[Aziego]

I need some help on possible causes of a slow coolant leak that I have been trying to diagnose for a couple months now. I have a 2017 330i xDrive F31. Every few weeks I need to add 4-8ounces of coolant to the large reservoir to top it off. I took it to a mechanic that specialize in BMWs, he pressurized the system and cannot detect any leak. I had replaced the cap to the reservoir as well since I read that is a common failure point. In the last week I put tracer dye but can't find any evidence of leak either.
I thought maybe a head gasket leak but the exhaust doesn't have the white smoke or the smell. I also changed the oil and did not find traces of the dye there either.
I am completely dumbfounded on where to look to fix this problem. Any suggestion would be highly appreciated!

[jsouth]

Is the system telling you to add coolant? Or do you see the level low in the reservoir?

My F30 with N20 is probably a slightly different reservoir than yours, BUT it is a classic symptom of the slow failure of the coolant level sensor to tell you to add coolant. In reality, the coolant is fine, the sensor is the problem.

Unfortunately, the sensor on my car cannot be replaced separately - it is an integral part of the expansion reservoir. It is a quick and easy repair, and the reservoir is plastic and not too expensive.

If you actually see a level drop, then you may need to look elsewhere. In my case, the error message became more and more frequent over a period of 2 years, so I replaced the tank. It was a good opportunity to replace the aging coolant anyway.

[Aziego]

It is actual coolant drop. The first time I discovered it, the reservoir was dry that ultimately triggered the sensor to tell me that the level was low. After refilling I checked it for 2 weeks after replacing the cap. It held until out of the blue it dropped again.

It is leaking somewhere, I just can't find it. A possible explanation is that there is something lose that only leak if I hit a hard enough bump.

[Owz88]

The B58 seems to vary between using no coolant, some supicions of a lot of them being underfilled from factory so one low warning before filling up and no further issues, to what you are experiencing of regular and significant coolant vanishing.

Mine is in a similar boat, main reservoir loses 200-300ml but on a roughly six month timescale. No evidence of leaks and it's parked in a clean garage with epoxy floors so it isn't going anywhere when parked and no obvious smoke etc.

This has been the case with mine for two years now, dealership have pressure tested the system with no issues, I haven't replaced the cap so it may be that causing my issues, either way a few hundred ml every 6 months isn't annoying enough for me to chase down further but if it is every couple of weeks for you i'd be wanting further investigation.

The second page of this thread might help - https://f30.bimmerpost.com/forums/sh....php?t=1738305

Some people in there in a similar boat (regular usage over tens of thousands of miles without any other symptoms) - some successes with small amounts of silicone grease where the o-rings seat for the res cap which I might try myself.

[Aziego]

I’ll give the silicone grease a try. Also I heard of the turbo causing the leak but I thought that impact the B58 not the B48. What is the best way for me to check the turbo if the leak is there?

[SeattleLSB]

Bigger or smaller tank? I had my air to water intercooler have a small rock hit it and slow leak.

The updated the design in mid-to-late 2017 and added rock guards.

When they replaced mine they added those so to won't happen again.

[Aziego]

My leak is from the larger tank.

[auburnf30x]

The newer two tank systems are seemingly becoming notorious for developing small leaks that will come and go...

At various times, I've had a small leak in both tanks that I was unable to locate despite replacing the caps, some hoses, even trying to track it down with UV dye and a black light. After all that, the leaks would continue and then suddenly stop inexplicably. I haven't had either tank leak in several months. Which probably means I'll be getting another one soon.

I've stopped worrying with it and just remember to check the level in both tanks at each oil change and top off as necessary. If the leaks were to suddenly become faster, then obviously that is a different story.

If you want peace of mind, however, the best way to check for mysterious coolant leaks that cannot be located with an obvious wet spot is with UV dye and a black light. Add the dye to the problem tank, drive it for a day or two (or more if your drives are short), and then shine the light around and see what glows.

[Aziego]

I already did the dye and can't find anything. I am just checking up on the level every few weeks now. As you said, the symptoms are very odd. I can drive 2-3 weeks and the coolant level stays the same. Then one day it just drops.

[auburnf30x]

When I dye checked mine, I found some light glowing around a few hose connections so I replaced them. It didn't stop the leak, however. That stopped on it's own a few weeks later for no apparent reason.

I always found some dye around the caps, but that's to be expected. Replaced both. At the time that did slow, but didn't stop the leak. Never any trace of oil in coolant or vice versa.

I honestly have no idea where it goes.

[Aziego]

3 weeks since I made the original post. Coolant went from max to min in the large expansion tank. Still no obvious sign of leaks, no traces of the dye at all in the engine bay. What's the best way to look for an internal leak. Oil analysis?

[JoHnMuir]

Check the weep hole of the water pump for dripping and heavy residue deposits. The leak there isn't obvious because it boils away on the hot engine. It's what happened to me, I only noticed because I checked the coolant level after running the engine for a minute and saw the side of the engine was wet.

[auburnf30x]

Check this thread out:

https://f30.bimmerpost.com/forums/sh....php?t=1889354

I just found out that there is a coolant line prone to failure on these cars, sometimes they start out as a slow leak from the main tank. There is an updated part. I'm replacing mine soon as preventative maintenance.

[Aziego]

Originally Posted by JoHnMuir

Check the weep hole of the water pump for dripping and heavy residue deposits. The leak there isn't obvious because it boils away on the hot engine. It's what happened to me, I only noticed because I checked the coolant level after running the engine for a minute and saw the side of the engine was wet.

I will check, is the weep hole on top of the water pump? How did you repair it? Was it just tightening a screw to close the weep hole?

[Aziego]

Originally Posted by auburnf30x

Check this thread out:

https://f30.bimmerpost.com/forums/sh....php?t=1889354

I just found out that there is a coolant line prone to failure on these cars, sometimes they start out as a slow leak from the main tank. There is an updated part. I'm replacing mine soon as preventative maintenance.

Thanks, I ordered a new hose to see if that does the trick.

[JoHnMuir]

Originally Posted by Aziego

I will check, is the weep hole on top of the water pump? How did you repair it? Was it just tightening a screw to close the weep hole?

On the B48, I think it's behind the lower half of the pulley. You'll need an inspection mirror to see, for sure. Repair involves replacing the pump, as the packing has failed if it's leaking. Slight moisture won't be a worry, but dripping is.

[F32Fleet]

Originally Posted by Aziego

I will check, is the weep hole on top of the water pump? How did you repair it? Was it just tightening a screw to close the weep hole?

The weep hole is there to let you know that the pump is failing.

[Aziego]

I had been looking at the wrong spot all this time. I finally found the location of the weep hole after I received a replacement pump in the mail. It is really difficult to get a clear view of and I had to use my phone almost as a periscope to view it. I am now confident the leak is caused by the pump. Now I just need to find time to replace it.

[1slowb58]

Originally Posted by Aziego

I had been looking at the wrong spot all this time. I finally found the location of the weep hole after I received a replacement pump in the mail. It is really difficult to get a clear view of and I had to use my phone almost as a periscope to view it. I am now confident the leak is caused by the pump. Now I just need to find time to replace it.

Where was the fluid leaking on the ground with this issue? I just noticed a pretty sizeable amount of green fluid about dead center of the hood.

[Aziego]

Originally Posted by 1slowb58

Where was the fluid leaking on the ground with this issue? I just noticed a pretty sizeable amount of green fluid about dead center of the hood.

No, my issue was there isn't any visible water leak at all. I believe when the pump starts failing maybe only steam or a very small amount of coolant is leaving through the weeping hole. It will cause my tank to go low every few weeks.

Your problem can be the vent line, check this out.

https://f30.bimmerpost.com/forums/sh....php?t=1889354

[JoHnMuir]

Originally Posted by Aziego

No, my issue was there isn't any visible water leak at all. I believe when the pump starts failing maybe only steam or a very small amount of coolant is leaving through the weeping hole. It will cause my tank to go low every few weeks.

Yep, that sounds about right. Mine was bad enough to empty the reservoirs every week and even then it was only leaking enough fluid to moisten the side of the engine.