Minor coolant loss

[Jon D]

As anyone had a very minor coolant loss with no apparent reason?

Car showed low coolant light added about 12oz everything checks out fine including pressure test. Any thoughts about what it could be or just one of those things?

[danewilson77]

Leak in the exp tank.

What mileage do you have on your cooling system?

Sent from my SCH-I545 using Tapatalk

[BCS_ZHP]

Try replacing your expansion tank cap, the rubber o-ring seal in there deteriorates over time. If you see some dried white/blue coolant residue drip marks on the side of the ET then it was likely the cap. Also check the coolant temp sensor on your lower radiator hose, the rubber o-ring in there goes bad over time, you'll see residue below it on your engine cover drip pan. And then there's a weep hole on the water pump shaft, it's hard to see that one unless you're on a lift, again the would be coolant residue on the engine block.

[danewilson77]

I did this as suggested, and it did not solve my slow leak. It's all good though because a new tank doesn't come with a new cap.

Sent from my SCH-I545 using Tapatalk

[BCS_ZHP]

Was thinking the ET cap because if it was pressure tested and no leaks appeared, then the only part of his cooling system that wasn't subjected to the pressure test was the ET cap itself. But a small leak off the w/p shaft is very hard to see too, unless they had the car in the air on a lift.

[Dave1027]

If you need to get a new cap, might as well go with the E30 cap which is 1.4 bar.

[ELCID86]

If you need to get a new cap, might as well go with the E30 cap which is 1.4 bar.

+1

I also learned something today on e46f:
"For information, the [ET] cap seals by two O-rings that slide down inside the opening of the expansion tank. It's not a seal at the top (like a garden hose connection). You could just have it finger tight and it would seal correctly. It has sealed well before it gets to this point. The only reason to have it hand tight is so it doesn't come loose."

[Jon D]

Thanks guys all of the above was double checked so will have to watch and see, there were 0 signs of drips etc and the car was on a lift. FWIW car is just under 50K


Sent from my iPad using Tapatalk HD

[LivesNearCostco]

I also have a slow coolant loss. In the past it was worse and I think it was either bad aftermarket ET cap or ill-fitting non-OEM replacement ET (both Myele). But I still get a slow loss after replacing ET and cap with genuine BMW. All coolant and heater hoses replaced along with thermostat and fan temp sender. Water pump has 110,000 miles but is reputed to be an EMP Stewart. Will check weep hole next time I remove undertray. No signs of coolant contamination in oil. Only other item I can think of not replaced was the base plate on which the ET rests.

[Daddyoh]

Also check the coolant temp sensor on your lower radiator hose, the rubber o-ring in there goes bad over time, you'll see residue below it on your engine cover drip pan.

As best I can tell, mine is leaking now at the coolant temp sensor. I was surprised at the puddle that was on the garage floor the other morning.
I guess the system needs draining to replace the sensor???
I just had it flushed in October.......crap.