Low Coolant Warning - can't find a leak

[MAE90M3]

Over the weekend I got a low coolant warning when I started the car up after sitting overnight. It has gotten pretty cold up here (not sure if that is related), but the message would go away for the rest of the day. The car's temperature remains perfectly normal, and the fans are not running hard/more than usual. The car got a new radiator/hoses, and a coolant flush in January of this year. Threw the car on a lift and it is dry, and there is no sign of a leak in the engine bay. I opened up the expansion tank and it was at or slightly below the minimum. I am going to top it off and see what happens.

There has to be a leak somewhere. Any ideas? No other codes are being thrown, the car runs great, and no white smoke.

[tbuck]

One of the nice things about the BMW blue coolant is that when it dries it leaves a tell-tale bright white crusty residue (unlike the green crap which leave a dark sticky film). Short of pressure testing the system, this has helped me find leaks many times over the years. And a sharp drop in temps will often show a leak coming from o-rings that are near failure and may be just fine at higher temps. If you had it in the air any leak should have left evidence for sure. Sounds like it is not much of a leak which always makes it harder to find. GL!

[Wally1552]

Longshot but maybe the coolant sensor went bad?. It located under the expansion tank. One Time I forgot to plug sensor back in and it gave me a low coolant warning. Just something to check.

[MAE90M3]

Originally Posted by Wally1552

Longshot but maybe the coolant sensor went bad?. It located under the expansion tank. One Time I forgot to plug sensor back in and it gave me a low coolant warning. Just something to check.

The expansion tank was at to a bit below the minimum, so there is definitely a loss of coolant from somewhere. Right?

[Helmsman]

Check the hoses going from expansion tank and in/out from the radiator (the hose connector to the Tstat housing is sensitive, marked in blue).
When I replaced Tstat I got a significant leak due I didn't get the o-rings from the Tstat housing right. The coolant leaked into the engine V, plenty of it so find it ways down to the floor. If the leak is small, maybe the engine heat is evaporating the coolant just enough so it doesn't poor down to the floor? if so, this is possible to see using a flash light into the V, without taking off the plenum.

[MAE90M3]

Originally Posted by Helmsman

Check the hoses going from expansion tank and in/out from the radiator (the hose connector to the Tstat housing is sensitive, marked in blue).
When I replaced Tstat I got a significant leak due I didn't get the o-rings from the Tstat housing right. The coolant leaked into the engine V, plenty of it so find it ways down to the floor. If the leak is small, maybe the engine heat is evaporating the coolant just enough so it doesn't poor down to the floor? if so, this is possible to see using a flash light into the V, without taking off the plenum.

I think we have a winner. I believe there is some coolant in the engine V. Not a lot, but I was able to get a sample with a thin plastic rod. There was a greenish blue fluid on the it (obviously the sample is contaminated with a bunch of stuff).

So this is most likely the o-rings from the Tstat housing?

[Helmsman]

Originally Posted by MAE90M3

I think we have a winner. I believe there is some coolant in the engine V. Not a lot, but I was able to get a sample with a thin plastic rod. There was a greenish blue fluid on the it (obviously the sample is contaminated with a bunch of stuff).

So this is most likely the o-rings from the Tstat housing?

Sounds like a good lead, it should be dry in the V. Not difficult but a bit of work to get there so if you want to try to make sure first I'd suggest you poor water into the V (there's a drain on rear side off engine so no problem with being generous with water), and get down there with some cloth on a stick and clean up as good as possible. Wait and see if some new arrives.

I haven not had any problems with Tstat O-rings leaking all of a sudden (in my case I screw up when mounting new rings), once seated I'd assume they are fine, but maybe can happen. Regardless if you confirm coolant coming this way there isn't any other place it can leak really.

If you end up mounting new rings better replace the Tsat as well ($30). And, be sure you grease the rings before assembling...

[MAE90M3]

Originally Posted by Helmsman

Sounds like a good lead, it should be dry in the V. Not difficult but a bit of work to get there so if you want to try to make sure first I'd suggest you poor water into the V (there's a drain on rear side off engine so no problem with being generous with water), and get down there with some cloth on a stick and clean up as good as possible. Wait and see if some new arrives.

I haven not had any problems with Tstat O-rings leaking all of a sudden (in my case I screw up when mounting new rings), once seated I'd assume they are fine, but maybe can happen. Regardless if you confirm coolant coming this way there isn't any other place it can leak really.

If you end up mounting new rings better replace the Tsat as well ($30). And, be sure you grease the rings before assembling...

Thanks! The Tstat was replaced a few years ago, but if it is only $30 then I might as well!

[us722]

Originally Posted by Helmsman

Sounds like a good lead, it should be dry in the V. Not difficult but a bit of work to get there so if you want to try to make sure first I'd suggest you poor water into the V (there's a drain on rear side off engine so no problem with being generous with water), and get down there with some cloth on a stick and clean up as good as possible. Wait and see if some new arrives.

I haven not had any problems with Tstat O-rings leaking all of a sudden (in my case I screw up when mounting new rings), once seated I'd assume they are fine, but maybe can happen. Regardless if you confirm coolant coming this way there isn't any other place it can leak really.

If you end up mounting new rings better replace the Tsat as well ($30). And, be sure you grease the rings before assembling...

Any recommendation what to use to grease the rings? Also, rings to grease are also the ones around thermostat?

[serranot]

Originally Posted by us722

Any recommendation what to use to grease the rings? Also, rings to grease are also the ones around thermostat?

Just use coolant.

[Helmsman]

Originally Posted by us722

Any recommendation what to use to grease the rings? Also, rings to grease are also the ones around thermostat?

Guess one "should" use silicon grease on rubber but I happened to have a good marine grease around which is petroleum based. Personally would not risk the new rings using coolant only, they are fairly fragile. Ask me how I know...