* Low Coolant Level: This is the simplest and cheapest thing to check first. Even a small leak can cause overheating, especially under stress. Check your coolant reservoir when the engine is cold.
* Radiator Fan Not Engaging: At highway speeds, your Blazer's radiator relies mostly on ram air (air forced through the radiator by your speed) for cooling. However, at lower speeds or when stopped, the radiator fan kicks in. If the fan isn't working, you might overheat only at highway speeds. Check to see if the fan comes on when the engine is idling and getting hot. There might be a faulty fan relay, a bad fan motor, or a problem with the temperature sensor that triggers the fan.
* Clogged Radiator or Condenser: Dirt, debris, or even insects can restrict airflow through the radiator and condenser (if you have air conditioning). This reduces cooling efficiency, especially at highway speeds where the ram air is less effective due to the blockage.
* Water Pump: A failing water pump might not circulate coolant effectively enough at higher engine speeds and loads, causing overheating.
* Thermostat Stuck Closed: A stuck thermostat will prevent coolant from circulating properly, leading to overheating.
* Head Gasket Failure (Less Likely): While less likely given the sudden onset of the problem, a head gasket failure *could* cause overheating, and symptoms often worsen under load (like highway driving). This is a more serious and expensive repair.
What to do:
1. Check coolant level: This is the absolute first thing to do.
2. Inspect radiator and condenser: Look for any visible obstructions. If you can see significant debris, try cleaning it out gently with water and a soft brush.
3. Listen for the radiator fan: Does it turn on when the engine is warm and idling?
4. Check the upper and lower radiator hoses: Are they hot when the engine is overheating? If the lower hose isn't hot, it suggests a problem with water pump or thermostat.
5. If unsure, take it to a mechanic: Diagnosing overheating can be tricky, and it's best to have a professional check it out if you're not comfortable inspecting the system yourself. The longer you drive with an overheating engine, the greater the risk of serious damage.
Ignoring the problem could lead to serious engine damage. Address this issue promptly.
