Q: Why is my truck running hot? Where is my coolant going?

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My truck is running too hot. If I am driving at a consistent speed it will stay around 5/8 on the gauge but when I am stopped at a red light, or idling for a minute or two it will get to around 3/4. Putting the heat on full blast makes it go down a little. When I start moving again it goes back to 5/8. The fan looks like it's running just fine. I have noticed I am slowly loosing coolant, I don't see any noticeable leaks any where. I have checked the oil and radiator cap and they both look good. There are also not any noticeable bubbles in the coolant reservoir or the radiator when I start the truck. My fuel mileage seems to be going down more and more the longer I've owned the truck, which may or may not be related. Any suggestions would be greatly appreciated.

My car has 135000 miles.
My car has an automatic transmission.

When a vehicle overheats when at a stop, this is a sign that the cooling fan on the front of the motor is not pulling enough air through the radiator. Of course, if you are loosing coolant, this can cause similar symptoms. Let’s begin with the cooling fan.

The cooling fan is attached to a fan clutch that locks up when the motor gets hot. These fan clutches do fail on a regular basis. The most accurate way to test it is to warm the motor until the gauge begins to get hotter than normal. It is at this point that the clutch should lock up and the fan will begin pulling air through the radiator. Simply feel how much air is being pulled through the radiator. Warning: Do not stick your hand by the fan blade, but stick your hand into the engine bay above the motor. There should be a large rush of air being pushed toward the cab. If not, turn the motor off and grab the fan. If it spins easily, you have found your problem.

If the fan checks out, you will need to dig into the cooling system further. Begin by checking to see if the radiator fins are clogged at the front. If they are, clean the radiator with a water hose. Make sure you are not bending the thin metal fins. If you do, the radiator will be permanently clogged. If it is clear, then move onto the coolant level.

The coolant level is best checked in the morning before starting the car. Remove the radiator cap and top it off if there isn’t coolant at the top of the radiator filler neck. Top of the expansion tank as well. The best way to test for coolant leaks is with the use of a cooling system pressure tester.

This will pressurize the cooling system in the same fashion as what happens when the car warms up. You may need to keep pressure in the system for an extended period of time. If there is a leak, you will soon begin to see drips on the ground. Follow them to their source and repair the leak as necessary. You can rent a pressure tester from your local auto parts store.

If the cooling system is free of leaks, the next thing to suspect is internal clogging of the radiator. If the cooling system is very dirty, then this is a definite possibility. Warm the car up until the thermostat is open. Drive it around the block. Come to a stop, turn the motor off and feel the radiator for cold spots. It is normal for the bottom of the radiator to be cooler than the top. You are looking for obvious cold spots in the radiator.

If you should require more help with this, I recommend the following inspection; Car is overheating inspection

Good luck!

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