Blue Plug

The Blue Plug is the Temperature Sensor on all fuel-injected engines. It provides coolant data to the ECU.

If you are looking for a solution to your Temperature Gauge problems, learn more about the Brown Plug

Operation

A cold engine needs more fuel pumped in than a warm one. Putting too little fuel into a cold engine will prevent the engine from starting, while putting too much fuel into a warm engine will lead to rich running, high carbon deposits and possibly pre-detonation of the fuel (pinking). To control the fuelling, the ECU needs to know how warm the temperature is, and it gets that information from the Blue Plug.

The Blue Plug is a simple thermistor - as the temperature around it changes, it changes its electrical resistance. In this way, it forms part of a simple circuit to the ECU, informing it of the temperature of the engine so that the ECU can adjust fuelling accordingly.

Faults with the Blue Plug can prevent the car from starting, or cause the engine to run too rich.

Location

The location of the Blue Plug is different on each engine:

• On M10 and M20 engines, it is located in the thermostat housing, at the end of the intake manifold
• On M40 engines, it is located on the cylinder head, under the intake manifold

Testing

Testing the actual Plug is a relative simple affair, requiring a multimeter.

• Locate the Plug and remove its loom connection
• Set your multimeter to measure resistance (Ohms)
• From cold, expect a reading in the region of 3500-4000 Ohms. Any more or less than this is not a huge issue.
• Reconnect, warm up the engine, and retest
• From hot, expect a reading in the region of 330 Ohms. If your Blue Plug has not changed its resistance significantly, then it is at fault and should be replaced.

However, for Blue Plug-related issues, it is also important to test that the information is reaching the ECU. To do this:

• Locate the ECU and disconnect its plug.
• Repeat the Blue Plug test, this time putting your multimeter electrodes onto pins 24 and 45 of the ECU.
• Confirm that the readings you got at the ECU are the same or similar to what you read at the Blue Plug.

If the readings differ, then the most likely cause is corrosion of the C191 plug.

Replacing

To remove the Blue Plug:

• Locate the Blue Plug and the Brown Plug on the engine.
• Disconnect the loom plug by pressing in the metal tab and pulling up gently
• Use a 19mm ring spanner or deep socket to unscrew the Brown Plug
• With this out of the way, use the same tool to remove the Blue Plug
• Replace Blue Plug and refit Brown Plug and loom connections

Cold Start Relay

Despite their technical knowledge, BMW had problems with cold starting on some M20 engines. To cure this without a fundamental redesign, a Cold Start Relay was devised to fool the ECU into thinking that the engine was already warm.

To identify whether your car was fitted with a Cold Start Relay, look for an extra relay fitted to the outside of your fusebox.

This relay is connected so that it switches on when the starter motor engages, and connects a resistor in parallel with the temp sensor, artificially raising the level of the coolant data to the ECU.