Where is the Coolant Going
 

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.

Is it possible that you have a small hole or a worn connection on the hose (small dia.) that goes from the expansion tank to the pump?

I only bring it up as I had mine (4.8iS) pop off while driving and didn't lose very much coolant eventhough I had driven 15km in 44c heat. I'm unfamiliar with the 3.0 so I'm not sure how your line are set-up.

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I had a similar problem with a truck. Three years and I could not find a coolant leak. One day, boom, head gasket failure. Bore scope the #1 cylinder. Does the piston top look cleaner than others? Does the plug look different from others? Even is the leak is small, steam cleaning will occur over time.

I had a pinhole leak where the belt ate through the coolant hose on the bottom left of the radiator. I had the system pressure tested several times no leaks. When we were replacing the clutch fan I spotted a few drops, kinking the tube revealed a nice stream.

I once had a tiny, hidden leak on my 2001 3.0i.

It was at the front end of the heater pipe (hard plastic, not hose) that runs along the left (driver's) side of the head / upper engine block.

It was so slow, and being hidden under a bunch of other stuff, and with a long way to drip and evaporate before it reached a visible area, it took a long time to find it.

Part number: 11-53-1-705-210

In my case, it was a new part I had put in there, which had defective o-rings. Replaced with Genuine BMW, as I should have used from the start.

I had slow leaks in the front end of the hard pipes as well. So slow as not to show on the driveway but I got a Harbor Freight Pressure tester and pumped the system up to 13 psi I believe and came back a bit later to find it had lost a pound or two of pressure. I kept topping off the reservoir until I changed the hoses and hard pipes while doing a larger repair and the problem became obvious when I looked at the ends of the hard pipes.

You need to rule out, or in, the idea that your head gasket is leaking coolant into a cylinder.

Here is how you do it...

1. Run the car to full operating temp one afternoon or early evening...

2. Let it sit overnight,

3. In the morning, before start-up, pull spark plugs and swab each cylinder looking for coolant.

I used a 1 foot piece of flex tube and (securely) taped 1/2 a q-tip to the end of it.

In my case, I found coolant in cylinder 6.

If you're getting that much coolant ingress, you should be able to scope it instead.

Assuming OP has a scope

$10 will get you one off Amazon by tomorrow.