If your underskirt is on, I'm pretty sure most fluids from wherever would drain back there. Assuming you have an automatic, you do have a trans. cooling line and thermostat...but otherwise you need to find the leak.

I can't recall a single heater core leaking here, though I'm sure they have. Far more likely another leaking site that's hard to see...like the hard pipes beneath the intake manifold.

Use a good led flashlight and you can see back there.

How much fluid are you losing...you're not overheating?

 

If your underskirt is on, I'm pretty sure most fluids from wherever would drain back there. Assuming you have an automatic, you do have a trans. cooling line and thermostat...but otherwise you need to find the leak.

I can't recall a single heater core leaking here, though I'm sure they have. Far more likely another leaking site that's hard to see...like the hard pipes beneath the intake manifold.

Use a good led flashlight and you can see back there.

How much fluid are you losing...you're not overheating?

Underskirt and reinforcement plate are both off. It's still up on jacks so I can get under there. It's distinctly coming from up high on the transmission, as opposed to just finding its way back there by running down something. Now, that's obviously not to say it's not running back there on top of something.

Yes I have an automatic. And I did the hard pipes when I just did my head gasket. It's probably ran about an hour after that job when this started. This started after I rebled the system because I wasn't happy with the first bleed (hot air from the vents wasn't boiling hot). So I went much slower this time and focused on getting air out of the core just by gently tapping/massaging the heater core pipes then this started, but I also have super hot vent air.

As far as how much I'm losing. Think of turning on your faucet to wear you just have a consistent stream instead of a drip, that's about what I have. Not overheating because I'm not driving it so I never let it get that hot (per OBD Fusion). Just starting in garage, backing out of driveway and back in, etc. Still have some other work to do so I'm just working through everything before I really turn it road ready.

I'll get out my borescope and fish it back to the hard pipes and I'll fish it onto the top of the trans to see where it's coming from. Thanks for the advice, sounds like heater core is a pretty rare failure.

 

The heater core is entirely inside the cabin, but its leaks drain outside via the condensation drain hole, outletting roughly on top of the transmission
This thread shows pics of it.

DIY: Getting to the heater core without removing dash

Hey everyone, I had recently tackled the heater core on my '03 325i and wanted to share a couple pics of how I got to it without removing the dash. Most DIY's I found had you removing most of the dash and I had read other threads that it wasn't necessary. After removing shifter surround...

www.e46fanatics.com

 

The heater core is entirely inside the cabin, so leaks (unfortunately) don’t drain outside.
This thread shows pics of it.

DIY: Getting to the heater core without removing dash

Hey everyone, I had recently tackled the heater core on my '03 325i and wanted to share a couple pics of how I got to it without removing the dash. Most DIY's I found had you removing most of the dash and I had read other threads that it wasn't necessary. After removing shifter surround...

www.e46fanatics.com

Thanks Bali. I think I incorrectly thought the heater core was integrated into the AC housing, and thus might have been leaking out of part number 9 in the attached. I'll go investigate further!

 

Alright, is there something I'm missing about the coolant system?

Checked and fine while engine running;
-Upper/lower rad hoses and water pump/thermostat
-Rear of head coolant sensor and heater inlet pipe that runs along the head in addition to the soft tube that it connects to
-Hard and soft pipe that run from ET to area right by OFH
-Heater core inlet/outlet pipes
-Block drain plug

Whatever it is, it's a somewhat large crack/leak. When I add more coolant to the ET it immediately starts leaking back by trans. Not like it leaks out of somewhere in front of engine then has to trickle back there. It's an immediate cause/effect.

What else should I be checking? All the stuff under the intake was checked with my borescope. I suppose my next step will have to be removing the intake to confirm all those pipes are fine.

 

I think the connection of the upper heater pipe where it connects to the heater rubber hose above the starter (under the intake) is the most natural place to start looking..Other than the heater core connection, that's really the only source of coolant back there.

Under the category of "why not", I would suggest you use a mirror and check where the head meets the block in the back of the engine. It's quite rare, but its very remotely possible the head isn't sitting flush?

 

I think the connection of the upper heater pipe where it connects to the heater rubber hose above the starter (under the intake) is the most natural place to start looking..Other than the heater core connection, that's really the only source of coolant back there.

Under the category of "why not", I would suggest you use a mirror and check where the head meets the block in the back of the engine. It's quite rare, but its very remotely possible the head isn't sitting flush?

Mike, I think you are referring to the same hose? I was able to get my borescope on both ends of this pipe and it's fine. It's also completely brand new within the last four weeks. I'm confident all the coolant hoses aren't leaking at fittings, but I'm going to have to pull the intake. If it's likely a hose instead of the core itself then I need to pull the intake manifold because it's likely cracked/broken in a mid-span somewhere.

And I thought of the space behind the head too so I did the mirror look back there and it's fine too. Mirror, finger, and towel was all dry when inspected back there.

 

Well, it just stopped. I'm just going to keep watching it. I'm wondering if it was pooled somewhere from when I pulled off my head and dumped coolant all over God's creation. But that wouldn't explain why leveling off the ET tank directly caused some to come out.

Anyway, exciting times, I'm down to only cyl 2 & 3 misfires and I get strong negative STFT under power. So smoke test it is for tomorrow!!

 

I get strong negative STFT under power. So smoke test it is for tomorrow!!

Negative fuel trims? That means running rich, which is rare. Most common cause is an over-reporting MAF sensor.
Positive fuel trims means running lean, which is common from vacuum leaks.

 

Leak is back.

Might be time for exploratory surgery? I took off everything down to the throttle body. That let me get a hand on basically everything under the intake. Bone dry. And what I couldn't touch I was able to see with my camera. All clean. For kicks I pulled off the center console to get to the heater core. When I pulled the cover off it gave me the coolant smell, but it's entirely possible that's just the smell coming through the box.

I've seen two other fanatics posts where people say they had a heater core leak drain out of the box (and onto the trans) it sits in without getting their carpets wet. I don't want to take out the core, but unless I've missed something I've put a hand on the other physical connections that flow coolant. Are there any other places? Any other internal passages in the firewall that would run coolant?

 

What about blocking off heater core...or connecting heater hoses to each other (there might be 3 of them?) to see if the leak goes away when you take the heater core out of the coolant flow?

I don't see how the heater core failed all of a sudden after you did all this engine work.

 

To be fair, it's possible this had been happening. I had never heard it run before I got it. I bought it with a toasted head gasket. I did run it a couple times before it happened, but to be fair it stopped itself after my first post.

Age, water & a cooling system pressure tester will go a long way. If it's the core, it should produce liquid draining out of the A/C drains in only a couple of pumps. Provided you've filled enough to get water into the core.

Just to clarify, pressure test as a complete system or pressure test just the core?

 

Age, water & a cooling system pressure tester will go a long way. If it's the core, it should produce liquid draining out of the A/C drains in only a couple of pumps. Provided you've filled enough to get water into the core.

 

KISS: Keep It Simple Stupid.
The whole system.

 

Fair enough. Not to sound stupid here (which I'm sure I will) but what does the pressure test get me that running the car doesn't? Am I more likely to hear the leak if the engine isn't running when I pressure test?

 

Yes. Also with the engine cold, you can perform the test multiple times to get closer to the source and not risk a burn.

 

Having rebuilt two E46 a/c systems in the last couple of years I want to point out a few things.

1. A leak from the heater core will come from above the transmission and drip below the car. The place the coolant will come from is the same place condensation drips out of when the a/c is operating normally in a humid environment. Have a look at the attached video. Time 16:30 you will see the bottom of the unit with the hole where everything drips out of. Time 19:15 you will see where this connects to a rubber boot in the chassis where the fluid is allowed to drip below the car. Your symptoms sound like a heater core leak to me.

2. Best way to check if it's the heater core is to disconnect as suggested below. I normally use a 3/4 inch irrigation connector to join the two heater core hoses under the bonnet so the heater core is bypassed. You can then fill and test to see if the leak continues.

 

A leak from the heater core will come from above the transmission and drip below the car. The place the coolant will come from is the same place condensation drips out of when the a/c is operating normally in a humid environment.

Thanks for the correction, I updated my post above.

 

Victory! Glad I stuck with my gut on this one about the core. When pressure testing (thanks MrMCar!!!!!!) there was an audible leaking sound from the core. And here's the pics. I didn't even need to put it under pressure, that crack is big enough to leak on it's own. The system pressure just exacerbated it. And thanks for the drain video jjrichar, I've added a pic of mine just for people to have future reference.

Thanks for all the help on this one guys. I don't know how to describe it but this leak just didn't seem/look/feel like a normal coolant line leak.

 

So is the heater core accessible from the engine compartment on an E46? or do you have to remove dash?
This is good info as we don't see many heater core failures.

 

I'm honored to be forging somewhat new'ish territory here. Completely accessible by only removing the center console components. Two pics below to show I pulled it out of the HVAC control/radio slot. It probably could have come out of the storage compartment slot if I had shifted the knob back to D, but that wasn't necessary. As an engineer myself I can say one of my Bavarian counterparts definitely did their modeling correctly on this one. It came out quite easily to be honest.

 

Glad you found the source and thanks for sharing your experience and photos.

 

I hope you didn't buy a Chinese Behr.

 

Just checked the FCPEuro site for an "05 330i. There is genuine OEM BMW ($150) and 2 aftermarket heater cores ($50). According to FCPEuro (and things change) The Mahle / Behr heater core was made in Czech Republic . The Nissens was made in China.

Obviously all are warrantied for life from FCP, but I sure don't want to do a heater core twice.

It's possible neither of the aftermarket choices are any good. Another example of how the quality of aftermarket parts supply for 15-20-year old BMWs is anybody's guess.

 

Just checked the FCPEuro site for an "05 330i. There is genuine OEM BMW ($150) and 2 aftermarket heater cores ($50). According to FCPEuro (and things change) The Mahle / Behr heater core was made in Czech Republic . The Nissens was made in China.

Obviously all are warrantied for life from FCP, but I sure don't want to do a heater core twice.

It's possible neither of the aftermarket choices are any good. Another example of how the quality of aftermarket parts supply for 15-20-year old BMWs is anybody's guess.

It's disappointing about the Nissens being Chinese, Sigh.....

 

I went as OE as you can get. A guy on the forums I've picked up some parts from is sending me one.

 

Well, I know why it was leaking. Look how bulged it was. Certainly experienced an over pressurized event at some point.

But on to my next problem. I'm very confident I followed the correct bleed procedure with the heater on low. But the new heater core stayed ice cold and I have no heat. So is my heater control valve not opening? Both lines to the core never warmed up, none at all. So now I'm wondering if my core was never actively leaking before because no coolant was getting there. Then maybe one time when I was cycling the heater it finally opened then started leaking? Thoughts?

 

''Withe the heater on low'' - could you describe exact procedure you went trough?
Low, hight - it doesn't make any difference as fan speed doesn't change anything but amount of air going trough heater core.
If bleeding done properly and no any air left in the system, then your valve either remains closed due-to fail of doesn't receive signal from control unit. Try disconnect wire from valve and see if there is difference. Disconnected valve is open by default.

 

To bleed you do not need to switch heater, as valve stays open when car is off and no key in ignition. Also, you can pour coolant as quick as possible, upto the point coolant shows up from bleeding hole.
As you decide to bleed with climate set for high heat, it is ok too, although ''usual sound'' has nothing to do with heater - you hear flaps opening/closing. Easiest way to check valve is to disconnect it and run engine and see if hoses are hot

 

And of course, I'm just an idiot. Re-bled and now it's fine. Thanks. For kicks I did pull out my valve and was playing with by putting voltage on it, it's a fast little sucker.