The cap should be tight. It is a problem if it isn't. However if coolant is being lost at that rate from the cap it would leave witness streaks all over the expansion tank, even if you never saw liquid coolant it would be impossible to miss those marks.

You can add UV dye to the cooling system and pressure test using a borrowed kit from Advance Auto or similar. The immediate suspicion I'd have are the plastic hardlines that run along the intake side of the block, underneath the intake manifold. A leak there can conceivably evaporate as it trickles down the hot engine block and leave few to no drips on the ground. But before you pull the intake manifold and go through that trouble, test the system.

 

the reason that the bleeder screw broke is that it was improperly tightened down some time prior yo the incident. The body of the screw is plastic, and has an oring that makes the seal. it shouldnt be tightened down all the way, just until sealing is attained. there is a way around this, replace the screw with a brass one, however as before, dont just torque it til it wont turn anymore, the body it attaches to is made of plastic as well.

 

I feel your pain. I'll go many months without any coolant loss and then get a small leak somewhere. It's been a long time since i've had one but these two stand out.

One of the leaks was from the thermostat housing. It WASN'T the gasket that failed but the O-ring that seals the electrical connector. I tried to replace the O-ring but couldn't find one that would seal. Thank you FCP lifetime warranty! I replaced it again and finally have a good one. Wahler that was replaced with another Wahler.
Check this thread: https://www.e46fanatics.com/threads/wahler-thermostat-leak.975417/

The other recent leak was from the upper radiator hose. Went thru 2 Rein hoses. (Again thank you FCP warranty). The plastic kept breaking at the lip by the O-ring. Both leaked after a year or so. Went with a Gates (Amazon special- trust me...better than socks...haha) and it's been holding up great.

Honestly best way to check is pressure testing as Shame said. Rent one from Advanced or Autozone. Get a bright flashlight and inspect.
I've had success with this dye when the leak was so small I couldn't find with pressure testing: https://www.amazon.com/gp/product/B008I2VOCA/ref=ox_sc_saved_image_7?smid=ATVPDKIKX0DER&psc=1

Pray it's not the hard coolant lines under the intake manifold. Not a horrible job, but not fun either.

 

Pray it's not the hard coolant lines under the intake manifold. Not a horrible job, but not fun either.

However, I would recommend replacing those hard lines regardless of whether they are the source of the leak or not, as it is long past time for them to be refreshed.

There is a lot of good advice in this thread so far. The biggest takeaway is to buy or rent a coolant pressure tester as it takes the guesswork out of the equation.

 

Search forums for cooling overhaul before you regret big time.

Use only brand names mentioned in these forums.

Stay away from AutoZone stuff...

 

the reason that the bleeder screw broke is that it was improperly tightened down some time prior yo the incident. The body of the screw is plastic, and has an oring that makes the seal. it shouldnt be tightened down all the way, just until sealing is attained. there is a way around this, replace the screw with a brass one, however as before, dont just torque it til it wont turn anymore, the body it attaches to is made of plastic as well.

HI I have a 2006 BMW X3.. I love the vehicle. I purchased it used 2 July 2022. I serviced the Transfer case, transmission, oil change, plugs, oil leaks fixed, valve cover gaskets NEW radiator, water pump, and new bleeder screw, which broke suddenly. change. The problem I seem to be having is loos of coolant, every 2 days or 1 day I have to add about less than a cup of coolant.. to keep the coolant pin-up. WHY am I losing coolant.. I notice the CAP is never staying very tight. This is a coolant cap from Autozone. COULD this be the problem? I have no leaks on the ground of coolant being lost...
Pleas someone can help with a solution.

I had a slow leak on my wife's 2008 X3 that took me a long time to discover, mostly because I wasn't looking in the right place. While replacing the oil filter housing gasket, I spotted the tell-tale white residue from coolant on top of the coolant tank. The old cap tightened fully, though offering somewhat high resistance to twisting on, and seemed okay physically. However, no denying the evidence of the leak, including white residue inside the cap but outside the seal once i looked closely. A new cap from FCP cured the leak; about as quick, easy, and inexpensive a leak repair as is possible on a BMW.

 

Do a cooling overhaul.

 

I am not getting any fluid leak from the coolant cap at all. This is what i do each day: open cap, add about 1/4 or less of water to make sure the rod is about 1" above. If i wait and drive a 2nd day without doing this...about 1 mile down the road the coolant light comes on. this is annoying. Because i should not have to make sure I have coolant in this car each and every day

 

Bring it to a shop.

 

Have you performed any diagnostic work to find the cause: pressure test the cooling system, leak down test, look for presence of exhaust gases in coolant, oil in coolant, or coolant in the oil?

 

Thank you for your reply. I will do what you have suggested. there is no oi in the coolant ..that we did check.

Say for instance that all of the above is OK..
Do you think it could be a bad coolant resivoir? I say that becuase. the top is constantly loosening on it's own. This is brand from AutoZone.. I have purchased a new cap and it still does the same thing.
Do you think I should get a better brand altogether?

 

I use either a Coolant UV dye kit ( avail on AMZN often have to buy UV dye separately- get the coolant-safe type) or a pressure test.

drop the belly pan and put fresh cardboard underneath to help spot leaks.

look for a puff of white smoke on the morning start-up...you may have coolant in cylinders.

Once in a while, you have to pay the man on issues you can't figure out yourself. Find a good non-dealer BMW specialist and bring it to him.

 

Your FLAPS is generally not a good source of parts for your BMW. With the cooling system of your X3 being arguably one of its weaker systems, it's even more critical to use either OE BMW or proven aftermarket brands. Most of us buy our BMW parts from online sources like FCPeuro, Eeuroparts, RMEuropean, AZautohaus, ECSTuning, etc. These vendors tend to offer parts from a very select few vendors that have proven reliable (not always, so still be careful). Having experienced the joy of an exploding aftermarket expansion tank on an e46 (same part used on your X3), I will only buy genuine BMW tanks as replacements. I DO use an aftermarket cap, but it is identical and from the same supplier as the BMW OE cap. I use a lower pressure cap like this: More Information for STANT 10246 (rockauto.com) . I have had good luck with REIN aftermarket hoses, but some posters will suggest only OE BMW.

At this point, follow duke's advice regarding testing the system. You can likely get away with some of the aftermarket parts for a while, but testing may reveal the need to replace something now due to leakage. Also, make sure you're following your engine's bleed procedures to a "T"! Bleeding a BMW cooling system is not like bleeding a Toyota.

AM.

 

@softdiamond This thread is nearing two months old and you aren't following any advice given here. Which is totally fine, it's your car, but why keep posting if you're just going to ignore everything? I'm sure you'll figure it out yourself eventually, just hopefully the realization is not accompanied by severe overheating and engine damage.

Good luck.

Edit: Re-reading the above I realize I come across as rude. My intention is not to make you feel bad or anything, just to point out that you're on a forum requesting advice and it took 4 members including myself, telling you the same thing for you to even acknowledge their advice.

 

I had a similar problem with a coolant leak. Turns out the hard plastic tube that runs from the radiator to the expansion tank was rubbing a wiring shroud and it wore a hole in the tube which leaked coolant. I think it’s a common problem since I was able to find a YouTube video of a guy fixing the exact same problem:

 

This video is for an F25 and therefore not applicable to the OP, who has a pre-LCI E83.
OP really needs to use a coolant pressure tester, as multiple people have suggested, instead of just guessing.

 

Agree with other posters, don't use non-oem expansion tank cap and especially a lower pressure one. The cap is designed to release pressure if it goes too high, and not release if it's below that. Raise the car, take the bottom shield off and look for white residues. Follow it up. I had coolant leaks from multiple locations over the years. Hardest to locate are the ones under the intake manifold. There are a few coolant hoses attached to the engine with plastic fittings that can break. The o-rings on those fittings can leak. Then oil filter coolant o-rings can leak too.

 

It's somewhat irrelevant to the OP's issue, but just to clarify... the substitution of a lower pressure ET cap is a fairly widely accepted practice on BMW's such as the E83 that inexplicably came fitted with a 2.0 bar cap. Prior and subsequent to this generation, BMW used a lower pressure cap. For instance, the LCI E83's were fitted with an N52 engine and a 2.0 bar cap, while the newer F25's and E70's that were fitted with an N52 used a 1.4 bar cap. 2.0 bar (29 psi) is very high for a cooling system and can contribute to some of the catastrophic failures that have been common in this generation (e.g., bursting ET's and hoses). The E30 model used the same cap, but in a 1.4 bar pressure release. By allowing pressure to begin releasing at 1.4 bar, you may be able to avoid a catastrophic failure, though even at 1.4 bar, your system is already overheating.

As I said, it's somewhat irrelevant to the OP's issue of leaking, as it's not the cause of his problem. I was just throwing it in in case he was buying a new cap anyway - might as well get a safer cap while he's at it.

AM.

 

Attacking Mid, I would agree with you and everything you said makes sense. Over the years I've also tried many of the recommendations for "improvements" people posted on the internet. Lower temp thermostats, hotter spark plugs, oiled air filters, etc. What I also learned is today's cars, like the BMW, have so many sensors and computers that work together that when I have issues or check engine light come on I can't be sure what the cause is. So, I've defaulted back to keeping it in stock as much as possible. My thought is BMW engineers are much smarter than me and have designed and thoroughly tested the car to work even in the harshest conditions. If the coolant system bursts, I think there are other issues, and using a lower pressure cap is just pushing the true issue down the road. If the N52 engine mistakenly came out with an incorrect pressure cap, I would expect BMW to issue a service recall to replace them.

I might still occasionally switch out to higher friction/temp brakes and poly bushings but only on the cars I drive alone or track.

Cheers

 

... So, I've defaulted back to keeping it in stock as much as possible.

This is generally wise, IMHO, and I agree.... there are a lot of "upgrades" that really are NOT upgrades! This particular one, however, doesn't involve any sensors and only has any effect once you've developed an overheat situation. There's no downside to releasing that pressure at 1.4 bars instead of holding out until 2.0, and releasing sooner may allow you to escape with a smaller parts bill. My theory as to why BMW spec'd that 2.0 cap was to reduce environmental impact of releasing coolant onto the ground. As with many well-intentioned decisions, I suspect the net effect was to greatly INcrease the volume of coolant released due to so many catastrophic failures resulting in near complete purging of the system. They redesigned the ET, reduced the pressure relief to 1.4 bars, and exploding ET's mostly became a thing of the past.

A year ago, I had a water pump failure in my E53 (virtually identical cooling system as pre-lci E83) that resulted in an overheating situation. I managed to escape with my ET intact to which I give likely credit to the lower pressure cap reducing the pressure spike that could have burst the ET. I learned about exploding ET's with my first BMW - an e46. I managed to blow 2 of them for a doubly-good education!

AM.

 

Most every water cooled engine pressurizes the cooling system to 15 psi. Stock caps for this chassis regardless of engine run 30 psi. I’ve gone even lower than @Attacking Mid and have gone to 1.2 bar (17.4 psi). When the cooling system fails, and it will, the lower pressure caps allow the cap to vent the excess pressure, rather than the tank itself, or in the case of N52, the Mickey Mouse flange. BMW is not going to retroactively call for a lower pressure cap on 20 year old junk; they don’t care about you or old junk, or the car outside the warranty period.

For the record, I’ve been running the 1.2 bar cap for 110k miles, and now at 206k miles, have had far less issues with mine than folks were posting about on these cars back when they were still under warranty 15 years ago. If you look at a graph of boiling point elevation due to pressure, you’ll see that coolant mix is more important than pressure. Plastic fatigues over time and can’t handle the increased pressure demands. Having a small split in an expansion tank has allowed me to get my vehicle the 12 miles home leaking coolant, whereas an exploded tank is a no-sail condition. These cars aren’t complicated.