I was getting a very strange problem with the coolant level light intermittently coming on and off in winter temperatures after the engine had fully warmed up with the cabin heating running. According to my expansion tank float the coolant level was perfectly consistent and always at the max mark. I did extensive searches and was unable to find a convincing explanation why that was happening. Although cases of this problem and minor variations of it were numerous. The cause of this problem was speculated to be anything ranging from a leaking tank to a bad level sensor. Having examined BMW's coolant procedure, I believe I located the problem:
The coolant level check using the expansion tank float is INVALID unless the coolant is previous filled according to BMW's procedure.
There are two possibilities when the coolant light is erratic:
1. There is an actual leak causing the coolant level to run low
2. The coolant is not filled according to BMW's procedure
As I am pretty sure about the condition of my car, and I did not fill the coolant according to the BMW procedure 4 or 5 years ago, that means the tank float is not giving me a valid reading.
PROBLEM ANALYSIS
The air pocket in the cooling circuit is compressed when the pump is running causing more tank coolant to enter the circuit making the tank level to drop to below MIN level. When the car and pump are off, the air pocket in the cooling circuit expands returning the tank level back to MAX. This produces a low coolant light while car is running, but the coolant level as indicated by the float is perfectly good while the car is off.
The correct procedure for filling the coolant while the engine is cold is as follows:
1. Fill coolant to MAX level while engine cold and off
2. Switch engine on
3. Heater blower to lowest speed
4. Heater temperature to maximum
5. Open expansion tank cap
6. Open upper hose vent screw
7. Fill expansion tank until air-free coolant comes out of the upper hose vent
8. Close upper hose vent
9. Fill expansion tank until MAX mark
10. Close expansion tank and run engine to operating temperature to open the thermostat
11. Engine off
12. Wait for coolant to cool down to cold
13. Open tank cap to add coolant as necessary
14. The correct level when cold should be anywhere between MIN and MAX
I would not recommend opening the vent screw once the coolant is properly filled as that would introduce air into the coolant circuit and invalidates the reading on the tank float. Further more it may degrade the cabin heating performance, and degrade the engine cooling performance.
FIXED
Since I topped up the coolant using the above fill method, the coolant light problem hasn't recurred. I lost a little coolant when I opened the vent screw with the engine running. I think the best way to do this is to not completely unscrew the screw. Once some coolant come out through the threads, that should be good enough indicator all air has been expelled. Anyway, the tank needed 3 small cups of coolant to top up and I am pretty sure that was more than the coolant I lost through the vent.
Last edited by GoingNuts; 02-27-2013 at 08:37 PM.
Hello, Thanks for great tips. Does it work with air conditioning version too ?
Thank you very much for the detailed explanation. I have been experiencing this exact issue with my 2004 330 ci conv. I look forward to my next day off to attempt this fix.
I've been having the same problem too. Has anyone had any luck with the OP's method?
I need to try this but I am not sure where the uppoer hose vent is located on a 2004 X3 2.5. Any help is appreciated.
CAVE LACERTUS
...like, a picture or a description or someone coming to my monster island and pointing at it. Really, any help is appreciated.
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Maybe this is the screw:
CAVE LACERTUS
Thank you!
CAVE LACERTUS
Thanks for the above! I have a few questions and concerns though:
1) step #2: switch engine on. You mean start engine, engine running right?
2) when I filled expansion tank until fluid flowed out of bleeder screw hole, the tank was filled to the very top! If I close up bleeder screw and tank cap as suggested, won't I overpressure system and maybe crack my new tank? I mean I poured slowly and by time the fluid came out of bleed hole my tank was full to the very top.
Any thoughts?
Also: I am replacing bleeder screw with a metal one. The stock plastic screw is too fragile.
And this intermittent light problem started after replacing the expansion tank. Going Nuts suggested the cause might be an air pocket. I agree with all your reasoning, but I wanna mention my heat is excellent. Once car is started and litlle more than halfway to full operating temp, I have heat. So I must not have too much air in there. Never had this intermittent light problem until the day after installing new expansion tank.
Is this an OEM expansion tank or Cheap Chinese Knockoff?
If not OEM, then there is your problem, as it will contain an incompatible coolant level switch:
https://www.realoem.com/bmw/photos/17137553919
I'm having this same issue. I'll be doing this soon!!! Just hope it's not damaging the engine in the meantime.
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Hello, I had a intermittent low coolant light, expansion tank had crack, mechanic replaced that, radiator, top hoses and I replaced the cap with a brand new one. Coolant light is now staying on constantly. Followed the flush procedure to the letter and still the same. swapped the level sensor with the one in the wiper fluid, still the same but the wiper fluid low level still works. When I was following the procedure, (high temp, low fan) I would hear a gurgle and then the coolant would rise then fall. Temp is constant 12:00 position when warmed up, heater blows hot air fine, no sign of any mayo in oil. Anybody know why the coolant light stays on? Also the mechanic was going to change the water pump but he says it is integrated on the same shaft as the power steering pump and would take a lot of labour. This true? Thanks all for any advice.
i had an intermittent issue with the low coolant light when it started getting cold outside, the problem was exactly the one remedied here (i had changed some cooling system items last year and was not aware of the correct bleeding procedure for the coolant system), i followed the instructions noted above, and it worked, no more low coolant warnings.
I've recently experienced the same problem, but at least three years after replacing my radiator and hoses. The light would come on and off intermittently, sometimes multiple times a day, but sometimes only once or twice. In all cases in same weather conditions and on the same route back and forth to work, therefore I could not point to any specific pattern.
I ended up replacing the coolant level sensor and the issue went away for about a week before reappearing again, but with much less frequency. I then found this thread and followed the instructions to fill the coolant. The problem once again went away for few weeks before it came back again. Never with the same frequency as the first time, but it is still occurring at a random.
I can't find any obvious leaks. The only exception was when I reached into the engine compartment (after removing the fan) and wiped underneath the expansion tank where the hose connects. I found some residue of the coolant on the paper. I once again topped off the coolant and will wait for the issue to occur again. When it does, I will check under the expansion tank to see if I find more coolant residue. To me it seems that if there's truly a leak, the light should turn on and stay on and not flip flop randomly. Does anyone know if the on/off behavior means something specifically? is it some sort of an early warning? Would it stay on solid only of the coolant level went down significantly? Does the hose that connects to the bottom of the expansion tank eventually develop a leak at the connector?
Edit: I just realized it looks like you have the N42/N40/N46 engine. On the M52/M54 I6 at least, the float in the tank has a magnetic disc that encircles the sensor when the level is too low. If the float is stuck or broken such that it's sitting in the bottom of the tank, it would be activating the sensor. It could also be a short in the line or the connector for the sensor. I'd check the wires for damage, and make sure the float/level stick work correctly. On the 6-cylinder, replacing the water pump requires only removal of the drive belt—took me maybe 30 minutes to DIY the first time. That said I'm not familiar with the 4-cyl and can't find a good diagram online; it could be totally different.
When the coolant is slightly low, the light tends to come on while the coolant is still cold and go off as it warms up. The expansion tank gives hot coolant a place to overflow from the radiator, thus the level in the expansion tank generally goes up as the car warms up. In a perfect world, assuming your level is only a little low, the warm coolant would expand into the tank, lift the float, and the light should turn off. Theoretically you could have some perfect amount of coolant fluctuating just enough to activate and deactivate the sensor at seemingly random times. It's also possible there's something wrong with the float in the expansion tank—they apparently get brittle from all of the thermal expansion and can be broken by moving the level stick in the wrong way.
In my experience, it can be almost impossible to pinpoint small leaks. It's likely only be leaking while temps and pressure are high, and at a small enough rate that most of it evaporates quickly. I recently had a small leak that I initially thought was the expansion tank, but it turned out to be the radiator. I bought a cheap radiator pressure tester kit and UV flashlight from Amazon for the next time I need to track down cooling issues. Did you replace the expansion tank when you did the radiator and hoses? Those are infamous for hairline cracks/leaks, and the float assembly that sets off the sensor is integrated. Also if memory serves, one of the hoses in that general area is rarely on the replacement lists (I think it's a heater hose, and particularly hard to reach the other end). Could be the o-ring on that hose, if it's original?
Last edited by smoj; 12-15-2017 at 01:21 PM.
Guys check your radiator caps... I
Went through hell and back and all it was just a radiator cap seals corroded( sigh of relief)
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Check radiator cap seals
Morning Guys, My car is a 328i 1999 manual transmission.
I have a similar problem but i have tried the above procedure to bleed the tank and the light went off and came after driving for 5 miles. i have checked if there are any leaks and there was nothing and the coolant is always at max. Secondly i have replaced the pump, thermostat and the sensor because some suggested that they are fucked up. My question to you is :
1.what makes that air inside the engine to stay and make as if the coolant is low but only comes out when bleeding and everything becomes temporarily normal?
2. what damage can that cause to the engine?
Keep more suggestions coming, i'm very frustrated with this.
Regards
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