All N55-powered BMWs use an electric water pump. They’re efficient and reduce parasitic engine losses, but they remain a source of headaches. More recently, owners have found their water pump’s aluminum ground bolt failing on them, shearing off for whatever reason and leaving the pump inoperable. Replacing the broken bolt with another aluminum fastener will revive the pump but will set you up for another failure down the road. Luckily, we know just the part to fix the issue permanently, and you can find it below.
Water Pump Ground Bolt Failure
Last Wednesday, I was merging onto a highway when the car suddenly went into limp mode with an iDrive warning message about the car overheating. Fortunately, I safely got the car off the highway to investigate the issue. I always carry an OBD2 Bluetooth scanner with me just in case I need to diagnose something on the road. I automatically suspected a failed water pump (less than 30K miles on it since I replaced it) and got an unexpected CD9010 LINBUS communication fault code stating it was missing. It was a confusing fault code, but seeing as I was 200+ miles from home, that was the least of my concerns.
Thankfully, my best friend made the trip to rescue me with his truck and trailer. Once I got it on the lift (6 hours later), I removed the splash shield and had the head of an aluminum Torx bolt fall out and land at my feet. I looked up, and seeing a ground cable hanging there, I traced the wire back to the water pump connector. Instantly, I understood the LINBUS communication problem. No ground means no communication.
It was still a very odd problem, though. I’ve never touched the bolt, and I doubt it has been touched since the car was built. Plus, the bolt has zero tension; it's not load-bearing and should not shear off on its own. After asking around, a few people who own/work in BMW shops have shared that they have seen a few of these bolts snap off; it seems like it could be a fatigue issue.
Water Pump Ground Bolt Replacement
For whatever reason, BMW used an aluminum stretch bolt as the fastener for the water pump’s ground strap. Aluminum, while lightweight, is pretty susceptible to corrosion from the elements and is generally quicker to wear. They’re also one-time-use items, so you must replace them every time they’re removed. I am considering using a stainless or zinc-plated bolt. The bolt itself is not the ground; the ring terminal is pushed up against the engine block, and that surface contact creates the ground.
The replacement bolt from BMW is p/n 12527557037. If you’d like to use a different fastener, one made of stainless steel or Zinc-plated/coated, look for an M8x1.25x20mm long screw. An off-the-shelf option from BMW is p/n 07129905423, which includes an encapsulated washer. Whichever you choose, the fastener won’t be expensive. If you replace the factory screw with a steel one and use blue Loctite, you will never need to worry about this screw failing again.
Water Pump Ground Bolt Torque
Torque the replacement bolt, p/n 12527557037, to 8Nm + 90°—the other option, p/n 07129905423, must be torqued to 19Nm. A dab of Loctite wouldn’t hurt, either.
With that, your N55 should be free of future water pump ground bolt-related failures. If you're looking for more info on the N55 or are interested in how to get more power out of it, we offer several resources, guides, and DIYs that will help you along the way. As always, don’t forget to follow our DIY Blog, and stay tuned to our YouTube channel for more great content! Happy wrenching!
