How do you tell if your BMW has a bad thermostat? Thermostats fail in one of two ways; they either fail stuck open or fail stuck closed. When they fail in a closed fashion its usually easy to tell because your car will likely experience severe overheating problems with the temperature needle buried in the red. This is the most dangerous type of failure because an overheating engine can cause major damage, including head gasket failure and cracks in radiators and cooling system components. In more severe cases, head bolts can strip the weak threads in the aluminum block of an M54 model engine.

If the thermostat fails stuck open however, it can be less obvious and can go unnoticed for a long period of time. That’s because the temperature gauge needle won’t bury itself in the red; instead the car will take forever to warm up to operating temperature. The downside of this is loss of fuel economy, and the engine to run rich for a longer period of time which can result in more unnecessary wear. Most modern electronic thermostats (such as the ones found on late model BMW’s) are designed to fail in the stuck-open position these days to avoid ruining the engine by overheating.

Most people notice their car is taking too long to warm up once the weather starts to turn cold. Fall and now Winter has set in here in Connecticut, and we noticed our 2005 X5 was taking forever to get to operating temp. It is even more noticeable to us in particular, because we also have an E39 530i with the exact same 3.0L M54 engine. The E39 warms up very fast, and the X5 was taking forever – so we knew something was up. I decided to do a test to see just how long each car was taking to warm up to operating temps from a cold start. I define operating temp loosely: where the water or temperature gauge needle is centered on the meter.

We took each car out on the same day, one right after the other so that the operating conditions and outside temps were the same. I took each car on the same route so driving conditions were also an unchanged variable. Here’s the video of the experiment below:

It turns out that the E39 reached operating temperature within about 7.5 minutes of a cold start. The X5 did not warm up fully even after 20 minutes of driving, so we know for sure that the thermostat is bad. Driving on a bad thermostat like this allows the computer to run the engine on a fuel map that is too rich. This map is intended to be run only at start up and for a few minutes until the car reaches operating temperature, at which point the ECU will modify or switch maps to run more lean and less fuel. That results in more fuel economy and more power overall. Running for 20+ minutes on a startup fuel map not only reduces fuel economy, but can increase wear on internal components because the fuel to air mixture is not ideal. Excess carbon buildup on internal components and maintenance items such as spark plugs can result.

In short, it’s important to replace a faulty thermostat like this as soon as the problem is identified. Faster warm up, proper running conditions and improved fuel economy are just some of the benefits you’ll experience once you tackle the job. Get a new BMW thermostat for your car here. See the next video below where I demonstrate how to replace the thermostat and water pump on an M54 6 cylinder engine:

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