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Swapping My BMW’s Serpentine Belt Turned Into a Whole Thing

Mission creep sets in a little too easily, even during the simplest of maintenance tasks.
bmw 128i maintenance mission creep
Peter Nelson

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I feel like the older I get, the more time I take to do my DIY-wrenching jobs. It doesn’t matter what I’m working on, be it suspension, brakes, or the engine, no matter how complex the task is, things are just taking longer. But this has nothing to do with wrench-turning taking a toll on my weary elder millennial bones. It’s all because of a common wrenching concept: “While you’re in there…”

I’m glad I made peace long ago with the fact that I’ll never be a quick wrench. I’d make a shitty lube tech at my working pace, but it’s not just because I’m slow. My recent 30-minute serpentine belt job turned into a nearly 18-hour ordeal because I always want to go above and beyond with the service, check for potential issues that could pop up, and make sure all surfaces are clean for future inspections and jobs.

Well, 18 hours is fudging it a little bit. When I went to quickly swap my 2011 BMW 128i’s serpentine belt, tensioner, and idler pulley a few nights ago, I noticed that quite a bit of schmutz had accumulated on the front of the engine during the course of its 85,000-mile life. Knowing that the BMW N52 inline-six engine is no stranger to leaks, the overachiever in me decided to degrease the front of the engine while I was in there. Around 20-30 minutes and a hell of a mess on my driveway later, the engine was looking far better. Most importantly, the metal surfaces around where the head mates to the block, the oil filter housing mates to the head, and the oil pan bolts up on the bottom were spotless.

However, I didn’t have a means to quickly dry it all and didn’t want to short anything, so I let it sit overnight. The following morning, the dry conditions of Southern California ensured a spotless surface, and swapping the three integral front-facing parts was a breeze. I like that there’s only one way for the tensioner to insert into the block, and because the bolts for the tensioner and the idler pulley require very low torque settings, they unbolted with ease. Before you ask, yes, I used a fresh aluminum bolt for the tensioner.

I could’ve then buttoned everything up and called it a day, but I already had the front intake piece removed. Because I was already in there, I decided to change out the air filter, as my Bimmer’s previous owner gave me a fresh one he had sitting around. This is an easy job, though kind of a pain in the ass for just an air filter. You unbolt the air filter housing, pull it out, undo the couple of Torx screws holding it together, swap the filter, and finally introduce it to its new home by doing the reverse of installation.

After removing the housing, I noticed quite a bit more road gunk and oil schmutz underneath it on the 128’s painted metal bits, and the housing itself was overall quite dirty. Even though none of this will ever really see the light of day, I figured why not clean it up… while you’re in there?

A BMW N52 engine's cabin filter
Schmutzy vs. brand-new, non-schmutzy Peter Nelson

After that, I moved on to the cabin filter, which is a hilariously large unit that unbolts via Torx screws (of course) underneath the hood up toward the cowl. Some grime had accumulated on its housing and around the area where it bolts up, so there I was yet again, a bottle of Simple Green in-hand, thoroughly cleaning bits of hard, tacky plastic. While you’re in there.

Finally, after I started the night before, everything was done. These small jobs were all easy and went quite smoothly, but, man, this kind of mission creep can take a toll on someone who works at a red-ear slider’s pace. Because the combined wrenching time came out to nearly 2.5 hours, I felt like I had accomplished something a bit more involved, but no, I just changed out $120 in easy-to-access parts. Again, my original intention was to just dedicate 30 minutes to the serpentine belt job.

However, at least I’ll have a better idea of what’s going on with the Bimmer’s various seals, and things look a lot nicer under the hood to boot. Who knows, if I do enough of these small jobs in the coming months, while also taking my time to clean stuff while I’m in there, my lowly base 128i could become a BMW Car Club of America Concours-grade show car.