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. That results in more fuel economy and more power overall. 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. Driving on a bad thermostat like this allows the computer to run the engine on a fuel map that is too rich. The X5 did not warm up fully even after 20 minutes of driving, so we know for sure that the thermostat is bad. It turns out that the E39 reached operating temperature within about 7.5 minutes of a cold start. Here’s the video of the experiment below: I took each car on the same route so driving conditions were also an unchanged variable. 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 define operating temp loosely: where the water or temperature gauge needle is centered on the meter. 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. The E39 warms up very fast, and the X5 was taking forever – so we knew something was up. It is even more noticeable to us in particular, because we also have an E39 530i with the exact same 3.0L M54 engine. Fall and now Winter has set in here in Connecticut, and we noticed our 2005 X5 was taking forever to get to operating temp. Most people notice their car is taking too long to warm up once the weather starts to turn cold. 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. 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. 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. If the thermostat fails stuck open however, it can be less obvious and can go unnoticed for a long period of time. In more severe cases, head bolts can strip the weak threads in the aluminum block of an M54 model engine. 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. 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. 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.
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