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?

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[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.

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[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


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[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.

[kayger12]

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.

You can drain and strain and reuse the fluid (or some people just drain into a clean bucket and reuse).

[danewilson77]

Temp sensor does not require a drain.

Pull the old one out and put your thumb over the hols, then install the new one.

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[kayger12]

Temp sensor does not require a drain.

Pull the old one out and put your thumb over the hols, then install the new one.

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Or that :)

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[danewilson77]

:)

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[Daddyoh]

Thanks Keith and Dane. Sorry for my delay in response.
I think I'll try the Ninja Quick-Thumb method, lol.
I also think I'll just order a new sensor since this one's got 90k + miles on it. (And I KNOW a refresh is due sooner than later........)
Cheers.

[Hornung418]

Experiencing loss myself. Car just came back from the shop after having the WP and Thermostat replaced. Hoses are relatively new and the ET might be OG. Will have to check the Thermostat and see if its flush. But it's very small coolant loss. Light comes on about once a month.

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[BavarianZHP]

I lost coolant (~100 ml) mysteriously for some reason months ago and reported it here somewhere. Ever since I've topped it off, it hasn't budged from that level. It's odd. I think it may have been the ET Cap letting off some steam. Can't ever be too careful with these systems though.

[BCS_ZHP]

I lost coolant (~100 ml) mysteriously for some reason months ago and reported it here somewhere. Ever since I've topped it off, it hasn't budged from that level. It's odd. I think it may have been the ET Cap letting off some steam. Can't ever be too careful with these systems though.

Might have been an air pocket stuck in your heater core. At a certain unique angle with your heat engaged, it finally got pushed out & the air pocket made its way to the ET. Then you found it, topped it off, hopefully problem solved.

[LivesNearCostco]

Has anyone experienced a cracked or seeping water pipe or heater inlet pipe? I am still losing coolant slowly after replacing radiator hoses, ET, cap (twice), and 4 rubber hoses that go between ET, heater core, water valve, and one or both of these pipes (including #5 in diagram below). But last night while replacing DISA, I noticed what appears to be dried coolant at a pipe connecting to the engine block near where the water pump sits. I believe it was one of these two plastic pipes, #6 or #8 on the RealOEM diagram, and it was near where part #7 (O-ring) is shown for each. Am pretty sure it was pipe #8.
Anyone else had to replace one of these pipes?

http://www.realoem.com/bmw/diagrams/i/g/239.png

[LivesNearCostco]

Nobody here experienced a cracked or leaking coolant return pipe or water pipe? I want to fix it if it's leaking, but I'd hate to remove my intake manifold only to discover that's not the leak.

[LivesNearCostco]

Minor update. I was pretty sure one of my hard plastic coolant pipes (either water pipe or heater pipe) was leaking because my coolant level was slowly dropping and I saw some dried coolant residue where one of them plugs into the front of the block behind the VANOS intake solenoid. However, more recently I also found dried residue on the water pump weep hole. After replacing the water pump, my coolant loss has slowed or stopped. So no longer sure the water pipe is leaking significantly.


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.

[Jon D]

Minor update as well, had the coolant loss again about 8 oz total in 5K miles. Pressure test this time found a leak, surprise :blink at the expansion tank a very very small crack where one of the hoses connect. Replacing the tank and cap should solve the issue once and for all.