Questions and Answers

BMW 325ic Slowly Overheats Q. Hello, I have a 1990 BMW 325ic. 2.5 liter, automatic transmission and 126,000 miles. I have an old overheating problem. I recently replaced the cylinder head, water pump, thermostat, heat sensor, the one that operates the auxiliary fan, cooling fluid, all belts, fuses and all relays. However, my car still slowly overheats in traffic or when I am at a stop.

The only thing that I have noticed was that the auxiliary fan doesn't work when the water heats up. However, the sensor, relays and fuses are new. When I unplug the sensor and jumper the pins together I can get the fan to work at either low speed or fast speed which means my harness and the motor is working. Please help! I am loosing sleep. I don't want to believe that its just a radiator flush or something like that. The sensor gets pretty hot but still doesn't start the fan.

Looking forward to hear from you...

A. It sounds like your auxiliary fan is definitely what is causing your car to heat up in traffic. You've done some of the troubleshooting steps but just need to perform a few more. You will need a thermometer and a multimeter with an ohms scale.

Warm the engine up to operating temperature and place your thermometer where you can get a temperature reading near the sensor's location. Note, the reading, is it hot enough to activate the sensor, usually 200° F? If not you may have an air pocket in your cooling system. The sensor itself won't work unless it is immersed in liquid.

Open the bleeder valve, near the thermostat housing while the vehicle is still running until a steady stream of coolant comes out, then close it. If this solves the problem your temperature reading will go up and the cooling fan will activate soon after.

If the fan doesn't activate, pull the connector from the sensor and measure the resistance across the connections (meter on the RX1) scale. The reading should be 0 ohms. If not you have a bad sensor. You can verify this by pulling the sensor and immersing it in boiling water with the multimeter leads alligator clipped to the terminals, check again for 0 resistance.