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Randomly overheating Toyota

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70herald

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Ok, I need help from the experts. My Starlet (4E engine, like the '90s Tercel in the US) has started to overheat on a random basis.

Last week, on the highway, the gauge started to climb, (hot weather) as soon as I stopped for a few minutes, the gauge quickly went back down to normal, and stabilized. After a few minutes of driving the gauge would climb back up again. I parked and went shopping and surprisingly, about a half hour later when I went to drive home, no overheating, nothing worked perfectly.

After this, I flushed the system, checked the radiator, hoses are in good shape, (I should replace the bypass hose but I think that requires pulling the starter?) Thermostat is about 2 years old.

Now suddenly today, in the city, cooler temps and it is suddenly overheating again.

Now what could be random like this? I can see water flowing, water flows through the block, and when it isn't overheating, the temperature stays quite steady on the gauge exactly where it normally is. I am pretty sure that the fan is turning on, it certainly turns on when the A/C is switched on. Could a thermostat be "random" I would have thought that either it works or not, but not sometimes.
 
Could simply be a sending unit going bad. Does the car actually show any other symptoms of overheating, or is it just the guage?
 
I have also been wondering about the gauge, but I didn't have a second thermometer with me to double check, and since it is random, I may have to wait until it starts showing hot while I am driving.

Edited:
Well the car just arrived home, nearly boiling so the overheating is real. Part of the problem is that apparently I failed to reconnect the sensor for the fan. (BIG OPPS!) But that still doesn't explain why it started to overheat in the first place, before I pulled off the thermostat cover to flush out the block/ flushed the rad etc. Hope I didn't do any damage.
 
My guess is an air lock somewhere in the cooling system?
 
Check Philman's idea, and also check for thermostat failed in the closed position(take it out, and set it in a pan of water, start the water to boiling thermostat should open - it should close again while it's cooling off), and/or waterpump impeller corroded/worn would be the first 2 places I would look. Then check for possible stoppages (scale buildup in the water jacket somewhere)
 
Looks like I have allot of work coming up. The timing belt needs changing anyway so this would be a good time to pull everything apart. Or maybe I will just have someone else do the job. Without a lift, getting to the water pump, and belts is a real pain.
 
Given that the radiator and block are clean and the coolant is fairly new. There are only a couple of things, thermostat, the heat sensing switch {the one that turns the fan on and off} and or the relay. {switch to relay to fan motor} AND or possibly the fan motor itself! When the temp guage goes past where it normaly runs, stop get out and open the hood to see if the fan motor is actually running or not. IF not then one or more of the four things mentioned above are bad.
 
A new thermostat seems to have done the job. I took the old one apart, and it looks like there is a small notch worn into it. Probably stuck if it hit exactly that location. That probably explains the randomness. Anyway after a few drives, it seems to be back to normal with no damage done.
 
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