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 www.realOEM.com, 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.