Car keeps threatening to overheat on me?

Question

My said car has this peculiar cooling system issue that has baffled me for a while.
What happens is after warming up, the cooling fan will come on but then will stay on and run at a high speed because it struggles to keep the system cool after some stop and go driving, which is evident by the temperature gauge reading closer to H rather than staying in the middle.. However when it sits at idle, it stays stable more or less, but the cooling fan still runs non-stop at a lower speed and it's pretty much the same way driving on the highway. 
A few other things to note is when completely warmed up, I feel no pressure in the hoses, the upper plumbing is super hot while the lower plumbing is considerably cooler and the temperature differential is on opposite sides of the thermostat housing (on this car, the housing is a remote part from the engine and is only connected via the hoses themselves), and inside the expansion tank, the coolant is ice cold even when the system is completely warmed up.
Now I have flushed the coolant several times due to having to replace several major components (radiator, water pump, thermostat) on different occasions. And each time I did the burping procedure, but despite it all, I'm still having these issues. I'm pretty sure there's not a problem with the head gaskets because the engine isn't running rough or thick white smoke pouring out of the exhaust or anything like that. So, what might I be up against? Faulty thermostat? A more thorough burp maybe?

Pedal2TheMetal45 · Answer

Nether of those caps creates pressure.... they just HOLD the pressure in the system. The pressure is created by the coolant heating up. If your hoses are not getting hard then something is not holding pressure. Be sure that the expansion tank is full when the motor is hot, if it isn't then fill it when the motor is hot/warm.. I say this because when the motor is hot it pushes coolant out of the motor into the expansion tank and as it cools it pulls it back into the motor.. so if you add the coolant before it cools off it well draw the coolant back into the motor and fill it back up.. then after it cools refill the expansion tank and then if it get low or overheats again then you have a leak in the system somewhere..
good luck
tim

andrew · Answer

make sure the earth wire is touching metal and not the paint this happened to me once

? · Answer

Definitely try replacing the thermostat, they're cheap enough.

Evil C · Answer

By the way, car is a 2003 Mercury Marauder. It has an expansion tank rather than an overflow tank due to being a closed system and yes I did try changing the pressure cap.

@Bazza, the pressure cap is located on top of the expansion tank of my car and not on the radiator. Also, the radiator cap doesn't pressurize the system, it only maintains a specific pressure in the system.  What creates pressure is the coolant expanding as it gets hot, and what raises its boiling point is the hot coolant compressing in on itself due to being in a closed plumbing circuit.

Anonymous · Answer

Have you renewed the radiator cap? This device pressurises the system and in doing so raises the boiling point of the coolant. Other than that either the radiator is still clogged or you have an airlock. The thermostat is easily tested by heating in a pan of water on a hob.

Car keeps threatening to overheat on me?

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