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Hi all! If its not one thing, its another... I've never really notice my temp gauge as it has always read the same regardless of the weather so I don't pay attention to it. (Have owned the car for 3 years). I read somewhere in this forum that a low reading contributes to poor mileage and that may be what is happening here.. My mileage sucks around town but on the highway @ 70-80 MPH I get around 24 MPG with 250,000 miles on the car. My gauge only reads about 1/4 on the dial but I still get great heat when I need it (Today temp is 42). Shouldn't it be reading about halfway at least regardless of the outside temp? Even at 95 degrees in July it doesn't come above 1/4.. Would it be a bad thermostat or something like a sensor? I believe that has something to do with my poor mileage. Thanks, Bobby

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Hi all! If its not one thing, its another... I've never really notice my temp gauge as it has always read the same regardless of the weather so I don't pay attention to it. (Have owned the car for 3 years). I read somewhere in this forum that a low reading contributes to poor mileage and that may be what is happening here.. My mileage sucks around town but on the highway @ 70-80 MPH I get around 24 MPG with 250,000 miles on the car. My gauge only reads about 1/4 on the dial but I still get great heat when I need it (Today temp is 42). Shouldn't it be reading about halfway at least regardless of the outside temp? Even at 95 degrees in July it doesn't come above 1/4.. Would it be a bad thermostat or something like a sensor? I believe that has something to do with my poor mileage. Thanks, Bobby

There is a temp sensor on the engine block that the ECU factors into the fuel programming of the engine. If it is faulty and reading too low it could degrade performance as the ECU may be incorrectly sending too rich a fuel mix to the engine. Don't rule out a stuck (open) thermostat either. That could cause engine to be too cold and also degrade performance. A lot of folks find it just easier to replace the temp sender than spend a lot of time diagnosing it. Is fairly inexpensive from 3rd party (around $20) so not worth spending hours diagnosing.

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Thanks curiousB for quick reply, now could you tell me where on the block I'll find the temp sender? Thanks again, Bobby

Hi all! If its not one thing, its another... I've never really notice my temp gauge as it has always read the same regardless of the weather so I don't pay attention to it. (Have owned the car for 3 years). I read somewhere in this forum that a low reading contributes to poor mileage and that may be what is happening here.. My mileage sucks around town but on the highway @ 70-80 MPH I get around 24 MPG with 250,000 miles on the car. My gauge only reads about 1/4 on the dial but I still get great heat when I need it (Today temp is 42). Shouldn't it be reading about halfway at least regardless of the outside temp? Even at 95 degrees in July it doesn't come above 1/4.. Would it be a bad thermostat or something like a sensor? I believe that has something to do with my poor mileage. Thanks, Bobby

There is a temp sensor on the engine block that the ECU factors into the fuel programming of the engine. If it is faulty and reading too low it could degrade performance as the ECU may be incorrectly sending too rich a fuel mix to the engine. Don't rule out a stuck (open) thermostat either. That could cause engine to be too cold and also degrade performance. A lot of folks find it just easier to replace the temp sender than spend a lot of time diagnosing it. Is fairly inexpensive from 3rd party (around $20) so not worth spending hours diagnosing.

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On the front coolant 'bridge' there are two senders, there's more towards the passenger side. One has a single wire, this is for the gauge, the second one, 2wire, is the ECU's sender. And indeed if it's bad or sending the incorrect signal the ECU is stuck in the cold fuel map, consuming a much higher rate of fuel.

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I get about 10 to 12 MPG around town and use medium grade gas and thats without the A/C ! I'm thinking my thermostat is bad and telling my ECU that the engine temp is cold and its running rich all the time since the gauge only reads one notch up from cold all the time....Does that sound logical??

*Edited for my lack of reading comprehension.*

What is "poor mileage" to you?

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I get about 10 to 12 MPG around town and use medium grade gas and thats without the A/C ! I'm thinking my thermostat is bad and telling my ECU that the engine temp is cold and its running rich all the time since the gauge only reads one notch up from cold all the time....Does that sound logical??
*Edited for my lack of reading comprehension.*

What is "poor mileage" to you?

10-12 MPG is pretty low so unless you have a chronic lead foot or 1200 pounds of deadweight in your trunk there seems to be something wrong.

The thermostat doesn't tell the ECU anything. The thermostat regulates the cooling systems temperature. The temperature sensor/sender sends a temperature reading to the dial gauge. As RDM states there is a second sensor that sends temperature reading to the ECU.

If your gauge is reading low it could be the temp truly is low because the thermostat has failed and is stuck open. This is a common failure and would explain why the block either never warms up or takes very long to do so.

The other possibility is the sensor for the ECU is truly faulty and sending too low a temperature reading to the ECU causing it to make an incorrect fuel decision resulting in rich mix and wasted fuel hence poor MPG.

Given your dash shows low temp and you are getting poor MPG maybe it points more likely to a bad thermostat. If you have a DVM meter you can measure the sensors resistance at various temps to see if the sensor is giving a false reading.

Since a thermostat and sensor are pretty inexpensive it may be easier to change both and be done with it. Depends how much you value your time. My hunch is to try a new thermostat first.

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Thanks for your reply.. I'll change out the thermostat first and see what happens. If not then I'll change the sensor.. Thanks again, Bobby

I get about 10 to 12 MPG around town and use medium grade gas and thats without the A/C ! I'm thinking my thermostat is bad and telling my ECU that the engine temp is cold and its running rich all the time since the gauge only reads one notch up from cold all the time....Does that sound logical??
*Edited for my lack of reading comprehension.*

What is "poor mileage" to you?

10-12 MPG is pretty low so unless you have a chronic lead foot or 1200 pounds of deadweight in your trunk there seems to be something wrong.

The thermostat doesn't tell the ECU anything. The thermostat regulates the cooling systems temperature. The temperature sensor/sender sends a temperature reading to the dial gauge. As RDM states there is a second sensor that sends temperature reading to the ECU.

If your gauge is reading low it could be the temp truly is low because the thermostat has failed and is stuck open. This is a common failure and would explain why the block either never warms up or takes very long to do so.

The other possibility is the sensor for the ECU is truly faulty and sending too low a temperature reading to the ECU causing it to make an incorrect fuel decision resulting in rich mix and wasted fuel hence poor MPG.

Given your dash shows low temp and you are getting poor MPG maybe it points more likely to a bad thermostat. If you have a DVM meter you can measure the sensors resistance at various temps to see if the sensor is giving a false reading.

Since a thermostat and sensor are pretty inexpensive it may be easier to change both and be done with it. Depends how much you value your time. My hunch is to try a new thermostat first.

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Make sure to just goto your local Toyota parts dept. and get an OE thermostat with the jiggle valve. Should be less than $25. I think the OE thermostat is far superior than an aftermarket part that's built to fit a hundred different engines. The jiggle valve is a pretty important part to the 1UZ cooling system and must be installed at 12 o'clock.

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