Q: Overheating to 3/4

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My car overheats to 3/4 of the way, stays for about 3-5 seconds then drops back to normal. There are days that it does this and other days it does not. What could it be? Would there be air in the system? I looked this up online and some say it could be the head gasket but my oil is not Milky, exhaust does not blow out white. I checked under the hood and the coolant reservoir seems like it overfillls and spills out if the cap. When I remove the radiator cap it seems like there is no pressure, it doesn't make any sound when I open it (I check it when the engine is cold).

My car has 238401 miles.
My car has a manual transmission.

Overheating can be caused by a number of things such as low coolant levels, a faulty thermostat, a clogged radiator or a failing coolant fan switch. As you may know the coolant fan switch helps to maintain the proper coolant temperature by turning on and off at specific temperature thresholds. When this switch is not working properly, this can cause the fans to come on intermittently, all the time or sometimes not at all. When this happens you will notice a temperature spike and drop occasionally as the fan comes on and off. When your thermostat is not working properly or is stuck closed, this will not allow the coolant to properly circulate through the engine, which may cause the engine temperature to fluctuate erratically or in some cases just remain hot. You may also have a water pump that is beginning to fail. Water pumps often times will begin to produce a bit of a whining sound when they are starting to fail. A common problem also associated with this is the water pump may begin to leak around the water pump shaft due to the seal being damaged. In addition to leaking around the pump, you may have a leak elsewhere in the cooling system coming from one of the hoses going to or from the radiator. Lastly, you may have a failing cylinder head gasket allowing coolant to leak from the water passages into the cylinders resulting in the coolant being burnt in the combustion chamber with the fuel. If this is the case, you may also notice a bit of white smoke coming from your exhaust which is a product of the coolant being turned into a steam like vapor. I would suggest having an expert from Your Mechanic come to your home to diagnose your cooling system.

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