When your 2018 Ford Explorer overheats specifically in slow-moving traffic but maintains normal temperature on the highway, the cooling system isn't providing adequate airflow at low speeds. This pattern points to specific components that can be diagnosed systematically.
Why Traffic Causes Overheating
At highway speeds, air flowing through the grille provides substantial cooling to the radiator without relying on the electric cooling fans. In stop-and-go traffic, vehicle speed doesn't generate airflow, making the cooling fans solely responsible for pulling air through the radiator. If the fans aren't working properly, heat builds up faster than the system can dissipate it.
Cooling Fan System
The 2018 Explorer uses electric cooling fans controlled by the PCM based on coolant temperature, A/C operation, and other factors. The fans may have multiple speed settings or variable-speed control. Fan failure—whether electrical, motor-related, or control module-related—eliminates the primary cooling method in traffic. Listen for fan operation when the engine is hot or A/C is on.
Other Common Causes
A stuck-closed thermostat restricts coolant flow at all speeds but becomes critical at low speeds. A partially clogged radiator can handle highway airflow but can't shed heat fast enough in traffic. Low coolant level from small leaks reduces cooling capacity. The water pump, if failing, may circulate coolant adequately at higher RPM but struggle at idle. Air trapped in the cooling system creates hot spots.
Diagnostic Approach
Start by verifying the cooling fans run when expected. Turn on the A/C—this should trigger fan operation. If fans don't run, check fuses and relays. Use a scan tool to command fan operation directly if possible. Check coolant level when cold and inspect for leaks. Verify the thermostat is opening by feeling the upper radiator hose as the engine warms—it should become hot as the thermostat opens.
Repair Costs
Cooling fan replacement typically costs $400-$700 for the fan assembly plus labor. Fan relay or module replacement is less expensive at $100-$200. Thermostat replacement runs $200-$350. Radiator replacement, if clogged, costs $500-$900. Water pump replacement is more involved at $500-$800 depending on engine. A complete cooling system flush to remove debris costs $100-$150.