While detailing my ES330 I noticed the coolant level in the expansion tank was slighting under the minimum mark. Looking closer I could see the cap on the expansion tank was not on tight, just resting on top like someone forgot to snap it close after the coolant was replaced at the last servicing or that it popped off. It has always been dealer serviced and had the last service about 3,000 miles ago which was at 100,000 miles. I drive on the freeway every day and it has never overheated. The tank is clean on top with no indication at all that coolant has ever overflowed. I put the cap back on without adding additional coolant and drove it about 15 minutes. The coolant level in the tank is now about an inch higher than the minimum mark. I can't believe I drove it 3,000 miles like that without any indication of overheating or leaks under the car. Is it possible coolant evaporated with the cap off like that?

 

That small amount of fluid difference would make no effective difference on coolant effectiveness. You would not notice a difference.

When it cools, use distilled water to top it up to 1/2 way between min and max. The coolant would evaporate a little bt easier with the cap off, but more likely was forced out with higher pressure, presuming it was full to begin with after the service. The small amount we are talking about is relatively inconsequential given total system volume, so using dH20 vs. coolant is fine.

 

go buy some pink 50/50 at toyota and fill it up! Or complain at lexus and get it done for free. "My cap was off and the fluid dropped low."