I've read through lots of threads already here about similar issues and symptoms to what I'm experiencing, but I just haven't been able to solve mine yet and I'm about out of ideas. I've even gone through the great thread about overheating here in the tech section.
Hoping someone might be able to help me out.

1998 Discovery 4.0 V8, 185k miles. New to me, unknown history.
Seems to run around town at about 215. I just noticed that on a long uphill drive that the temp started to climb past halfway on the gauge , my Torque app on my phone said it was hitting 230 and climbing so I pulled over and let it cool. I popped the hood and the coolant was boiling in the expansion tank and venting out the cap.

So far I have replaced a few things and there has been no change.

New, out of the box items installed this weekend-
Radiator
Water Pump
Viscous fan clutch
Thermostat, 160 degree
New serpentine belt.

So tonight I started it from cold and let it idle about 30 minutes in the driveway. Temp finally got to about 192, and I took off for a drive. On the freeway for a few minutes and the temp seemed to stabilize about 185. Then when coming off the freeway the temp got to about 206. By the time I got back to the house and let it idle again it was about 215 and climbing steadily.

I believe I have the air bled out a much a possible. No white smoke from the exhaust. The upper hose is very hot to the touch but not extremely hard with pressure. I also verified that the belt is routed properly, and my a/c fans are working fine. They even kick on without the a/c when it's too hot.
So I've ordered a block test kit to make sure there is no exhaust in my coolant. But other than the possibility of a HG problem, I'm pretty much out of ideas. I really expected my new radiator to be the silver bullet and it wasn't.
I appreciate any more ideas from the pros out there, thanks!

 

Well I guess this is good news. I'm actually really surprised. And so the search continues..

 

I wouldn't rule out the thermostat, first new one I put in mine was bad. With the parts you replaced you should be running mid 180's for temp although the 160 tstat is way too cold. Congrats on the block check, I wasn't quite so lucky.

 

Yeah thanks, I've got a new one on the way, a 180 this time. I have to add one more thing that is confusing to me. If I start from cold and take off the expansion tank cap and wait for it to reach operating, all seems fine until it starts to hit about 200 degrees. Then the expansion tank will start to overflow coolant heavily, like a toilet overflowing. Then once I shut it off, the coolant of course stops overflowing and disappears back down into the rad and leaves the expansion tank empty.
I know the system wants to build pressure, but it doesn't seem normal to me. I mean if there is no pressure in the system yet, what is causing the tank to overflow?
Almost seems like something is blocking the flow maybe or a huge air pocket, even though it's has been bled and burped about 20 times.
Thanks all.

 

Personally I'd get a leak down tester and give all the cylinders a once over. Yours is acting exactly like mine was. The fluid tester I used had a light blue color and it was just barely noticeable that it was changing color (went more clear than yellow) but as soon as I put air into #6 it was obvious that it was causing the pressurization.

 

Is the belt routed correctly?

 

Check your radiator cap, ,it should not be boiling at 215 degrees. The pressure should force the boiling point to be higher. Also, make sure there is a bleed hole in the thermostat, that makes a huge difference in preventing air pockets.

 

If you are not running 50/50 mix it will overheat I had this issue, be sure the right front tire is up on a ramp or curb with cap off to bleed

Did you flush the engine, drain all coolant and hook garden hose to heater core lines flush both ways till clear on engine and heater core, can be sludge, have you pressure tested to be sure there is not an external coolant leak

 

I wanted to update those following this thread, it looks like I have solved my problem(s).
I had two things going on. One was that my reservoir tank cap was not working, it was letting pressure through at very low pressures. New cap now and no more bubbling or coolant being pushed out of the tank.
The other issue was my brand new fan clutch was just not pulling enough air through my new radiator. So I replaced my brand new fan clutch with a BRAND-brand new fan clutch that was rated as 'severe duty', and now my temps sit right where they should be.
Of course the severe duty fan clutch is locked up a majority of the time so there is more fan noise, but that is a small price to pay.

Here is the one I bought. It fits perfect and sits a bit closer to the rad. The only mod was to open the 8mm holes on the fan to closer to 10mm to bolt to the clutch.
CARQUEST or Imperial Fan Clutch - Severe Duty 215158: Advance Auto Parts

 

Two things besides the almost constant noise, more stress on the pump bearings and lower fuel mileage. You might want to look into electric fan, bigger area, less strain and less fuel use. There are good threads for which one and install, Tom's comes to mind.