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Discussion Starter #1
I have a 2006 get mustang with overheating issues.
I’ve changed the thermostat, new fan assembly, heater hose had a leak so I replaced that with a silicon hose and replaced the sensor under the intake manifold, checked the fuses, flushed twice and am now I’m stumped.
 

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Make sure the fan is coming on -- you can just let the car idle and watch/listen, or I think the fan should come on immediately if you turn on the A/C

If the car does not overheat on the highway, but overheats in slow or stop and go traffic, that is a sign that the fan is not coming on.

If it is not coming on, most likely cause is the relay . . . I think there are two of them . . . there are some threads on here with the details
 

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Discussion Starter #3
The fan comes on, I’ve checked the connection and it looks good. This has been an issue for several years but has slowly been getting worse. I live in the mountains and it only overheats while I drive uphill when it’s slightly warmer out (75 degrees or more).
 

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How high is the temp getting? Did you replace the reservoir cap? Had to replace mine last year. And then I had to replace the 90* quick disconnect under the manifold last week. It had a small fracture.
 

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Discussion Starter #5
I replaced the cap last year. Today was the first day I got the gauge a bit past the H and the overheat light came on( for the first time). I’m going to check tomorrow morning if the fluid has gone down at all. I replaced that whole set up under the manifold with a silicon hose and two hose clamps because it did have a leak but is not the reason why I’m overheating. While I was under the manifold I replaced the sensor as well.
 

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Make sure all the air has been "burped" out of the system. The easiest way I have found is to use the upper passenger side radiator hose as "primer" pump. Grab the hose with both hands squeeze it rapidly several times until you no longer see/hear air bubbles come in to the degas bottle. The rad fan has 2 speeds.I believe low speed comes on around 190* and high 205~210. It also should go immediately to high speed as soon as the AC is turned on. The best way to determine this is to turn on the AC immediately after starting the engine from a cold start. If the fan doesn't turn on immediately then the high speed relay is probably at fault. You can verify this by swapping the the low and high speed relays. Sorry, I don't recall exactly which 2 they are in the fuse/relay box under the hood. Also check for blockage between the AC condenser and radiator. You may have to get a pressure test of the system if all these check out. Or you can buy a kit at Harbor Freight or any auto parts big box store.
 

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Another way to bled the system is to take the main cap off when car is cool of course. Leave the cap off, start the car and turn the heat all the way up. Do have a bucket under the car or something to catch coolant as it will bubble and spill out as it purges the air out. When this no longer happens shut the car down and after cooled apply the cap. It will overheat if there is air in the lines. I had to do this back on my 96 when I had some coolant lines fail... Got it all fixed just to keep randomly overheating like you are explaining. Bled the air out and was good to go.
 

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the remote thermostat housing set up is a PITA to bleed. there is a plug on the right side of the coolant crossover, just right of the alternator. it takes a allen wrench to loosen. remove that while the engine is cold, as that's the highest point in the cooling system. if you can get the front end elevated on ramps or just higher than level that will also help but not entirely necessary. start the car and let it come to temp and add coolant into the crossover as necessary. good luck
 

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If you remove the bleeder on the crossover there's an o-ring on the plug. I can't remember the size, but mine split when I unscrewed it the first time I flushed the system a few years ago.
 

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I recently changed the coolant in my '06 and it was easy enough to bleed. Just leave the cap off the overflow reservoir, start the engine cold, turn the heater on full, and top the reservoir up slowly while squeezing the upper radiator hose. Once it stops bubbling, replace the cap and go for a drive. After a couple of drive cycles, any remaining air should have been evacuated and you can top up the reservoir as required.
 

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How high are those mountains? Are we talking Appalachians or Rockies. I know the thin air of the Rockies will decrease performance but I'm not sure about over heating. Maybe others might know. I owned a 72 Torino when I moved up there in Colorado and that weak a$$ engine did not like the climb but I don't recall the temp going up during it.
 

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Possible Sources. 4.6 3V

Thermostat
Coolant pump
Radiator ( clean A/C condensor core and radiator )
Cooling fan
Pressure relief cap

Stock fan settings
Lo fan turns on 216℉
Lo fan turns off 206℉

Hi fan turns on 228℉
Hi fan turns off 216℉

Normal engine temperature 202℉ to218℉
Fail safe 250℉
Shut down 330℉

Thermostat opening temperatures
Starts to open 175-182℉
Fully open 202℉

Good luck.
 
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