If you’ve noticed at some point that your temp needle has been a little saggy, indicating your engine is running cold, you have a problem you shouldn’t ignore. This is almost always caused by a faulty thermostat, and luckily this common wear item is usually not a big task to replace on most cars. The thermostat is a temperature controlled valve that sits right in the coolant steam. The housing it sits in is almost always at the end of one of the main radiator hoses, making it relatively easy to locate.
When the engine is cold, the thermostat is designed to be closed, blocking coolant from circulating through the radiator, hoses, and engine. This helps your car warm up faster, because an internal combustion engine really hates running outside of the specified operating temperature. Once the engine reaches that temperature, the thermostat opens up, allowing coolant to get to work, maintaining the engine at the temperature it prefers.
So why is my engine running cold?
If your engine is struggling to get up to temperature, this is most likely due to the thermostat failing. These parts are designed in a way that when they wear out, and fail, a spring causes them to fail fully open. That way you don’t suddenly find yourself with a stuck closed thermostat and quickly overheating car. With the thermostat stuck open, however, your engine will have trouble warming up as it starts out circulating cold coolant through the radiator immediately. Without the ability to really gain its footing, most cars will struggle to get past the 1/4 hash mark on the temp gauge.
The main problem with this is that shortly after the internal combustion engine went mainstream, car makers started devising ways to allow the engine to get up to temperature faster and faster, as this lowers emissions, improves fuel economy, and makes for a more efficient burning motor. However, if your thermostat is stuck open and the ECU (on more modern cars) or the warm up valves/injectors (on older cars) will be constantly feeding high doses of fuel. This is generally referred to as the warm up cycle, which purposefully causes the car to run a rich fuel mixture in order to warm up faster.
If you perpetually have an engine running cold, it will be stuck in it’s warm up cycling, causing damaging side effects. These are all caused by an overly rich fuel mixture and include poor fuel economy, carbon buildup, and in extreme cases the raw gasoline in the exhaust stream can foul a catalytic converter.
It’s important to note that some cars have different systems, especially German cars which may have thermostats built into other assemblies. Sometimes these could be can be integrated into hose assemblies and water pumps, so keep an eye out. Also, when doing a thermostat it’s an industry norm to replace the engine’s coolant temperature sensor at the same time, just to be sure. While a bad temp sensor can cause anomolies with engine temperature readouts, at least in my experience whenever I have an engine running cold it’s because it actually IS running cold, due to faulty thermostat.
eEuroparts.com offers a variety of kits to help you replace your thermostat, temperature sensor, coolant, and associated components to get your cooling system in good shape with enhanced reliability, which we are all shooting for. Some cars may be trickier than others, so before doing this work yourself it’s always a good idea to get an adequate repair manual.