Tribute: Mystery of the Disappearing Coolant

[ZooOOMM!]

I have to refill coolant about every 100 miles with no apparent leak. Temp remains in middle until fluid becomes low then rapidly rises.

I still have to perform more "run it and stare at it" trouble shooting, but here are some details:

First look:
1. I don't see any fluid on ground, or pooled anywhere. Thought I saw a tiny bit at top right of radiator, but could have been residual from servicing or perhaps very small seep.
2. Fluid is clean, as is the engine oil.
3. There is no apparent white "steam" leaving the exhaust.

Second look:
1. After filling expansion tank, I run until warms, then watch tank began to lower as thermostat allows coolant to flow. I continue to add until level stays at full line and absent low light on dash. I have now run it at approx 1500-2000rpm for a while with cap off to bleed air out.
2. Now, after running at 15-2000rpm with stable coolant level, when releasing throttle cam and allowing it to idle, coolant quickly back-flows into expansion tank and overflows. It is possible that this is happening out on the road, but unseen (I don't drive the car often) thereby explaining no fluid on driveway.

Next up:
1. Stabilize coolant level, then run it and stare.
2. Considering getting a pressure leak check at a shop.

Possibly related? or not?
1. Just started getting a "whiring" noise near belts and an acrid burn smell.

Any troubleshooting ideas?

Filler:
-2001 Tribute, 6cyl 3.0 litre, 170k miles
-already replaced thermostat

Thanks
.

[StealthWyvern]

First of all do a coolant system presure test and report back... you can rent them from autozone.

[ZooOOMM!]

First of all do a coolant system presure test and report back... you can rent them from autozone.

Great news!! Had planned on pressure check in the morning - but already had the equipment for another "run-it-and-stare-at-it" test. Found a crack in the radiator. It was barely visible and appears to be mostly sucking rather than leaking. The fluid loss is coming from the overflow on the expansion tank as it gurgles the air out periodically. Had seen this before, but expected it was simply bleeding air after refilling the system. Think it is happening out on the road - so no fluid in driveway, and evaporating before I get a look at it. Will replace radiator tomorrow and hopefully be done with it.

Thanks for your response - thought it was going to be more of an interesting mystery.

[StealthWyvern]

Glad to hear you figured it out... sucks its the rad and not somethine like a loose clamp.

[ZooOOMM!]

True, would have preferred -- clamp. But still, quite a bit better than my gut feeling of -- head gasket with the ... oh wait ... you also get a bonus warped head for the overheat -- discovery.