A common problem area on the early BMW E30 3 Series cars is constant idle problems. In my tireless quest to solve this annoying gremlin, I have found that one possible culprit is the coolant temp sensor. This sensor is used to provide the idle control unit with information to keep the car idling before the oxygen sensor heats up enough to take over mixture control. In my case, I had found that my car would idle at about 1500RPM when cold, the drop to normal. I started checking everything, and found that this switch was faulty. I ordered the new switch from Pelican and once installed, it cured the idle problem.
Keep in mind that this article is specific to my 325is, however the article applies to all BMW models in general. That said, I decided to do a small tech article showing how to replace this sensor and hopefully provide you with some insight on this common idle problem and how to correct it.
The first step is to open the hood and locate the sensor. In this case, the sensor is located in the thermostat housing. Locate the sensor and remove the electrical connection from the top by pushing in the metal clip and pulling it off.
Now use a 19mm deep socket to remove the old sensor. It should come out with no resistance, however if there is a lot of corrosion around the switch, it's a good idea to clean all this off prior to removing the sensor. I've heard a lot of different ways to remove rust, but believe it or not, the best way I've found is to go to your local vending machine and buy a can of Coke. A toothbrush and some Coke will clean up just about any rust you can imagine. (this is another convincing argument why soda pop is bad for you.) just get it clean and pull the old sensor out.
Take a look at the new and old sensors side by side. You can see the discoloration on the old sensor. This is most likely an indication that years of hot coolant passing over the sensor has eventually caused oxidation on the surface, which in turn causes the sensor to build up internal resistance, causing it to eventually fail.
Now look at the thermostat housing make sure the area around the housing is clean and free of any dirt. You don't want this to get into the coolant. Just wipe away any grime or grit. Now, place the new sensor in the housing and use the socket to tighten it in place. Do not over tighten, as you can damage the threads. Now, reconnect the electrical plug for the sensor and start the car. Your idle should now be between 700-900 RPM and smooth like Shaft.
You may also want to bleed the cooling system to get rid of any air that may have entered the system. There is a small screw on the top of the thermostat housing. Start the car and loosen the screw. This will allow any air to bleed out. When you see straight coolant coming out, the system is bled.
Well, there you have it - it's really not too difficult at all. If you would like to see more technical articles like this one, please continue to support Pelican Parts with all your parts needs. If you like what you see here, then please visit our online BMW catalog and help support the collection and creating of new and informative technical articles like this one. Your continued support directly affects the expansion and existence of this site and technical articles like this one. As always, if you have any questions or comments about this helpful article, please drop us a line.