93 LS400 90,000 miles. Coolant level is low. I was going to add some but have a concern: When cold the coolant level is at the bottom of the res tank (about 1/4 inch), at normal operating temp the level is at the very top of the res. tank. Is this much of a variation normal ? I'm afraid that if I fill to the cold level marks on the res. tank the fluid will overflow as soon as the engine warms up. Thanks for any thoughts, comments

 

I get the same occurrence on my '01 ES, 73300 miles. I believe this is normal for all cars, unless you see a lower reading at normal engine temp or a color difference of the coolant.

 

Update - I found a leak. A pressure test of the cooling system indicated that I have a leak where the coolant level detector screws into the tank. Does anyone know if this is usually the sensor o-ring, or, something more involved like the tank itself. Looks like there is a threaded metal insert attached to the tank that the sensor screws into.

Further investigation indicates that the overflow tank is different than most other cars I have seen. Looks like coolant is actually pumped into the tank on top and pulled out the bottom back into the cooling system circulation. Most other cars I have seen have a single connection to the coolant overflow tank which simply catches volumetric expansion overflow.

Anyone know why Lexus did it this way? Perhaps to avoid stagnation of the coolant in the tank?

Perhaps this explains the big variation in level?

 

Yes!
Coolant flows in and coolant flows out. Why have good coolant seat around waiting to be used? The design serves many good purposes. The drawback in this design is higher wear-n-tear rate on the reservior (subjected to higher temp&pressure) which is one reason why your reservior leaked. Check for hairline cracks and repair where appropriate.