850 Heater Core PSA: potential for catastrophic coolant loss Topic is solved

Post by **atucker1** » 04 Jul 2025, 01:00

This may be documented elsewhere, but wanted to provide a warning of how quickly a leaky heater core can progress. Although the typical heater core leak may be a slow drip, it has the potential to be a source of massive coolant loss, as happened to me.

My 1996 850 had a slow heater core leak (a few drops here and there, appeared pretty minor) so I was waiting for a few months to replace it. Suddenly, while driving onto the highway, the leak blew open with a gush of coolant. Probably lost 2-3 liters into the cabin in a matter of 30 seconds! Thankfully was able to pull over before the car got close to overheating, and the car is now dried out and with a new core installed. But a reminder not to let a leaky heater core sit since a slow leak can turn into rapid, potentially catastrophic coolant loss without warning! Don’t be lazy like me and let it sit!

Post by **abscate** » 04 Jul 2025, 02:47

Im guessing it was an end cap failure that did this? If you can , please post pictures of the old core so we can see the gory details

Post by **atucker1** » 07 Jul 2025, 00:12

Will do when I can! It actually isn’t particularly impressive visually, no clear separation or anything. Essentially it just started leaking around the entire circumference of the end cap.

Post by **FireFox31** » 09 Jul 2025, 01:21

Where did the fluid leak out? I'm always terrified that a heater core leak will spray burning coolant at my legs while driving at highway speed. The shock and pain would surely make me lose control. Please tell me it doesn't play out that way.

Oly850 · Post by **Oly850** » 09 Jul 2025, 13:37

Well this is terrible isn’t it? I was much more interested in seeing the grenaded heater core than anything, and it was just a pedestrian leak that turned into a very large leak and left us none the wiser.

Post by **abscate** » 15 Jul 2025, 05:45

Here’s a picture of early heater core seep on one of my 1999s, found during a hvac evaporator R/R

I might have added 100 ml coolant every 6 months to this car?

I think it is dated 2014 and we’ve added about 80k miles to that, estimated.

I didn’t notice that the Al frame is lifted where it is seeping, I wonder if that’s related?

Where did the fluid leak out? I'm always terrified that a heater core leak will spray burning coolant at my legs while driving at highway speed. The shock and pain would surely make me lose control. Please tell me it doesn't play out that way.

Rest assured, it does not. Generally it sprays out into your crotch area. Sleep easy

5underpressure · Post by **5underpressure** » 16 Jul 2025, 09:34

On the upside, bypassing the heater core is a reasonable roadside repair.

Post by **Sveedy** » 16 Jul 2025, 13:16

Rest assured, it does not. Generally it sprays out into your crotch area. Sleep easy