Cracked intake manifold
A cracked intake manifold can cause coolant to enter the combustion chamber and ignite with the fuel/air mixture. This will result in white smoke exiting the exhaust. It may also lead to a reduction in engine power, as the fuel/air mixture may leak out.
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If the intake manifold is made of plastic, it has a different tolerance and response to engine heat. As the engine temperature rises, the metal expands. The manifold may not expand at the same rate, or the stress of the expansion may be too great. This force would warp the manifold and can cause it to crack.