What causes white smoke or steam to come out of the exhaust?

No,
White smoke is steam
Blue smoke is oil.
Black smoke is rich mixture.

We see 'white smoke' questions every year at this time of year when the weather changes. Your gasoline contains water at molecular level and the engine burns it off as steam which exits the exhaust pipe.

he causes of white exhaust smoke can vary; however, it is common to see white exhaust smoke when first starting a car, especially on cooler days. This is generally steam caused by condensation. As the engine warms up and the condensation dissipates the white exhaust smoke (steam) is no longer seen. If excessive white exhaust smoke is present well after the engine warms up, it is necessary to have the car inspected for possible internal coolant leaks. Indicators of an internal coolant leak include billowing white exhaust smoke accompanied by a sweet odor or a low coolant reservoir level. An internal coolant leak can also contaminate the engine oil giving it a frothy, milky appearance. Even small amounts of coolant entering the combustion chamber will produce white exhaust smoke.
One of the main causes of white exhaust smoke and coolant loss is a cracked or warped cylinder head, a cracked engine block, or head gasket failure caused by overheating. A cracked head may allow coolant to leak into one or more cylinders or into the combustion chamber of the engine. Dirty coolant, a poorly maintained cooling system, a low coolant level, or a non-functioning cooling fan can cause engine overheating. In addition, engine wear can eventually cause the gaskets to lose their capacity to seal properly allowing internal coolant loss. Intake manifold gasket and head gasket failures are two of the most common sources of internal coolant loss caused by engine wear.
Never remove the radiator cap or coolant reservoir cap while the engine is hot or running as it can cause serious injury; always allow the car to cool down completely first. Checking for a low coolant level in the reservoir is the first step in determining if coolant loss is causing the white exhaust smoke. If the coolant reservoir is at the proper level but excessive white exhaust smoke is present, a cooling system pressure check is required to determine where, if any, coolant leaks are located.

What is year--make--model?
Are you losing coolant? The white smoke may be coolant in the combustion cylinder, possibly caused by a head gasket problem. If head gasket problem, fix that first.

With idle issues, clean/check anything that has to do with idle air system. Any applicable trouble codes?

that is steam, not to worry

It is actually steam. The head gasket has failed and the knock is either the pistons trying to compress liquid or the oil has been thinned out from the coolant.

It is common to see white exhaust smoke when first starting a car, especially on cooler days. This is generally steam caused by condensation. As the engine warms up and the condensation dissipates the white exhaust smoke (steam) is no longer seen. If excessive white exhaust smoke is present well after the engine warms up, it is necessary to have the car inspected for possible internal coolant leaks. Indicators of an internal coolant leak include billowing white exhaust smoke accompanied by a sweet odor or a low coolant reservoir level. An internal coolant leak can also contaminate the engine oil giving it a frothy, milky appearance. Even small amounts of coolant entering the combustion chamber will produce white exhaust smoke. One of the main causes of white exhaust smoke and coolant loss is a cracked or warped cylinder head, a cracked engine block, or head gasket failure caused by overheating. A cracked head may allow coolant to leak into one or more cylinders or into the combustion chamber of the engine. Dirty coolant, a poorly maintained cooling system, a low coolant level, or a non-functioning cooling fan can cause engine overheating. In addition, engine wear can eventually cause the gaskets to lose their capacity to seal properly allowing internal coolant loss. Intake manifold gasket and head gasket failures are two of the most common sources of internal coolant loss caused by engine wear.
Never remove the radiator cap or coolant reservoir cap while the engine is hot or running as it can cause serious injury; always allow the car to cool down completely first. Checking for a low coolant level in the reservoir is the first step in determining if coolant loss is causing the white exhaust smoke. If the coolant reservoir is at the proper level but excessive white exhaust smoke is present, a cooling system pressure check is required to determine where, if any, coolant leaks are located. THESE LEAKS WILL CAUSE SEVERE ENGINE DAMAGE! Have the car inspected immediately.

I
Internal coolant leaks can and will cause

all engines do cold. its water vapor and or the hot exhaust, causing the air, to change dew point, (looks like steam)
on the white smoke smells like burned oil , it is, and is slow to dissipate like steam, (see moms tea pot , and steam fast dissipation?)

if it is oil , its most likely bad valve guides and seals, or both.
if a compression leak down test passes, its not rings, (mostly)

Steam is not smoke. Smoke would be from oil burning, the steam would be overheated coolant which is 50% H2O. Oil making white smoke, more than a little would have a smell that irritates the eyes and nose
while the steam from overheated coolant will smell sweet. The mixing of coolant and oil will be apparent on the oil fill cap when major damage has occurred and looks like coffee with cream.

if you have white smoke in your exhaust, you have a blown headgasket, and if you 626 is turbo'd and depending to the tubo is cooled eg antireeze, then coolant could be leaking out that way too. also it would be helpfull to know what the steam is coming from. BUT since you mentioned that is coming from the exhaust, it woud wager that its a blown head gasket. you can veryfi this a number of way. take out your spark plugs one at a time and have a look at them. if a couple look "too clean" or seam to "steam cleaned" compared to the other ones = lack carbon build up etc, then the plug that you took out is the cylinder where the engine coolant is leaking into. also, you can do a compression test = which ever cyclinders are super low = blown gasket is located on that cylinder. PS if you do have a blown head gasket, you should replace your oxygen sensor as well because the glycol in the antifreeze wrecks the O2 sensor.