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Discussion Starter · #1 ·
Hello, recently my car has suddenly decided not to warm up all the way. I still get a decent amount of heat out of the vents, however the gauge is only moving up a quarter of the way. I thought it was the thermostat (even though it and the housing were replaced back in December), so I just pulled it and it looks perfectly ok. The coolant was flushed back in November 2013. Checked the antifreeze protection with a prestone gauge and I'm good to -35F. I plan on checking the hoses running into the heater core tomorrow after I arrive at work (15 mile drive that takes about 20 minutes), and see if the hoses are the same temperature.

What's confusing me and a mechanic friend is the engine will get close to normal temps while sitting with the heater off, however either of the 2 will drop it to about the 1/4 tank mark if you use the fuel gauge markers as a reference.

This seemed to start after the housing was swapped out so if there a way to get air out of the system? I've ran the car with the heater and fan cranked and still haven't gotten any extra air bubbles out of the system.
 

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Heater control won't help, as they went to a "blend door" to control air flow between hot & cold instead of having a water control valve that changes coolant flow through the heater core.

Was the thermostat double checked for the bleed hole at the top? That could trap some air if positioned differently. Hot water immersion with a thermometer is really the only way to test a thermostat, other than seeing obvious problems such as stuck open when cold.

I don't think air entrapment is a likely issue for variable temp readings on yours, as the gauge registers off a cylinder head sensor (CHT).

Just for some data, you could try both putting a thermometer in the dash vents (kitchen style works) & using the "test" mode of the Odo. to check reported temps (gauge usually averages a lot, tending to show Cold, warm, normal, hot & overheated - spending most time at cold/normal/over.
 

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For the price of a thermostat i'd just toss a new one in there. When I had air in the system of my old 4runner I just took the cap off the rad, jacked the front end of the truck up and let it idle or about 30-45 minutes. That got rid of all the air bubbles.
 

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Discussion Starter · #4 ·
I'm waiting on the ring to show up at Ford. I figured I'll install both from them to make sure it's working right. Right now one of my smaller hoses is cold even though I have slightly warm air coming from the vents.
 

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Can you tell us what the engine temp is while it's running? You can do that with the electronic odometer trick. You can even watch the temp rise up to over the thermostat rating, then watch it fall when cold water enters, then rise up again to repeat the cycle until it settles on a temp. If you don't see the rise/fall/settle cycle, then you know your thermostat is stuck.

Key off: press and hold the trip odometer reset button. Continue to hold the button, turn the key on, wait until the odometer reads [test] before releasing the reset button. Now it will change to [gage], and the needles will bounce around on the dash for a few seconds. Now each time you press and release the reset button you'll scroll through thirty test functions. The one you're looking for reads like this depending on your ambient temp and whether or not the engine is cold [15 C]. Now, without turning the key off, start the engine and drive. Now your odometer will display the engine temp in C. Look for the heating cycle. Let us know what temp you rise to, and the conditions in your area- for example, if it's -20 F outside, we need to know that. Thanks.
 

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The ODO trick is just going to say exactly what the gauge does. Both run off the same info.

There is only 1 source of info, the Cylinder Head Temperature Sensor. When it fails it's common for the gauge to only work part way. This is because of how the CHT is designed, dual resistance.

So, I'm going to say the CHT is the problem. Or A problem.


If I'm not reading correctly and you are getting reduced heat from the vents also, then I'll have to amend my diagnosis.
 

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Ahhhhh I'm having this same exact problem! Started yesterday. I just made a post about it too...I thought it had something to do with the thermostat sticking with the crazy cold weather we've had over here recently.....hope we figure out what's going
 

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The ODO trick is just going to say exactly what the gauge does. Both run off the same info.
Yes, but you don't get exact temperatures in any scale on the gauge, and that's what I wanted so that he could see the thermostat opening if it was. If you don't see a heating/cooling cycle, then the thermostat is most likely bad. It might not be bad if the ambient temperature is so low that it won't heat up at all, but I've never seen that.
 

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Discussion Starter · #10 ·
Ok, after sitting at work for 8 hours (forgot to check first thing this morning). It read 5 C when it was 0 C outside, allowed to run and idle to 100 C before my shift was over. As soon as I began to drive it began to drop to a floating range of 70 C to 74 C. So I'm guessing bad thermostat?
 
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