Stumped by coolant loss

[RenaissanceMan]

Hey folks-

My '95 525it is losing coolant and I can't figure out where the leak is coming from. The level goes from the cold fill line to triggering the sensor about every 4 or 5 days. The radiator appears to be leak-free, all hoses appear to be intact, there are no leaks coming into the interior, and there is no coolant in the oil. I also just changed the water pump and the thermostat and there are no leaks coming from either of those components. Any suggestions on what to check for next would be greatly appreciated. Thanks!

[Joefresso]

Check cap and bleed screw. I just had same problem where it was coming out p the bleed screw because it wouldn't tighten down (piece under screw was broken and just kept spinning). When coolant got hot it would come through there slowly and burn off so you don't really notice it. Same thing could happen if you have bad rubber washer on expansion tank cap. Good test is tighten everything and give the top radiator hose a squeeze. You shouldn't hear or feel any air come out of bleed screw or cap. If you do, you have leak. I just replaced my radiator. Pretty easy task, but bleeding system is pita

[RenaissanceMan]

Check cap and bleed screw. I just had same problem where it was coming out p the bleed screw because it wouldn't tighten down (piece under screw was broken and just kept spinning). When coolant got hot it would come through there slowly and burn off so you don't really notice it. Same thing could happen if you have bad rubber washer on expansion tank cap. Good test is tighten everything and give the top radiator hose a squeeze. You shouldn't hear or feel any air come out of bleed screw or cap. If you do, you have leak. I just replaced my radiator. Pretty easy task, but bleeding system is pita

Thanks for the reply-

I tried the radiator hose squeeze suggestion and I'm not seeing any leaks. I've actually already had to replace the radiator last year, as the expansion tank had a leak. It is still in great shape. Wherever the leak is, it must be a very small one. BTW, I agree on the bleeding job being a huge challenge. Have any tips on making that job go a bit quicker? :)

[Russellc]

check the aux water pump near the firewall. Mine was leaking and losing coolant while driving so it was not apparent when parked.

[Joefresso]

Only advise is give yourself enough time. Need to warm engine to bleed, then level goes down but have to wait until completely cool to fill back. And that process might need to be repeated several times. Also, might want to carry around some coolant with you for a day or two in case you need to top off

[RenaissanceMan]

check the aux water pump near the firewall. Mine was leaking and losing coolant while driving so it was not apparent when parked.

Thanks for the reply, Russell. I actually found and fixed a leak from that exact same location several months ago. I've checked that area again, and it's totally dry. :95


Only advise is give yourself enough time. Need to warm engine to bleed, then level goes down but have to wait until completely cool to fill back. And that process might need to be repeated several times. Also, might want to carry around some coolant with you for a day or two in case you need to top off

Thanks again, Joe. Seems it's pretty time consuming no matter what. :)

[RenaissanceMan]

Ok, I need some more feedback from y'all, if you don't mind... I finally found the source of the coolant leak. There is a tiny amount of weeping near the bottom of the attached expansion tank next to a seam. Now, this is the 2nd radiator that I've had develop an expansion tank leak with this car in the last 5 years. The place I get these from offers a lifetime warranty, but after they replace this one they say they won't do another replacement because there must be something else wrong with the car that caused the radiator to fail, alluding to the fact there must be something causing there to be too much pressure in the system. Now for the record, this car never overheats nor behaves erratically and otherwise runs as well as can be expected for a 200k BMW that has been well-maintained. Would you agree that there is something else causing this problem, or am I just experiencing a case of bad luck with replacement radiators? Could there be a problem with the head?

[RVAE34]

I think it is possible you have a problem with the head/HG. I know this problem all too well as I had a cracked head when I first got my car (M50 non vanos) and had the same problem recently with a blowthrough from HG failure at cylinder #6. I went through 2 radiators before I found out.

The first thing you need to do is a compression and preferably leakdown test. That will tell you for sure whether it's the head/HG. If not then you can look at other things.

[Russellc]

Not so sure anything unusual with your car. The e34 cooling system is well known weak point. Water pumps and radiators fail quite often. Many repace these parts every 60,000 miles. Mine is on it's third radiator and at least 2nd water pump at 204,000 miles. I accept it and go on. BTW, you could repace the expansion tank and seal. This may help.

[91e34M5]

Have you tested or changed your radiator cap? Maybe it's allowing too much pressure to build.

[RenaissanceMan]

Have you tested or changed your radiator cap? Maybe it's allowing too much pressure to build.

Haven't changed it or tested it yet... However, it is relatively new. How do I test it?

[91e34M5]

If it's new, then it's probably OK. About the only way they can be tested is in a shop with a pressure tester for the colling system.