Noticing your coolant reservoir boiling without any engine overheating is indeed perplexing but not uncommon. The bubbling you’re witnessing is likely due to air bubbles entering the system rather than the coolant itself being overheated. A common culprit for this is a leak in the radiator cap, which can allow air into the system and cause the coolant to bubble.
Other potential sources of the problem include leaks in the water pump, overflow tank, or even the head gasket. Your vehicle’s coolant system operates under high pressure and is designed to be airtight. Any breach in this system can lead to a decrease in its efficiency and the bubbling effect you’ve noticed.
Given the variety of potential issues, it’s crucial to get a professional diagnosis. Delaying a visit to the mechanic can lead to more severe damage and, subsequently, higher repair costs. We recommend scheduling an appointment with a trusted automotive specialist to pinpoint the exact cause and remedy the situation promptly.