Thermostat failure ?

1994 4cyl. 198k. Now that it is much colder outside. The temp. gauge reads very low, but after awhile it goes up to the normal reading of approx. 185 , but it fluxs
from normal to low. When it's in the low reading it gives less heat. I had changed the waterpump this spring w/timing belt. Changed the antifreeze also. Hadn't changed the thermostat, would this be the problem? Or is it the temp. sensor ? Either way it's not reading the correct temp. to the computer. It running very good but its got low heat. Any idea what is wrong? Thanks, Dan
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Its the thermostat, use a genuine toy unit I put in an aftermarket one and for years was cold. I replaced it and found it never sealed properly, the aftermarket one was smaller.
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odyssey wrote:

Because the heat output corrresponds to the reading, I think the sensor is ok and the thermostate is sticking at least partly open.
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Because coolant temp is such a vital parameter in so many aspects of the operation of your EFI engine, my recommendation would be replacing the thermostat at a MINIMUM of every 2nd year. Thermostats are simple mechanical controls. Your thermostat will begin functional failure in one of two ways. It will bind in the more open position causing cooler than required temps ... or it will bind in the more closed position increasing temps. Both types of partial failure have a significant effect on a number of critical ECU measured parameters.
Maximum two years on a thermostat ... coolants formulated for the system construction and design ... a little attention to this very important closed system ... and you're doing right by the people who sweated to develop the technology.
Oh yeah. Since the foundation of your cabin heating system is also your engine cooling system, you can't even address your "comfort" complaint until you ensure the health of the cooling system that everything is based on.

fluxs
the
wrong?
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RACEGUY wrote:

That seems a little...anal. Thermostats typically last many years, and when they fail you can tell it's happening. 5 years would be more like it for preventive maintainance on Toyota thermostats. Our '95 sedan still has the original, and it still works fine. Uisng Toyota coolant is a good idea.
(...)
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Agreed, I should have added that I always use Toyota RED.

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Thanks for all your help. It had turned out that it was the rubber seal around the thermostat, it had corrosion around the edge of the housing causing the coolant to leak past the thermostat. Changed it out with Toyota brand (with the "Jiggler" in it ) blasting warm comforting heat. Thanks again. Dan

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