The telltale stains on the driveway told me something was amiss, and it became clear it was coming from the lower radiator hose, or the coolant sensor on the lower hose. The car also had been flashing a service-engine-soon light with a P0128 code, which means the coolant isn’t coming up to temperature quickly enough, so it was clear that the cooling system needed attention.

When it comes to a cooling system, you can either repair it at your convenience before it’s absolutely necessary, or repair it at great inconvenience — and potentially added expense — when it finally gives out completely. I prefer the former over the latter.

On a 15-year-old BMW, you have to take the “while I’m in there” approach. So, between the leak and the SES light, I decided to replace the thermostat, which comes with a temperature sensor, the upper hose and the lower hose and another temperature sensor that snaps into it. Solve problems before they start, right?

Before we go on any further, the last part of the job is refilling the car with coolant and bleeding the system, and it is critical you do it right. If not, the car will overheat. We don’t offer any photos, but it’s not difficult. Because it’s a BMW, it is a little weird.

To fill the radiator, remove the radiator cap — duh — and the bleeder screw on the hard plastic fitting that snaps onto the radiator and the expansion tank. Fill it till coolant rises to the top of the opening of the bleeder screw, then wait. The coolant will sink a bit. Keep filling it till that doesn’t happen anymore. You might see some bubbles in the opening, which is some of the air you are trying to bleed out. Next comes the weird part.

When the coolant is full, turn on the key to the run position, turn the heater all the way up as hot as it will go, which is 91 on the display, and be sure the fan is on. Also, roll the thumbwheel between the center dash vents to the red mark. Let the car sit that way for five minutes. You do this to circulate coolant throughout the full system, heater core and all.

You have to turn on the ignition because there’s an electrical pump that circulates coolant to the heater core. As you’re letting the car do its thing, keep an eye on the coolant in the radiator neck and the bleeder hole. You might see more bubbles.

After five minutes, there should be no more bubbles. Put the bleeder screw back in place and the radiator cap on, start the car and watch the temperature gauge for a bit. If it climbs above normal, you still have air in the system.

Like all work on this car, it begins with removing the air filter housing. Well, it begins with disconnecting the battery, but I forgot to do that, so do that first. There’s a coolant drain fitting on the bottom of the radiator. It’s color-coded blue, so it’s easy to find.

The job itself isn’t terrible, even if you have never done it before. I can’t say I’m a fan in theory of how much plastic BMW uses on this part of the cooling system, but it does help it come apart and go back together pretty easily. I don’t know if the plastic bits would hold up to the rigors of racing, but for daily driving and an occasional HPDE day, they seem to work.

Of course, while I was under the hood, I noticed some other coolant hoses that ran from the radiator to the engine and to a fitting on the firewall. In truth, I should have replaced those too, but they’re next.

If you have the VIN for your BMW, specifically the last seven digits, you can look up your own parts online at, then Google the OEM part numbers to find the best deal.

Even if you take your time, the job should only take you about two hours. Here’s how it unfolds.

The front air duct has to come off to be able to remove the fan and shroud assembly, which is necessary to be able to access the thermostat and the radiator hoses.
BMW makes the radiator drain plug easy to find because it’s blue. It’s also and easy to remove because it’s made of noncorroding plastic.
Like almost every other job under the hood of an E46, you have to remove the air filter housing. I’m half convinced this is why aftermarket cold air intakes are so popular on these cars.
Loosen the clamp from the intake duct to the MAF sensor, and unplug the connector before removing the air box.
There are two electrical connectors on the fan shroud that need to come off before you an remove the fan assembly.
The fan assembly is a really clever design. It fastens at the top with a screw and a push pin, and slips in and out of tabs on the bottom of the radiator like a cassette. It’s really simple, which is surprising on a BMW, a company that seems to have a penchant for making some things more complex than they need to be.
OK, all of the hoses and most electrical connectors involved in this repair are held on with wire bales. The sensor on the thermostat has a wire bale that comes off and allows easy removal of the connector.
The upper and lower hoses have wire clips that hold them in place. Just pry the clips up with a small screwdriver, then coax the hoses off the fittings with a little muscle.
You can use a big-blade screwdriver to pry the hoses off, but we only used it on the thermostat housing because we knew we were going to replace it. If you use a big screwdriver to pry the hoses from the radiator, be careful not to break the plastic fittings on the radiator and coolant expansion tank.
Here you can see the upper hose separating from the radiator and coolant expansion tank. You also can see the grooves where the wire bales snap in to hold the hoses in place.
You know the drill here. Remove the bale, unplug the connector. Lift the bale on the lower hose. This is undoubtedly the source of the coolant leak, so I thought it best to replace the hose and the sensor.
Here are the new OEM parts we found online using the part numbers from and good old Google.
The new sensor presses into the lower hose and lets you know it’s installed correctly with a solid and reassuring click.
The thermostat is held on with three 10 mm bolts and one 13 mm bolt, which for whatever reason, ties in with the engine lifting bracket.
The thermostat and housing is all one piece and it includes the fittings for the upper and lower hose. The new one comes with the rubber gasket and a temperature sensor.
One advantage to the plastic parts and rubber gaskets is that there is no need to scrape leftover paper gasket material off when removing the old parts. We just sprayed the head with a little brake cleaner and wiped it down.
The new thermostat comes out of the box ready to install. Note the tabs on the hose fittings that fit into the slots on the hoses.
The hoses slip on much easier with the bales in the “out” position. Once the hose slips into place, you can push the bale in. The slot in the hose fits over a tab on the radiator. It only goes on one way, which is nice.
If I ever need to remove the hoses again, using Eaton assembly lube will make the job much easier. I use this stuff on coolant and vacuum hose connections all the time. Racecars get torn down a lot. This assembly lube helps keep you from busting knuckles on stuck hoses that let go all at once.
Slip the hose over the thermostat and snap the wire bale into place so things stay put. It’s really easy to tell if the hose is on right because it snugs right up to the inner lip on the fitting.
All the hard work is done. Just put back on the peripherals (air box, fan and air intake) and refill the coolant reservoir and you’re finished. Be sure to bleed the system!
Image courtesy of Eric Green

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