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2000 BMW leaking water

I have a 2000 323i BMW and have had water leaking out of it on and off for the past few years. I have tried replacing the hose which has not helped and have been unable to find a leak. Sometimes I refill the car with water and it will be empty in a few hours and there is no clear pattern. It will lose water when it is parked but has also happened when I am driving. Any ideas?

Are you sure it’s not condensation from the AC system? That would be normal.

If you are losing coolant, you need to find a shop that will do a coolant pressure test.
You should replenish a 50-50 mixture of water and coolant, not pure water.

Do you see water under the car after it has been parked? Can you tell what area the water is leaking out if it from?

I’ve seen water pumps that leak intermittently only when the engine is running.
I suggest filling the system when cold, starting the engine, then watch carefully under the hood and under the car while it warms up.

As the owner of four 3-series BMWs, I fear that I have some bad news for you. It does not really matter where the leak is coming from. At 14 years of age, virtually every plastic or rubber part of your cooling system is due for replacement. Any part of that cooling system that you have not already replaced is overdue. It would be nice to figure out where the leak is coming from so you are certain that one of the parts you replace will fix it, but at this point, unless you are about to trade off the car, you are wasting your time and money fixing one part at a time.

Common leak points on an e46 BMW:

  1. The temperature sensor in the lower radiator hose. You can buy a new sensor, but you cannot find the correct size metric O-ring in the US unless you are willing to buy 144 of them. A sise #30 O-ring from the hardware store is a close fit, or you can try wrapping the sensor in teflon tape to make the O-ring fit tighter.

  2. Expansion tank cracked. The crack will open up when the system is hot (pressurized). It will generally leak little or not at all while the car sits cool. There are several fittings at the bottom of this tank that can leak as well, especially if your car is an automatic.

  3. Water pump. My wife’s '04 330 water pump failed at only 90k miles, but generally these water pumps last about 150k miles. Most folks replace them before they fail because you want to replace it while you are in there replacing all the other cooling system parts. I doubt that your problem is the water pump because once a water pump starts leaking, catastrophic failure is only weeks away, not years.

  4. Thermostat cover. Made of plastic. Shrinks and warps as it ages.

  5. Radiator. This will fail at the nipple where the top radiator hose attaches. The whole nipple will blow off, emptying the cooling system in seconds. This will happen when you are on your way to the airport to catch an important flight, because life is like that. Also, the seal between the plastic end tanks and the aluminum core can dissolve is someone puts Dexcool antifreeze in it.

  6. Any of the rubber hoses to the engine or heater. The press-in fittings that they use on BMW water hoses make the car quicker to assemble on the assembly line, and they work well for the mechanics doing warranty work during the first four years of the car’s life. After about 8 years they are a PITA to get apart and they don’t seal too well if you reuse them.

When you say the car is “empty”, I presume that you mean that the overflow tank is empty, not the whole cooling system, right?

Don’t fool around with this. Running this engine just a few minutes with no coolant in it will turn an expensive German engine into an expensive German boat anchor.

My whole family loves their BMWs, and does not want to drive anything else, but BMWs do have some weak points, and you just found the biggest one.

Oops, correction. That is a #31 O-ring for the water temperature sensor.