My 93 SC 400 seems to be over heating, I changed out the coolant, changed out the thermostat, bleed the system, worked fine, but after sitting a week, I started it up, noted that no heat from heating system, even on high, then after about 4 blocks, started to overheat again. I don't see any visual leaks. Please advise

 

when was the last time the car got a new water pump?.

 

All the belts ok? Bad / broken belt = water pump not pumping

 

funny as it may seem I started out with a non factory thermostat and I had the same problem. Please if you haven't get a Lexus(toyota) thermostat

 

heat in the car works by taking coolant and pushing it through a minature radiator, called a heater core. Lack of heat, could be lack of coolant moving.

the car overheating also points to coolant not moving.

Logical explination: Water pump isn't moving the coolant.

 

I had a similar problem.
Water pump was the culprit.
It will cost you some.