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Q: radiator fluid boils on 1993 Toyota Previa

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In a hot day ( 100F), When I use the air conditioner, the radiator fluid start to boils and leak 3 to 5 liters. When I am in the highway the temperature keeps at the middle of the gauge, but when I stop or in the city it strart heating and the liquid boils and fall.
In a day I had to complete twice, 3 liters each time.
Thanks
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A partially restricted radiator could also be an issue if the Previa can cool adequately for the most part on the freeway but on a hot day and air conditioning loads added the radiator cannot dissipate the extra load/heat generated.
After experiencing some overheating, I recently self drained & flushed my 1993 Previa's radiator and reservoir tank with interesting results. I noticed something was wrong when my temperature needle moved to hot, my radiator reservoir tank was full and remained cool, and the heater did not normally come on to heat inside the van. I think the hot coolant was not flowing past the heater core because of a blockage in the cooling system. So I guessed the reservoir was the bottle neck keeping the system flowing.

First, I removed the top plastic air duct intake cover. Then I removed the radiator reservoir tank. After unscrewing the bottom radiator drain plug to drain coolant, I ran a yard water hose (with on/off hand grip for control) from the top radiator inlet hole and flushed out the old coolant.

The interesting part came when I completely cleaned and flushed the coolant reservoir tank with a the yard water hose. I noticed that one or two of the three inlet/outlet nozzles had been clogged with debris and/or coolant sealant. So I cleared the reservoir's thin nozzles with a Q-tip and thin ice pick or screwdriver. It appeared that the reservoir tank had been clogged which made the cooling system inefficient, especially during warm weather with the AC on.

To flush out the old coolant from the engine, I ran the yard water hose from top radiator hose with the engine on for about 1 minute. Eventually I saw old water/coolant flush out from the hoses that connects to the reservoir tank (which had been removed.)

After 45 minutes of draining, cleaning, flushing, I connected everything back up and refilled the radiator with fresh coolant. Be careful when removing the hoses... the nozzles, nipples, or hose connection tips can easily be broken if the rubber hoses are pulled the wrong way.

For now it seems the radiator is keeping cool. If your temperature needle is showing up hot past the middle mark, try flushing out the reservoir tank, radiator, and hoses to make the cooling system more efficient. In my case, it seems the reservoir tank was clogged because 3 of the 4 inlet/outlet holes are very thin and can easily get clogged with old coolant, debris, dirt and/or sealant
I found my problem... The coolant reservoir cap was damaged and that generated the leak.
Thanks all who answered
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