@guy Yes, I understand what you meant. I've always kept cold level at the top line after every coolant service/repair and it's never ever been an issue. I don't recall exactly where level would be when hot but the tank was always full, never sucked out or empty, when hot. I've been using an airlift for quite some time as bleed and fill would always take me hours of driving around local roads, bleed, fill, drive, bleed, fill, etc. The airlift removes all air, collapsing all hoses to -25psi and then vacuum fills with fresh coolant with no air.

Have you noticed if the level on your changes depending on whether you've set at max heat or max cold?

 

Well, it just got weirder

Went for another drive for about 15 minutes. ECT perfect, upper hose firmed up. And coolant level remained normal.

Returned home and parked so I could start further investigation of the water in the floor issue.

Checked upper hose and it was soft. Checked coolant tank and it was full nearly to the cap. Opened the bleeder and nothing happened, no air, no coolant and level in the tank remained nearly to the top. Tried turning the cap and it was stiff and would not click as it had just done before the drive. Remember this cap's only about 2 weeks old, its predecessor's about a year old. Both function as they should.

EDIT: New coolant pump fitted a couple of months ago, its predecessor started leaking from the shaft even though it was just a year old. It appears to be working correctly and has been as far as I can tell but what are the odds perhaps something's amiss with the impeller? Maybe the impeller's sliding on the shaft and not circulating coolant properly?

 

I think I'm going to have to give up at the moment. I removed the cap and the coolant level didn't change, still very high. Put the cap back on and it clicked this time like it's supposed to. Started the engine again and let it run. Upper hose firmed up and coolant level dropped to just above the top cold fill mark.

Went for a long drive in heavy traffic. ECT A stayed between 190-201F, ECT B stayed between 171-212F, much higher than normal. Ambient temp 35-36C. Returned home, upper hose hard and expansion tank nearly empty. Let it sit for 3 hours, no change.

 

That is weird. Its as if there is a one way valve interfering.
I've been over and over this... I can't put my finger on why its not flowing properly.
You can see from my reservoir pic that coolant is constantly fed to the reservoir through the bleed hose.
Or is it the thermostat is not working properly... you did say your hoses stay hard a long time. Mine don't.
Check valve not operating properly? Probably not as your engine temperatures are ok.

 

Also considering recent repairs, that's why I was thinking perhaps the new water pump intermittent (slipping) impeller might exhibit similar behaviour, coolant not flowing. But I've never dissected one and not certain the likelihood of this.

 

I can't tell if coolant comes from the overflow hose connected to the top of the radiator. There's coolant there but don't see it flowing in any significance as far as I can tell

Confident the t-stat's working because it takes a really long time for the ECT B temp to go over 100F.

Upper hose often remains hard, sometimes softens overnight but not always.

Check valve? You mean the one in the t-stat, a bypass I think?

Engine temps ok most of the time though as I've noted here, they're all over the place some days at 200 +/- 10 degrees, some days 220 +/- 10 degrees and occasionally on the first long drive of the day, temps have been going into the high 230s and once or twice low 240s only to come down straightaway. No cluster warnings but I do see the cluster gauge move up then down.

 

One more update to this mystery. Yesterday morning upper hose rock hard overnight, coolant level at the bottom of the tank. Drove about 30 miles for errands and temps remained at 190-200F. Upper hose stayed hard, low coolant warning came and went. This morning, upper hose still rock hard, coolant level still at the bottom. Went for a 60 mile drive, stopped for fuel, upper hose hard and coolant level still at the bottom. ECT A climbed up to 242F then came down to 213 then settled at 226 for the remainder of the drive (ECT B was under 100F and stayed in the 90s for a very long time) . On the return drive after exiting the interstate, I sat at a traffic light for along time (long cycle times and we have them about every quarter mile despite 55mph speed limits) and watched ECT A climb up to 244F (unusual because I'd just driven over and hour and typically long drives maintain consistent temps) and then drop to 217 and then back up to 226. After parking in the garage I checked the upper hose and it was firm but no hard and had some give and the coolant level was just above the max fill line. I watched the level for a few minutes and saw it rise slowly to about an inch above the max fill line and then stop rising. Checked an hour later and found level had dropped to just above max fill and upper hose soft.

 

This afternoon went for another 30 minute drive, everything okay. Stopped for an hour and then headed back. Ambient temp 44C, watched ECTs and noted ECT B was climbing to 170 then 180F, whilst driving and ECT A in the 190s. This isn't normal as ECT B's normally much lower and not climbing when in motion. Returned home, found upper hose now soft and coolant level low too with some residue on the expansion tank. Left engine running, collected what I needed and went for a short drive. ECT B kept climbing and ECT A stayed low in the 190s but heater working fine. Stopped to check and found coolant climbing to the cap, turned around and headed home and saw ECT A start climbing to the 240s and then come down and heat started blowing cool. Found coolant gushing from the cap again. Let it cool down and manually refilled with a gallon of fresh coolant. Let it run for a bit, heat now working again and all appears to be okay again.

 

good grief. This cyclical situation is hard to keep on top of. Surprised no one else has piped in.
Its interesting that there are such large quantities of fluid involved. Which differs from mine.
Incidentally, you had asked earlier; no, the coolant levels remain as they are with the aux heater running.

 

@guy

Engine cooled down, tank empty, topped off with about another half gallon of coolant. Made a new, concerning observation. Squeezed upper hose and noticed the tank cap hissed, then when I released the hose, it hissed again, essentially squeezing and releasing the hose was pushing air out and sucking air in through the cap. This's the new cap, it's quite stiff to tighten or loosen and clicks loudly. I put the old cap back on and it's quite easy (little effort) to tighten or loosen and clicks faintly but it doesn't hiss like the new cap. Keep in mind though the old cap was allowing coolant to leak which's why I replaced it.

Thought's?

EDIT: Note for the last couple of days the system's been holding pressure, until this afternoon and I did wash and dry the new cap tonight because they don't work when soaked with coolant (they just slip).

Could there also perhaps be an issue with the expansion tank neck threads, deformed due to heat or the like? If the caps aren't functioning correctly that would match what I'm observing, intermittently not venting vs. not sealing. Perhaps I should fit yet another tank?