Symptoms:

Heater works fine as long as car is moving. Once at idle, either in gear or on park. Heater blows cold air, even when set on HOT setting.

Target vehicle is the 2000 LS 400 with about 150k miles on it. Only had for about 6 months so not entirely familiar with the vehicle.

Possible solutions found were air gap in the coolant and bad thermostat.

My Findings:

Opening up the hood I found at least two things that pointed to air gap in the cooling system.
1. The overflow reservoir was only half full. I thought it was fine, but if you look closely at the tank the 'low' and 'full' marks are about 1/2 inch apart at the very top of the tank. The reservoir needs to be completely full with red coolant to operate correctly. I was not aware of this.

2. When squeezing the top radiator hose with my hands... there was no feel of fluid in the hose and air was easily pushed back into the reservoir. I could see and hear the air bubbles in the reservoir.

Solution:
Using Toyota coolant I filled the reservoir to the top level. I then powered up the car and turned the heater on full 'hot' and the defrosters on. It was about 20 degrees outside at the time and slightly warmer in the garage with the door open. I then proceeded to squeeze the upper radiator hose to get the air out of the system. Then I'd refill the reservoir and repeat. I did this until the car got to operating temperature and could easily feel liquid in the upper hose as opposed to air. Took about 1/2 gallon of fluid.

When I finished, with the car never being driven and only idling in the garage, the heater was blowing hot air. I then replaced the cap on the reservoir and all things look good a few days later.



Hope you find this helpful.

 

Good writeup. I had this same problem!

 

If you make sure to properly orient the jiggler hole in the thermostat to the 12 o'clock position, and put a fresh radiator cap on from the dealership, fill at the fill bolt, and then monitor the system overflow reservoir through some hot and cold engine cycles, topping up as the level falls, then the system will self-bleed, no hose squeezing and other incantations are necessary.

(although I will admit to being a bit of a hose-squeezer myself when doing the initial fill - lol)

 

no, they won't. as someone who chased a small but irritating air bubble for over a month only to find it liked to hide in the top end of the motor, that simply isn't true on '98-00 cars.

you have to crack the bleed point on the top of the throttle body if you get a bubble stuck up there, it's the highest point in the circuit. OP got lucky in that his wasn't a small bubble, he was just a fair bit low on coolant.

 

I have never squeezed the radiator hose but the trapped air was easily and quickly removed from the fill hole. My method is to use a funnel as is shown below. The diameter of the nose is just the size of the hole and you can fill the LLC without worrying about the leak from the hole. All you need is to fill the LLC to the level of the blue mark after the reserve tank is completely filled and the cap is set.

After staring the engine, the top level of LLC is getting lower and lower as bubbles are coming out of the hole. Then I fill the LLC some more. Repeating this bleeds the air. Bubbles are coming out next to next for 10 minutes and the filling and bleeding are completed simultaneously.

 

i too used the filled hole to remove the air bubble (not on the throttle body but on the upper radiator hose inlet, on my 97). but i admit there was hose squeezing involved as well !