Mysterious loss of coolant....looking for ideas please

[redwagon]

So I'm looking for a list of things to check.......I'm currently losing coolant at a rate of about 2-4 ounces every 100-150 miles. Essentially if I top it off at the lower mark on the expansion tank indicator, after 100 miles or so it has sunk down so the top is flush with the lip of the tank.

1. I pressure tested (cold/cool engine) and no loss of pressure at 15psi.
2. Looked for stains and cant find anything anywhere of significance.
3. Its not getting worse, a stable rate of loss so far.
4. Only cooling related issues- water pump was replaced about 5K miles ago, and a couple weeks ago I replaced coolant and inspected for any indications of leaking. The 04 ZHP has about 64K total.

I'm thinking perhaps to pressure test when its hot, but that would entail hooking up the pressure tester when cool and getting it hot I assume?
The coolant tank cap is original AFAIK, might replace that but I dont see stains around that. Hoses all look good and no evidence of leakage, but I know at some point soon I know I need to do the major cooling system overhaul, but I want to know the offending part first.

What else to do?.......I dont want to have a small leak suddenly turn into a gusher at a bad time for obvious reasons so trying to get to the bottom of this.

Tim

[ZHPRegistry.net]

The cap is a REAL possibility. When the car is fully warmed up, how tight are the rubber hoses when you try to squeeze them?

[zj96sc]

what does your oil look like?

[telijah]

Your plan of action seems pretty spot on. You'll want to check things while hot as you know, thats when things start expanding and such. The concerning part is you cannot find the signs of the leak, which is likely like it was suggested to check your oil. If coolant is getting into your oil, it may not be a cheap fix.

[danewilson77]

On my old 325i it would only leak when hot. I had these same indications. I think it was due to crack swell when warm. Would the crack below hold 15 psi when cold?.....dunno.

[kayger12]

I'd be taking a hard look at the cap and expansion tank.

[Sockethead]

My 325 coolant loss was due to a cracked head. I could tell that's what it was because the coolant smelled like exhaust. there were no other tell tale signs...

[LivesNearCostco]

In your case did the exhaust also smell like coolant?

Edit: Right before I bought my car, we discovered a very slow coolant leak from one of the radiator end-tanks. But this was at 135,000 miles, so it's not likely to be the cause at 64,000 miles. Still I suspect it was similar to that ET crack... something small that only seeps when hot. Mine was diagnosed because there was dried coolant on the side of the radiator. You could only see it from under the car with the plastic undertray removed.

[Newjack]

I'd be taking a hard look at the cap and expansion tank.

+1
It may have cracked somewhere underneath the expansion tank so you wouldn't necessarily see it. Also a very common failure on the E46.

[ranger]

+1
It may have cracked somewhere underneath the expansion tank so you wouldn't necessarily see it. Also a very common failure on the E46.

Yep - I had that happen with my 325.