I might try that trick hopefully next week when I should receive the coolant heater (with pump) and install it in one of those two hoses.
Can't remember right now which one.

Went for a drive, coolant got to a bit under 80 C, ~180 F and moved around that.
I have the 180F thermostat.

Still not very hot blowing, but on this nice -27 C, -17 F, day, I could feel it is warm what it blows.
But it was a cold drive.

Is there something other than the temp sensor next to the mirror adjuster next to the steering wheel that could be messed up?

 

I think if you set the HVac temp to HI instead of 28, it bypasses the temp sensor (to help troubleshoot if it’s your sensor or not).

 

You also need to check the heater matrics to make sure its not blocked up with debris, is the air flow strong throught it, and do all the leavers work

 

I just helped a friend with his 04 like mine 4.6l did the water pump and new tank and cap and aluminum t for the top hose. Did rhe normal bleed procedure for 30 minutes and his wasn't for some reason not filling the heater core so I removed the top and bottom core hoses and flushed the core out. I pour distilled water taped my air line and blew it out so no clogs. Then attached inlet hose and backfilled the exit hose with 50/50 coolant. Ran the procedure again 2 time til heat and no more waterfall effects. Ran obd to check thermostat operation Temps took for ride and got flow once exempt was hit it dropped to 186 then gradually ran with good heat and runs at 199 to 201 pretty consistently no back flow out the tank anymore either. Hope this helps if not resolved. This thread has plenty of solid feeback.

 

One item to note, factory water pump with stock idle speed does not move enough water to circulate water through the heater matrix with factory thermostat. You can confirm this by removing heater hose going back to thermostat, nothing will come out. A little better with inline thermostat as it forces the flow through the heater matrix if the thermostat is not open (no option for flow to go through the bypass line).

 

Got the coolant heater (with a pump) installed and tested.
Installed so it will pull coolant from the heater matrix.

Coolant level in the expansion tank is just on the full cold line, ie on the seam of the tank.
Had the heater running for a couple of hours, coolant at +36C (~96F), outside around -7C (19F).
The level of the coolant in the expansion had risen a bit over the seam line.
I opened the cap and there was pressure in.

Removed just a tiny bit of coolant so the level was back on the line.

Had it idling for some time, so the coolant heated to ~70C (158F) and went for a drive.
Now had the cap turned one rotation open.
I've usually tightened it, and turned maybe half a rotation open so it's loose, if I turned it full rotation it's tight again.
Dunno if that mean anuthiung but something I've noticed.
Ice on the windscreen melted as I had the blow to the windscreen while idling. So it does give heat now, maybe better, but it's warmer outside too so...

Went for a warmup drive, some calm driving, some kickdown sport mode full accelerations when it was properly warmed up etc.
I could actually warm my fingers on the vents with the air blowing.

Waterfalls are still there, but just.. You need to know to listen to it now to hear it, it's faint.
So it's getting better now.

Coolant level was a bit lower that the small input hose in the upper bottom section.
There was some spillage on the expansion tank and the fusebox cover, not much, just some drops, and they weren't there when I left.
I also wiped clean the tank exterior before driving and there was some coolant on the tank.

I'll see some more tomorrow for a bit longer calm drive to town and back..

What's the expansion tank coolant level supposed to be, when the engine is warm?

 

They will spit out a bit while running if it is much over 25% full. That's not what the line says but that's what they do.

 

Well it took a while for to send me the coolant system pressure tester kit...

Put the tester on the expansion tank and pumped 1 bar in there.
Aaaaand it started to creep down..

Found four hoseclamps loose that started leaking. Three of them dripping and one that just flowed nicely.
Tightened them up and new pressure in.

Checked after ~2 hours and the pressure was down to ~0.75bar.
Re-applied pressure if it's just flexing hoses.

Checked again after some 2 or 3 hours and it was around 0.75 bar again.
Did not see any puddles underneath.
New pressure and left it overnight.

Aaaaand in the morning (4am today) the gauge said almost to zero pressure.
Took the tester off the expansion tank and there was a little of pressure relief.

Will need to investigate more. Maybe fill coolant, try to bleed it, get her warmed up etc.

I have her SORNd now as there is too much fordium in the rear chassis, not going to pass MOT like that.

 

So you suspect a leak, you could try and put some red food dye in the coolant to help you locate any leak points or a florescent dye if you have a black light

 

Be careful with pressure test, pull all the spark plugs before you turn her over. We have had forum members with hydrolocked engines bend connecting rods.