Safe to have inverter coolant slightly overfilled?

You're good to go.

 

It doesn’t have to be precise. The tank is there to provide room for thermal expansion and reserve supply for thermal contraction. Just keep it close to the line and get it checked if it starts to get lower.

 

If inverter fluid levels drops suddenly and it is a large quantity, you may have a head gasket leaking fluid into the heads. Please have your car checked ASAP or if you start noticing hard starts/knocks, engine coils misfiring!! I waited to have mine checked and had to have a partial engine rebuilt.

when the techs plugged in computer diagnostic, all they got was misfiring engine coils. we changed out all the spark plugs,wires and engine coils-- but I forgot to tell them about the inverter fluid drop cause I had refilled it. 1 week later, engine light comes on and some rough starts and misfirings start over again-- I was going to drop it off at shop after work, but it was too late, Overheat light came on..I turned off car right away and coasted my way into a parking spot. Inverter coolant was ALL gone! I refilled and slowly drove it to shop where I got the bad news.

Lesson learned the hard way.

 

Inverter and ICE cooling systems are totally separate. Inverter coolant cannot leak into the cylinders. If the coolant in the ICE radiator drops suddenly, on the other hand, that could indicate a leaking head gasket.

 

@davecook89t is absolutely right. Inverter and ICE coolant are not connected. The inverter coolant cools the inverter, not the engine.

If the inverter coolant level is falling, it's simply leaking. Most likely a hose connection.