2008 V6 temp guage maxed but engine is not hot - Ford Mustang Forum
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post #1 of 7 (permalink) Old 06-28-2019 Thread Starter
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2008 V6 temp guage maxed but engine is not hot

After driving many Mustangs myself, my twins now have their first Mustangs. One started showing a maxed temp guage so I asked my mechanic nephew to take a look. The engine wasn't hot even though the guage was full H. He said the lower radiator hose was hot and the top hose cold so we replaced the thermostat.

The guage still shows max H but the engine is still not hot. The top radiator hose is fairly hot but the bottom hose is cold. The coolant in the reservoir on top of the engine is cold as well.

Does anyone know if the water pump pushes water from the bottom of the engine to the top or pull it from top to bottom?

Any suggestions on wht the problem could be?

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post #2 of 7 (permalink) Old 06-28-2019
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Water from the thermostat goes into the top of the radiator.
The best way to remember that would be that there can always be air at the top but there will as long as there is fluid in the system will be coolant at the bottom.


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post #3 of 7 (permalink) Old 06-28-2019
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hmmmm . . . . does anyone know where the water temperature sensor is located?

If you let the car cool off, like overnight; and then start it up, does the gage immediately go up to full hot? If yes then it must be a bad gage or sensor, since the engine and water cannot get hot instantly.

If the gage is actually functioning correctly, then it sounds like the water is not circulating through the radiator. That could be caused by:
> a bad thermostat (does not open) -- you said you already replaced that
> water pump failure -- not likely, but possible
> an air bubble that needs to be bled out???? IIRC there is an "overheating" thread in the GT section that says how to bleed it

Edit: here is bleeding procedure copied from that other thread

Quote:
Originally Posted by Bullitt95 View Post
1. Fill the cooling system from the expansion reservoir until the level reaches the COLD FULL line.

2. Start the engine and let it idle with the cap off. This will evacuate more air so top up the reservoir as required.

3. Loosen the Allen head bleeder screw on the coolant crossover to evacuate any remaining air (apply a rag around it prior to opening to avoid creating a mess). If there are no bubbles, retighten it.

4. Make sure the expansion reservoir is properly topped up, screw on the cap, and go for a 10+ minute drive. Once finished, park the car with the front pointing uphill if possible.

5. After a minimum 4 hour cool down, recheck the coolant level in the expansion reservoir and top up as required.

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post #4 of 7 (permalink) Old 06-28-2019
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Quote:
Originally Posted by WhO2 View Post
After driving many Mustangs myself, my twins now have their first Mustangs. One started showing a maxed temp gauge so I asked my mechanic nephew to take a look. The engine wasn't hot even though the gauge was full H. He said the lower radiator hose was hot and the top hose cold so we replaced the thermostat.

The gauge still shows max H but the engine is still not hot. The top radiator hose is fairly hot but the bottom hose is cold. The coolant in the reservoir on top of the engine is cold as well.

Does anyone know if the water pump pushes water from the bottom of the engine to the top or pull it from top to bottom?

Any suggestions on what the problem could be?
1. The water pump draws cooler water from the lower radiator hose, pumps it around the engine where it absorbs heat, and the hotter water exits the engine from the upper radiator hose.

2. The water in the lower radiator hose is always cooler than the water in the upper radiator hose in a running engine, as the water circulates from the top of the radiator to the bottom and sheds heat.

3. It's extremely rare for a mechanical water pump to fail completely. They will usually leak from the weep hole when the pump shaft seal perishes.

4. A thermostat stuck closed would prevent coolant from reaching the upper radiator hose, so that would remain cold and soft.

5. The coolant temp. gauge measures cylinder head temperature, and the sending unit is in the valley underneath the intake manifold.

6. If the gauge shows H right from a cold start, either the gauge sending unit is faulty or the gauge stepper motor is stuck. With the latter, the gauge would still be on H even with the ignition off.

7. Use an infrared temp. gun to check the temp. of the upper & lower radiator hoses as the engine idles from a cold start to normal operating temp., and report back.


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post #5 of 7 (permalink) Old 06-28-2019 Thread Starter
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Thanks for the replies. To answer a couple of questions...

The temp guage does not start out hot. On a cold start the guage is below C and slowly (3-4 minutes) works up to the 1/2 way point. The guage stays at 1/2 for a while (3-4 minutes). At that point it will go to H and the high temp light will come on. At this time the coolant in the reservoir on top of the radiator is cold (not even warm).

We did replace the thermostat but the one in the car when I bought it was in perfect shape.

There are no leaks of any kind anywhere in the cooling system.

Thanks again for the help.
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post #6 of 7 (permalink) Old 07-03-2019 Thread Starter
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It ended up being the temperature sensor (a.k.a. sending unit?). Once we replaced that the temperature guage seems to be showing correct ly.
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post #7 of 7 (permalink) Old 07-03-2019
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thanks for closing the loop with us!

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