My 2002 Mercury Mountaineer Idle increase from 750 rpm to about 1250 rpm when the temp outside is 70 degrees or above. when it is really hot, his causes the car to overheat.
Thermostat changed, radiator flushed and fluid changed, idle air control valve changed,air filter changed. what could it be that’s causing this problem ? I have no codes so dealership can’t help.
The very first thing that comes to my mind when reading your email is “is this engine overheating to start with”? The onboard computer is probably raising the idle speed to compensate for the excess coolant temperature.
Just like your body sweats to cool itself off, your computer shuts down all non essential items like you’re a/c compressor and increases the engine idle speed to get more coolant circulating in the radiator.
First thing I would do is check the coolant level inside the radiator, and watch the coolant flow inside the radiator (if you can some radiators are hard to look inside of) while the engine is running to see if there is coolant circulating.
My guess is that there is a restriction in the cooling system somewhere, possibly inside the radiator itself. I am assuming you have an old time manual radiator fan (and probably an electric fan for your air conditioner condenser).
When the engine is running hot and the idle speed has increased, I would expect the air the radiator fan is sucking across the radiator and blowing onto the engine to be VERY hot. If this air is cold, then you don’t have coolant circulation inside the engine or radiator.
I might even remove the thermostat just to rule out a potential problem with it. When the engine is hot, you can also run your hand along the center of the radiator to see if you can feel a drastic change in temperature from the top of the radiator to the bottom, it SHOULD be uniformly hot from top to bottom.
Today’s thin radiators can easily get restricted with rust, which is almost impossible to flush out. Radiator replacement is the only solution.
Keep me posted as to what you find out.