* No cabin heat: Obviously, you'll lose heat in the cabin. This is the primary reason people *use* the heater core.
* Potential for overheating (in rare cases): While the engine coolant will still circulate, bypassing the heater core removes a small amount of cooling capacity. In extremely hot conditions or with already marginal cooling system performance, this *could* contribute to overheating, though it's unlikely.
* Air in the cooling system: Bypassing the heater core can make it harder to properly bleed the cooling system of air. Air pockets can impede coolant flow and lead to overheating or uneven cooling.
* Potential for corrosion: Coolant is designed to prevent corrosion. If coolant doesn't circulate through the heater core, that section of the system becomes stagnant and more susceptible to rust and scale buildup. This is more of a long-term concern.
In summary: The engine itself is unlikely to be directly harmed by a temporary bypass, but it's better to fix the underlying problem (a leak in the heater core or hoses) rather than bypass it. A bypass is a temporary workaround for troubleshooting, not a permanent solution. If you bypass the heater core, closely monitor your engine temperature gauge.
