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| − | The Blue Plug is the Temperature Sensor on all [[Fuel|fuel-injected]] engines. It provides coolant data to the [[ECU]].
| + | #REDIRECT [[Blue_Temp_Sensor]] |
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| − | If you are looking for a solution to your Temperature Gauge problems, learn more about the [[Instruments#Temp_Gauge|Brown Plug]]
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| − | __TOC__
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| − | =Operation=
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| − | 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 [[Fuel|fuelling]], the [[ECU]] needs to know how warm the temperature is, and it gets that information from the Blue Plug.
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| − | 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.
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| − | Faults with the Blue Plug can prevent the car from starting, or cause the engine to [[Fuel#Rich_Running|run too rich]].
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| − | =Location=
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| − | The location of the Blue Plug is different on each engine:
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| − | * On [[M10]] and [[M20]] engines, it is located in the thermostat housing, at the end of the [[Intake#Intake_Manifold|intake manifold]]
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| − | * On [[M40]] engines, it is located on the cylinder head, under the [[Intake#Intake_Manifold|intake manifold]]
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| − | =Testing=
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| − | Testing the actual Plug is a relative simple affair, requiring a multimeter.
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| − | * Locate the Plug and remove its loom connection
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| − | * Set your multimeter to measure resistance (Ohms)
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| − | * From cold, expect a reading in the region of 3500-4000 Ohms. Any more or less than this is not a huge issue.
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| − | * Reconnect, warm up the engine, and retest
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| − | * 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.
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| − | However, for Blue Plug-related issues, it is also important to test that the information is reaching the [[ECU]]. To do this:
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| − | * Locate the ECU and disconnect its plug.
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| − | * Repeat the Blue Plug test, this time putting your multimeter electrodes onto pins 24 and 45 of the ECU.
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| − | * Confirm that the readings you got at the ECU are the same or similar to what you read at the Blue Plug.
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| − | If the readings differ, then the most likely cause is corrosion of the [[C191]] plug.
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| − | =Replacing=
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| − | To remove the Blue Plug:
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| − | * Locate the Blue Plug and the Brown Plug on the engine.
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| − | * Disconnect the loom plug by pressing in the metal tab and pulling up gently
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| − | * Use a 19mm ring spanner or deep socket to unscrew the Brown Plug
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| − | * With this out of the way, use the same tool to remove the Blue Plug
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| − | * Replace Blue Plug and refit Brown Plug and loom connections
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| − | =Cold Start Relay=
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| − | 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.
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| − | To identify whether your car was fitted with a Cold Start Relay, look for an extra relay fitted to the outside of your fusebox.
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| − | 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.
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