up into the expansion tank and causing it to pee.

Hearing your story...I am thinking that if your pump went out, there might be bits in the heater core.

A good way to test this is to feel or get a thermal scanner and check the inlet and outlet temps on the heater core. The outlet will be the closest to your coil packs. If it is cold or cooler than the inlet, you have a heater core issue (you may also hear the waterfall in the cabin). If not, your T stat might be causing pressure to build as well.

 

The truck was at operating temp....you can't make that call until engine is complete cold and coolant is back in the bottle.

So I recommend, let engine get COMPLETELY cold. Check reservoir, fill if necessary, and replace cap while filling etc. Make sure you raise coolant reservoir and bleed system as filling to ensure no air in system.

 

Ok, so I'm going to grab a new cap and see if the new one I just bought was somehow defective. As far as the T-Stat theory, I did a boiling water test on the stove with a couple different thermometers and it opened at the correct temperature (around 195f).

In response to Friday Night Disco, I used to always get the waterfall noise, but haven't since redoing all of these components. Per the workshop manual, when refilling the system, I open the bleed screw all the way, then fill the reservoir and raise it 8" above the engine to force it into the system until the bleed screw is flowing, then tighten that down, fill the reservoir again and run it for about 10 minutes with the heater on full, let cool then fill to cold level. No waterfall noises.

So yeah, at this point, I think I agree with the cap idea, but wonder if something in the chamber where the overflow line goes into the bottle is clogged, so instead of going down into the bottle, it goes out through the cap and overflow tube instead.

When I took the system apart, I noticed there was a bunch of glitter in some of the smaller lines, so I'm guessing someone did a leak stopper on it a long time ago, and it never got properly flushed until now.

 

Have you ever seen coolant level increase once it's cooled? If level was as low as it appeared in video the call can be made.
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Understood on the bleeding with my suggestion, however, humor me and check the temp on the heater core lines with your hand. That is a cost free check that can eliminate any problem with the heater core. Also the heater core is awesome at trapping little bits of debris