Where is the Coolant Going

[zonefive]

I'm losing coolant at the rate of about 2 oz per 100 miles for about the last 2,500 miles. I've taken it to two BMW indies.

The first could find no leak and said he was stumped. Pressure tested the car overnight. The second said he found a radiator and a thermostat leak via pressure test and he replaced both.

After the radiator and thermostat were replaced, the low coolant light came back on after 400 miles. The indy who replaced the radiator/thermostat told me there were no external leaks after a second pressure test but that they car smoked on start up after the pressure test and told me I need a new motor because the engine is consuming coolant somehow. Compression of all cylinders was over 200 except for #1 at 175.

I've owned the car since new. 2001 3.0 207,000 miles. The car has never overheated. The engine has no power issues, never smokes on regular start up, idles smoothly.

The second mechanic told me that it cannot be the expansion tank, which was replaced 35,000 miles ago, because it did not leak under pressure. The water pump was replaced 65,000 miles ago. They said they checked the coolant pipes going in to the engine and they had no leaks.

The first mechanic told me to keep driving the car until the source of the coolant loss revealed itself, and he doubted there was a crack in the head or block, or a head gasket failure because there would be other symptoms besides a slow coolant loss.

It's tough because I feel nervous driving the car for extended distances -- don't want to get stranded by the problem ultimately getting suddenly much worse.

Tempted to replace the rest of the cooling system --- hoses, expansion tank, and water pump and see if it stops the problem.

Advice appreciated.