What Causes Fluctuating Car Engine Coolant Temperature?
3 Answers
The reasons for fluctuating car engine coolant temperature include: 1. Faulty coolant temperature sensor: The sensor directly contacts coolant to measure engine temperature. If it fails, the ECU will use substitute values when detecting unreliable signals, causing unstable temperature gauge readings. 2. Damaged thermostat: This affects normal engine operating temperature, leading to unstable temperature gauge. 3. Cooling system malfunction: Check for leaks in the engine cooling system and whether the water pump continues working. 4. Insufficient coolant.
My old car had this exact issue, and it took several attempts to figure it out. The temperature gauge needle would swing like a rollercoaster—sometimes hitting the red line, which was terrifying, and then dropping back down. Upon inspection, we found the thermostat was stuck. When it gets jammed halfway, opening and closing intermittently, the coolant circulation goes haywire. Another time, we discovered the radiator was half-clogged with willow catkins, and even with the fan running at full blast, it couldn’t dissipate heat. The scariest scenario was low coolant levels—air trapped in the system can cause false boiling. My advice: pull over immediately and feel the radiator hose. If it’s scalding hot, don’t push your luck—calling a tow truck is far cheaper than repairing an engine.
Running a repair shop for twenty years, I've seen this issue commonly fall into three categories. First, check the coolant level - if it's below the MIN line, air can enter the system causing vapor lock. Second, mechanical failures, particularly focus on whether the thermostat spring is worn out; it should open at 87°C but gets stuck, only suddenly releasing when temperature spikes. Finally, test the radiator fan's high/low speed settings - poor connector contact can cause intermittent operation. Last week, a Tiguan had its fan resistor burnt out; at 110°C coolant temp when the fan should've been at full speed, it only ran at half, causing the dashboard gauge to fluctuate wildly.
