How a Thermostat Works in the E46 - How to Diagnose a Broken Thermostat

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You can check temp from your instrument cluster. Search for YouTube videos on "E46 temperature instrument cluster", E46:
1.) Turn car on with engine running.
2.) Hold the instrument cluster button for 10 seconds until 1.00 appears, release and press repeatedly until you get to "19 unlock", then leave it there for a few seconds to select it.
3.). Now press the button repeatedly until you get to the function you want.
7 - Coolant Temperature readout.
9 - Battery voltage readout.
4 - Fuel consumption (press the button to cycle to different readouts, l/h, l/100kms, etc)
2 - Instrument cluster test

How I removed thermostat:
1.) Drive the car onto a ramp.
2.) Remove the push pin center fasteners/philips screws from the lower engine protection pan.
3.) Drain the coolant from the radiator drain screw, open bleed screw and cap to increase flow. Optional: Drain the coolant from the engine block by removing the 13mm bolt on the block below cylinder #2(2nd from the front, rear cylinder is #6 closest to the rear of the car), MAKE sure you close the all the other bleed screws and cap before doing this, so that the coolant flows slowly, once the 13mm bolt is out, then you can open the top bleed screw by the radiator cap to increase flow.
4.) Remove the plastic intake duct to the filter box.
5.) Use a 32mm open end wrench and turn clockwise on the fan nut, if you don't have a fan clutch bar to hold it steady, use a hammer to hit the wrench in the clockwise position to unlock the nut, it should come loose with a medium hammer hit, if it is siezed on there, you'll need a clutch bar to hold it steady. You can retighten the same way.
6.) Remove all the bolts for the thermostat, remove the hoses for the thermostat by pulling the locking pins, a flat head can be used to pry the hoses away from the thermostat.(The plastic will break if it's brittle).
7.) Clean the surface with a flat scraper, be careful of the sharp edges of the aluminum block.
8.) Use an RTV silicone meant for water pumps, and create a bead along the outside of the new gasket on the new thermostat. These thermostats have been known to leak due to a poor rubber gasket(BMW should us a high quality silicone gasket), so the RTV silicone is a must if you don't want a leak.
9.) Install everything as you removed it, except the radiator cap and top bleed screw, leave those loose since you'll be adding coolant.
10.) With the engine off, fill the system with approximately 8.5L of 50/50 antifreeze. Any antifreeze meant for new engines(aluminum) blocks will do. To avoid a mess, wrap a rag around the bleed screw and radiator fill port to catch radiator fluid.
11.) Turn on the engine, the coolant that was full will now be drained by the water pump, top up the coolant once again.
12.) Drive the car down from the ramps, put the cabin heater on high, fan on high. Rev the engine a bit(~3500RPM max, or the radiator fluid will splash out of the fill port), pump it a few times to move the air out from the system to the radiator cap/bleed screw location. Top up the coolant once again as required.
13.) Give it 5 minutes to heat up and open the thermostat, rev the engine a bit and add coolant as needed.
14.) Once coolant starts to leak from the bleed screw, close it. Now the engine should be warm, the thermostat should be open, and the air bubbles should be out of the system. Now the system should be full of fluid, and have no air bubbles.
15.) Install the radiator cap, drive the vehicle around the block a few times, getting it revved up to about 4500RPM should get the air bubbles out, nothing crazy like 6500RPM is required.
16.) Park the car, and let it cool for at least 2 hours, then open the bleed screw beside radiator cap again, it should release air, as soon as fluid comes out close it, and your system should operate properly.

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