Losing Coolant - Any Advice?

[AudiB5owner]

Greetings B5ers. I haven't been active in a long time but I thought I'd pose a question for y'all.

Issue: Mysterious coolant leak
Symptoms: Low coolant, heater not working

Tests performed
Compression test: 170, 170, 170, 175
Visual: no external leaks, pass floormat dry
Coolant: Dark colored, smells like rust. No oil in the coolant.
Oil: Normal

I ruled out a bad headgasket. Does coolant always circulate through the heater core even when the heater is off? I haven't used it in over a year until a few days ago and discovered that it wasn't blowing warm air. Is it possible for coolant to leak inside the K03? Any thoughts on where the coolant went?

Thank you for your input!

[Wrath And Tears]

With age its often common for the heater core to become clogged with debri, reverse flushing the heater core with a garden hose will often clear out the gunk and let coolant flow better. Doesn't explain the coolant loss though. Sounds like you have ruled out internal leaks so external seems likely. Are you able to pressure test the cooling system? With the amount of dried coolant by the overflow hole it could be your coolant cap o-rings are flat and you are slowing loosing coolant from the cap, or the coolant reservoir has an issue and isn't letting the cap seal properly. Both are fairly common issues.

Slim chance your thermostat could be stuck closed but then you would probably have overheating issues with the coolant loss.

[redline380]

How long does it take for the coolant to go low?

I would first pressurize the system and see if you can find any obvious leaks.

[nynoah]

I would also add that: that coolant looks like it needs to be changed out. So I would look at doing that first and flush your heater core. That fluid looks so old that it looks like it is dirty which will cause issues. Also when fluid is that old it tends to evap off too. I would also look at the bottom of your expansion tank. They can crack and fluid will slowly leak out the bottom.

[AudiB5owner]

Thanks for the feedback folks. I will double check the seal, but it should be fairly new as the tank and cap was replaced probably 20k miles ago. I just noticed that the coolant was low a couple days ago so I will have to monitor the time it takes to drop again. Will definitely do a pressure test. A flush is on my list as I'm certain the core is clogged.

Another person told me that some headgasket cracks cannot be detected with a compression test but I'm not sure. I will keep an eye on the levels.

[Avant Nate]

That white residue looks like coolant is getting by the cap.