Your car is RHD, right?

Assuming it does give, you can easily get a RHD inverter from Japan - the newest one (from the first Crown hybrid and the absolute last models of the 3GSh). Shouldn't have much degradation.

@Lwerewolf this is my attempt at mocking up the 4th gen expansion tank... I got me a GWL10 tank from the parts shop... it's is twice as big as the GWS191 tank

Looks like the feed hose will plug right in, I will need an S-shaped hose for the return. Have not figured out how to modify the bracket yet.

The overflow hose to the main radiator expansion tank will be deleted (red cross). The inverter already has its own small radiator, so I'm assuming that this will completely separate the 2 coolant loops?

Any idea how high I should fill the thing?

Uhh...

As high as the "high" line goes? Not sure if this is really necessary. Might do better with a different tank given where it has to fit. Honestly, nowadays the main advantage to this method IMO is the lack of ambiguity over which loop is losing coolant if the main reservoir is getting low - even though it's almost always the engine loop (my rad cap had disintegrated >_<).

I still stand by my statement that you should swap the inverter for a "fresher" one if you're having trouble with it. RHD ones should be plenty. LHD ones are problematic, as usual.

On the bright side, looks like I'm not the only one with a dirty engine bay (don't ask for pics, somehow the engine is mostly clean but the rest is as dusty as it gets )

On 2nd thoughts, I think you are right --- I might keep this current cooling system as theoretically it is more efficient, albeit more complicated. I've already ordered a GWS204 Crown inverter as a spare. My current inverter seems to be holding up but it's getting old.

For the GWS191 - I studied the manuals, the 2GR-FSE in our cars run 2 coolant circuits, 2 expansion "tanks", and 1 common recovery reservoir.

For the GWL10 - the 2GR-FXE runs 2 coolant circuits but they are separate - for the engine 1 expansion tank, 2 recovery reservoirs (separate sealed coolant system for the inverter circuit with a recovery tank).

Our system is more efficient because of the dual expansion tanks, but has more hoses than the GWL10 as a tradeoff, and as you said the common recovery tank means leaks are harder to diagnose. I guess Toyota judged that with the improved inverters it was OK to run a sealed independent coolant circuit for reliability and simplicity.

I guess I could add another reservoir to the inverter circuit and redirect the coolant overflow pipe to the engine reservoir.. but that's even more hoses and brackets