Your thermostat is stuck open. Buy a new one they are usually really cheap and you can usually repair it yourself. They are easy to get to even in compact engine space.
spark plug gap Can also be a vacuum leak.
The cooling fan should run continuously when the A/C is on. See if it is running, and if not check the fuse, and thermal relay. If they are good, see if the fan motor itself it good. If all this checks out, the drain and flush the radiator with a chemical flush. Also make sure the front of the radiator is free of debris.
Most likely it is a belt on your car. Usually the squeeling is from a loose belt while idling and will also make noises while driving.
Also, the Toyota Tacoma 4X4 SR5, without Keys, parked, no one in it?
Yes, idling an engine puts more wear and tear on an engine (especially diesels) than normal driving does. It's also bad for the environment, not to mention just plain wasteful.
you will proberley find the vehicle has either a misfire or air intake leak(petrol engine)/vacc pipe , i can only go as deep as the question appears, in a nut shell the question needs to be more in depth.
Often this happens when a tune up hasn't been done on the car in a long time. It also may happen when one of the cylinders misfires or when the engine overheats.
You may have the problem of your intake manifold gaskets. They are a problems on the lumina. I had to have mine replaced due to the same thing. Also check if your coolant is mixing with your oil if it is. Than it is definitally your intake manifold gasket.
check your fan clutch if its weak it will overheat
Sounds like you are getting engine compression in to your oil jacket.
The windings melt. Also may start a fire if surround conditions permit.
A bad coolant temperature sensor is one thing that could cause an idling speed problem with a Peugeot 406. There could also be a problem with the idle control valve.