Overheating & low coolant in 2017 GMC Yukon XL Denali. Coolant flush or thermostat?

The symptoms you're describing with your 2017 GMC Yukon XL Denali – overheating, especially in specific conditions, and low coolant warnings – point directly to an issue within your cooling system. At 90,000 miles, both a coolant flush and a thermostat replacement are common maintenance or repair items.

Here's a breakdown of potential causes and actions:

1. Diagnosing the Issue:

   • Low Coolant: The first step is to identify why your coolant is low. Check for visible leaks around hoses, the radiator, water pump, and coolant reservoir. A pressure test of the cooling system can help pinpoint leaks that aren't immediately obvious.
   • Overheating Symptoms: If the temperature rises quickly and stays high, it could be a thermostat stuck closed. If it fluctuates, or only overheats under load, it could be a partially clogged radiator, a failing water pump, or air in the system.

2. Coolant Flush:

   • Recommended Interval: GMC typically recommends a coolant flush every 5 years or 150,000 miles, whichever comes first, for Dex-Cool coolant. At 90,000 miles and 7 years, your Yukon is due based on time.
   • Benefits: A flush removes old, degraded coolant and sediment that can reduce cooling efficiency and cause blockages. It ensures fresh coolant with proper anti-corrosion properties is circulating.
   • DIY vs. Professional: A coolant flush can be a DIY job, but it requires proper disposal of old coolant (which is toxic) and careful refilling to avoid air pockets. Using the correct type of coolant (Dex-Cool, specifically) is critical. For a first-timer, especially with overheating issues, professional service is often safer.

3. Thermostat Replacement:

   • Function: The thermostat regulates engine temperature by controlling the flow of coolant to the radiator. A failing thermostat can get stuck open (engine runs cold) or stuck closed (engine overheats).
   • Symptoms of a Failing Thermostat: Rapid overheating, especially after starting, or the engine taking a very long time to warm up. Your symptoms of overheating in traffic suggest it might be struggling to open fully or at the correct temperature.
   • OBD Codes: A failing thermostat can often trigger a Check Engine Light with codes like P0128 (Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)) or other temperature-related codes.
   • DIY Replacement: Replacing a thermostat is a moderately difficult DIY task. It involves draining some coolant, removing hoses, and replacing the thermostat housing. Ensure you use an OEM-quality thermostat and gasket.

When to See a Shop: Given the combination of low coolant and overheating, it's highly recommended to have a professional diagnose the exact cause. They can perform a pressure test, check for exhaust gases in the coolant (indicating a head gasket issue), and accurately assess the thermostat's function. Addressing a cooling system issue promptly is crucial to prevent severe engine damage, such as a blown head gasket or warped cylinder heads.