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General E46 Forum
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Old 01-13-2015, 10:37 PM   #1
searedtuner
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Low Coolant Light; Low Coolant, but No Leak?

So today, after I thought I was done with my cooling system for awhile, my low coolant light came on. It turned on briefly when I started the car this morning, but it started to come on while driving later in the day. I also noticed at one point the coolant temperature fluctuate from the middle position on the gauge to about the middle of the cold side of the coolant temperature gauge.

I did open the cap and I noticed that the coolant level did look like it was low. I then proceeded to check for leaks under my BMW, but was unable to locate any (aside from the small oil leak from my oil pan gasket I've known about).

Now the weird part is that I have replaced every component of my cooling system in the past 4 months, everything from the water pump, to the thermostat, to the radiator and radiator hoses and even the expansion tank and radiator fans.

I am kind of at a loss as to what is causing the missing coolant.

I suspected that maybe I had an air pocket trapped in the heater core, which was released since I have been using the heater a lot in the past few days (cold blast in Texas :/), but that wouldn't make any sense because the last person to touch the cooling system was a BMW mechanic from a BMW dealership; you would expect a BMW mechanic to properly bleed the coolant system. It also has been close to a month + a thousand or so miles since that happened.

Tl;DR: Low Coolant light comes on intermittently, Looks like low coolant, No leaks, one time temperature fluctuation to the cold side but stays in the middle , New cooling system parts installed by BMW
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Old 01-13-2015, 10:45 PM   #2
Koninkaulus
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Either the dealership didnt bleed it well and the coolant got sucked into heater core (or back of the engine) or you have a leak. There really can't be any other explanation as you said it. Add more coolant and bleed it well. Make sure there is no air in the system. If the same problem occurs you know you have a leak Wouldnt be the first time ppl think theres no leaks but there still is a hole somewhere in the system.
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Old 01-13-2015, 11:02 PM   #3
searedtuner
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Do you think it would be worth it to have a different BMW dealership flush my coolant system?

They want $150 for a diagnostic, but I'm not sure I want to go with that because I can't imagine a leak would form in parts that are only 1 to 4 months old. I have already dumped a lot of money into the cooling system.
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Old 01-13-2015, 11:42 PM   #4
lszlszx
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If it's a slow leak, you won't see any coolant underneath.
Jack up the front of the car and look up toward the water pump and thermostat.
Look for white residue, that gives the leak away.

They must have bled it well, otherwise it would have overheated over 1000 miles.
Another possibility is a bad head gasket and the coolant is burned in the combustion chamber.
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Old 01-14-2015, 12:05 AM   #5
Koninkaulus
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As lszlszx stated the leak may be inside the block. I doubt this, but its possible. If its leaking and mixing with engine oil you may smell the coolant odor coming from the exhaust pipe. Sadly its not the other way around so that you could just check the coolant for oil marks

I am not sure how much coolant you need to drain for the sensor to light up, but the engine wont overheat easily if theres only under 1 litre of coolant missing. I mean you cant say for sure it was bled well in the dealership. The bad thing is that if the engine has overheated (air bubbles in the back of the engine, temperature difference -> crack) then the head gasket is propably cracked and you know where the leak is. The gasket may be cracked anyway. Why did you replace the cooling system in the first place? Did the cooling system fail and the engine overheated? :-|
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Old 01-14-2015, 05:50 AM   #6
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Did you replace the lower temp sensor or temp sensor O-ring in the lower radiator hose?

Very common point for slow leak that is very hard to find.
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Lower hose temp switch O-ring - BMW #13621743299
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Old 01-14-2015, 11:27 AM   #7
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There is very likely a leak, you just won't see it easily. Also with your temp guage on the cold side, your thermostat is getting stuck open and is likely on its way out as well. You may want to look at all the connections from underneath the car as previously suggested
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