BMW Hard Starting When Warm – Engine Temperature Sensor
This answer is applicable for many BMWs in addition to the one listed below.
I have a 1998 528i touring with 115,000 miles that starts fine when cold, but starts hard (cranks about 2-3 seconds) when hot. If I start it in the morning it starts immediately. When at operating temp (after driving to work), if I turn it off and then try to restart, it will start right up. However, if I let it sit 30 minutes to a couple of hours, it takes some cranking to fire up (will sometimes kick when cranking but takes much longer than is normal). Problably been this way for 3 or 4 months. When it does start, there a fuel smell in the exhaust like it’s running rich …. only on start up. When it’s running, it runs fine; smooth, good idle, good power. No lights lit on dash (Check engine, etc). Spark plugs and all coils (from you guys) were replaced about six months ago. Air filter is a K&N drop-in filter and is clean. Everything else is stock. Fuel filter is about a year old. O2 sensors were replaced and Mass Air Flow sensor was cleaned three weeks ago. I replaced a couple of cracked air hoses around the intake and idle control valve. I’m all out of ideas. Any help would be greatly appreciated.
The symptoms that you’ve described would be consistent with a faulty engine temperature sensor. All BMW models with electronic fuel injection (L-Jetronic, Motronic and all late model applications) use a temperature sensor that monitors the engine coolant temperature and reports back to the ECU (Engine Control unit). In this case, the net effect is that the sensor is “cooling down” too quickly and causing the engine management system to richen the fuel mixture (because it thinks the engine is colder than it is) which floods the engine. The short shut down periods are OK, as the sensor has not started to detect a slight drop in engine temperature yet. However the mid-length shut down periods allow the sensor to start to detect that the engine temp is decreasing and the faulty sensor changes its resistance too quickly …. showing a signal to the ECU that is incorrect. Finally, the longer periods, allow the engine to actually cool down to the point where it can handle the sensor’s resistance change (and corresponding ECU mapping),
Of course, the above points are made without any actual diagnostics, but this would be a common issue that seems to line up with your symptoms.
The Bentley repair manual covers the engine temperature sensor replacement. Basically, you just remove the harness plug , unscrew the sensor and screw in the new one. Note that the sensor is installed into an area where it is exposed to the engine coolant, so you will lose a small amount of coolant during the interchange. Most sensors are relatively easy to access. Again, the Bentley repair manual covers the location. On your 98 528i, the sensor is mounted in the cylinder head, on the intake side, below the intake manifold runners for cylinders #1 & #2. Some other models have the sensor mounted in the thermostat housing or (on V8s) in the water crossover pipe at the rear of the cylinder heads. Additionally, there are some other less common applications.