About 6k miles ago I blew the front coolant crossover pipe. Pulled over within a minute and had the car towed.

I pulled the supercharger and replaced the front crossover pipes with the updated design, replaced the water pump and every hose and gasket I touched. Everything was torqued to spec as I subscribed to the service manual.

Car runs fine, no check engine light or codes. Has lots of power.

I've been chasing what I believe to be a very small coolant leak, initially after the replacement I refilled everything according to spec and had to top off a few times once all the air worked its way out. I did not vacuum fill. It seems that the coolant level settled and was stable for about 1k miles, checking on the same flat service ice cold overnight.

Now it seems to be dropping from MAX after 2k miles about 4-5mm.

I've looked the car over for any signs of a leak TOP and BOTTOM and can't seem to find anything. The valley behind the water pump is bone dry. I can see the plastic waterpump rear fitting, dry. Rear crossover looks dry no marks on the transmission housing. Heater core connectors dry, thermostat dry, both radiators and drain plug looks dry.

I do seem to smell a slight coolant smell near the fuse box on the passenger side where the bleed off plug is (replaced this too) but everything is dry top and bottom. Heater in the car blows hot no issues there.

Going to be taking it in to have a pro look it over and pressure test it but anyone have any ideas?

I pulled the cap off the expansion tank and dripped that water so ignore it.

 

With my 2012 5.0L, I topped up the coolant to line up with the top of the "Cold Fill Range " letters. The level has not moved in 3700kms( about 2600miles). I always check the level in exactly the same place/position as when I topped it up. I have noticed that both the water and oil level measurements are very sensitive to whether the car is positioned at the same degree of levelness. It is possible that you are slightly overfilling the coolant.

 

So after doing a few pulls, parked the car let it cool overnight and it looks like the coolant level rose back up to where it originally was set 2000 miles ago if not maybe a tick lower.

 

Have you checked to see if there are exhaust gasses in the coolant?

 

Brought a combustion gas tester. Sucked some coolant out, hooked up my scan tool and monitored the coolant temp with the cap off until it reached 95 C and pumped the tool for a couple of minutes until the coolant temp reached 100c (212 F) then shut the car off.

Didn’t change color.

What temp is the thermostat fully open? 90c?

 

According to rock auto a non-oem thermostat says the oem temp is 203 degrees F or 95 C.

I also tested the fluid after monitoring it for 20 minutes on the exhaust directly and it turned color instantly.

 

I'm having your same situation in my xf-r.

On another car i had it was a small pinhole in the radiator, fixed with a radiator sealant additive, did the job for years, never came back until i sold the car.
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It evaportes instantly as it touches the radiator, took me months to find.

i have checked absolutely everything in my xf-r, so I'll be trying the additive.
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I have also read that these symptoms are indication of early stage of water dying.

 

According to the Service Manual for my 5.0L NA engine, the thermostat begins opening at 88-90C, or 190-194F and should be fully open at 102C, or 216F. Note that there is a by-pass arrangement so that some coolant is always flowing on the 5.0L. I have no information about the 3.0L supercharged cooling system.
I have a cheap permanent reader to monitor eng oil temp, eng coolant temp and alternator voltage.
Typically I have seen between 194F-202F for coolant temp and 225F-245F for eng oil temp during the hot weather. Generally the higher temperatures appear at slower speeds and the lower temperatures at speeds up 150kph

This shows what I monitor.

 

Thanks for the info, I just wanted to confirm I did the test correctly (thermostat open). Once the coolant hit 98-100c I noticed the level in the tank started to rise (went from cold level low mark to full) so I assumed coolant was circulating and pumped the tool rapidly. I’m pretty sure the SC engines have different temps but they should be similar.

Found this searching the forum,
"2.7l TD: "The thermostat opens at a nominal temperature of 82 °C (180 °F). This opening is delayed at lower ambient temperatures to improve heater performance."

3.0l V6: "The thermostatbegins to open at 88 - 92 °C (190 - 198 °F) and is fully open at 102 °C (216 °F)."

4.2l v8: "On vehicles without supercharger, the thermostatbegins to open at 88 - 92 °C (190 - 198 °F) and is fully open at 102 °C (216 °F). On vehicles with supercharger, the thermostatbeginsto open at 84 °C (183 °F) and is fully open at 98 °C (208 °F)"

I am assuming the 3.0 V6 listed is the non-SC version and its probably similar to the SC 4.2.

 

Did you ever get this sorted, i have the exact same issue.

I did a compression test and a head gasket leak test, both turned out well.