Thermostat/cooling problem

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Mark
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My tercel:: 1984 Automatic, 1981 sedan
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Thermostat/cooling problem

Post by Mark »

I've been having a problem with the cooling system on my '81 sedan in the last few days. I was driving on the highway, when the temperature gauge started dropping like the thermostat was stuck open. It eventually went all the way to the bottom of the gauge. When I stopped at a light for awhile the gauge would eventually creep back up to about 1/3. When driving again, the needle would go back down to cold. A classic stuck-open thermostat right?
So I bought a new thermostat and took the old thermostat out. The old one wasn't stuck open. I tested it in a pot of water on the stove and it opened at around 80 degrees Celsius as it should. When the water started to cool, the thermostat closed. So in my opinion, the old thermostat was fine. I tested the new thermostat the same way and it was fine as well so I installed it. With the new thermostat installed, the cooling system behaved the same way. The temperature took a long time to rise and it wouldn't go above 1/3 on the gauge when driving around town. I started from cold in my driveway and idled the engine. The upper and lower rad hoses gradually warmed up equally along with the radiator. It still seems like the new thermostat is stuck open as well and allowing the coolant to flow through the radiator when the engine is cold.
Anyone have this happen before? Any ideas? I'm hesitant to tear it apart and change the thermostat again since the original one and new one tested fine.
xirdneh
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Re: Thermostat/cooling problem

Post by xirdneh »

Maybe the temp gage on the dash is acting up
Love those Tercell 4x4 wagons but they sure suffer from road noise.
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Mark
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Re: Thermostat/cooling problem

Post by Mark »

I thought about it being a sensor/gauge issue, but the flow through the radiator even at low temps is what is confusing me. I'm still going to check the wire run from the sensor and see if there is a partial ground somewhere. I might borrow a laser thermometer and verify temps as well.
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Petros
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Re: Thermostat/cooling problem

Post by Petros »

you can buy an an after market temp gauge at an auto parts store and temporarily install it to verify engine temperatures. get the mechanical type (not electronic) and you get a direct measure without any ground or other electrical issues. they do not cost that much, and are good for testing the factory temperature gauge.

a simple test of the gauge is to pull the wire off the sensor and ground it, the temp should go to full hot, when not grounded it should go to cold. If the engine temp seems normal, it sounds like it could be a bad connection somewhere in the sensor/temp gauge system.
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Mark
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Re: Thermostat/cooling problem

Post by Mark »

I just used a laser thermometer to check temperatures in the cooling system. The temps started at around 35 degrees everywhere. As I idled the engine the temps very gradually warmed up everywhere equally (the upper hose/lower hose/radiator/thermostat housing/etc.). There wasn't the sudden temperature increase through the radiator that I'm used to seeing in a car when the thermostat opens. It's still behaving exactly like a stuck-open thermostat. I checked the wiring run and found some oil in one of the connectors. I cleaned it up, but it didn't make any difference. The coolant eventually warmed up to 80-90 degrees C and the gauge was showing just below the middle. At 80 degrees, the resistance across the sender was 68 ohms. I couldn't find a resistance table for the Tercel, but an 86 Camry table said 80 degrees = 65-89 ohms.
I'm still leaning towards a defective thermostat allowing flow past it for some reason. Even though it opened and closed at the right temps when I tested it in a pot of water...
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Re: Thermostat/cooling problem

Post by dlb »

If both t-stats and the sensor passed their respective tests, the gauge itself would be my next guess.
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Mark
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Re: Thermostat/cooling problem

Post by Mark »

The reason I'm leaning towards a thermostat issue is that the temperature on the gauge drops to low gradually when I'm driving on the highway, just like as if there was flow through the radiator. I don't think a bad sender or gauge would act like this. I wonder if the spring is weak allowing coolant pressure to push it open. I tried starting the engine with the radiator cap off to see if I could see evidence of flow in the coolant, but the level slowly rose and started overflowing so I can't run the engine without the cap on for more than maybe 5 seconds.
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Re: Thermostat/cooling problem

Post by marlinh »

Mark, are you saying that when you start a cold engine you can only run it for 5 seconds before it starts overflowing? That sounds weird.

I would suspect the new thermostat. The boiling water test gives you a general idea that the thermostat will open but isn't definitive for it's function in the car. Even though it is a PIA I would try another good quality thermostat.

If the thermostat is wide open while driving down the highway you will see the temp drop on the gauge. Ram air is cooling the radiator and the thermostat isn't modulating for the changes in temp. In your earlier test the upper and lower rad hoses should not be at the same temperature. When the coolant finally warmed up to temp, what was the temp of the lower rad hose. Also when you drove the car and the gauge was reading low, did you pull over and check all the temps to compare your readings?
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dlb
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Re: Thermostat/cooling problem

Post by dlb »

Mark, your last post got me thinking about the rad cap -- the symptoms you mentioned do sound more like a stuck open t-stat than anything else but it seems weird that two t-stats would misbehave in the same way, and rad caps are hella cheap anyway so I would try replacing that. Have you noticed any coolant loss or overflowing of the reservoir since the symptoms started?
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Mark
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Re: Thermostat/cooling problem

Post by Mark »

No, no coolant loss. I haven't had the time to do much investigating this weekend, but maybe today or tomorrow I'll take it for a run on the highway and see if the temperature still drops with all the extra airflow across the radiator. If so, I'll just try changing the thermostat again.
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Re: Thermostat/cooling problem

Post by Petros »

you might try a new cap as well, those are considered replaceable items like the t-stat, air filter, etc. and they are inexpensive anyway. if it is not allowing the cooling system to pressurize property that might cause it to behave as you describe.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
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ARCHINSTL
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Re: Thermostat/cooling problem

Post by ARCHINSTL »

Ditto on the cap.
While not experiencing this on Goldie, I had a similar experience on my late '88 Dodge/Mitsu p'up which was solved by a new cap.
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Mark
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Re: Thermostat/cooling problem

Post by Mark »

I'm trying to wrap my head around how a radiator that's not pressurized enough can cause a too-cold cooling system. I always assumed it would cause overheating?
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Mark
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Re: Thermostat/cooling problem

Post by Mark »

I replaced the thermostat again today and now everything is back to normal.
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Re: Thermostat/cooling problem

Post by marlinh »

Glad to hear that. Defective parts sent you on a wild goose chase.
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