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Thursday, February 29, 2024

Doug's Domain

Doug Vetter, ATP/CFI

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Sunday, January 21, 2007

Tensioner Pulley Bearing Failure

Last Monday I started the car to go to work and I heard a muffled rattling noise from the front of the vehicle. I can best describe it as the sound that might be produced by a couple ball bearings rattling around in a foam-lined can. The sound was intermittent, but occurred often enough to allow me to localize the source to somewhere around the oil filter cannister. The first thing that ran through my mind was bad bearings of some sort, but I couldn't tell exactly what was causing it. Since it was raining, I turned on the lights before I pulled out of my driveway and I noticed the sound became more pronounced and nearly continuous. At that point I figured the alternator bearings were on the way out.

The engine was running smoothly and there were no warning lights illuminated on the dash, but I figured I'd play it safe and drop by the dealer to see my tech. He agreed to leave his nice, warm shop and brave the weather with a smile to help me isolate the problem. It didn't take long for his tuned ear to suggest "sounds like a problem with one of the belt pulleys". He then grabbed a can of brake cleaner and a long (12" shank) screwdriver and asked me to start the car and turn on the headlights and the A/C. Once again that made the sound much more obvious and nearly continuous. He sprayed the cleaner on a few of the larger pullies but that didn't do anything, so he grabbed the screwdriver, carefully laid the tip on a few of the engine parts and put the handle to his ear, thus creating a crude but effective technician's stethoscope.

(Image: Closeup of failed tensioner pulley)As he carefully routed the tip of the screwdriver onto the main drive belt tensioner it began to vibrate in concert with the offensive sound. He handed the handle of the screwdriver over to me and I felt the screwdriver jumping up and down as the tensioner bucked from the change in bearing load. Sure enough, the main belt tensioner pulley bearings were on their way out.

I breathed a sigh of relief at the prospect of an easy (read: inexpensive) repair, particularly since I knew I'd get the replacement part for free under warranty. I'd replaced this very pulley during the cooling system overhaul last year. My tech said he was booked for the day so if I wanted him to do the work I'd need to bring it back to him, but I offered that I'd try to do it myself and call him with any questions.

Fast forward to Saturday. I brought the car into the garage, fired up the kerosene heater again, and got to work. About an hour later, I had the new pulley back on and had run up the engine. It's completely silent now. On a scale of 1 to infinity, this is a one-curse job (I only let out one curse due to crampt space in which I had to work), so it's a pretty reasonable task for the DIYer. This job is identical to that required to replace the belts, so I'll document that later in a DIY article.

When I took a closer look at the old bearing there wasn't any obvious damage to the bearing races like bluing of the metal due to heat stress. In fact, there were only a few differences of note:

How long could I have run the bearing before it failed? I really don't know, but since a failure of this bearing might cause the belt to come off, and that could cause some real damage to stuff like my brand-new radiator, I wasn't about to test that boundary. For what it's worth, the sound did become noticeably worse over the course of the week, so I imagine this is a task best taken care of in that timeframe.

More Tools

I went to Eppys again today to pick up some tools.

Lesson learned: never put your mirror tool in the same toolkit with other loose tools. Fortunatley, the old one was actually a magnet / mirror combo, so it will live on as a magnet tool. I decided to buy a dedicated mirror tool since it was slightly cheaper and that's the one I used to do the pulley repair.

I have to admit that I take most of what my 30 year BMW technician says as the gospel, but I was always a bit skeptical of the technique he taught me to R&R the engine driven fan. Somehow just tapping on the end of a wrench didn't seem like the right thing to do, so I bought a $32 generic tool designed to hold the water pump steady by grabbing the bolts that hold the pulley on. I never took it out of the original packaging as my technician's technique worked. I plan to return the tool. Summary: all you need to R&R the engine driven fan is a large 32mm wrench...and it doesn't even need to be a "thin" wrench.

In prep for the valve cover work, which requires a very light torque, I decided to spring for the smallest torque wrench SK offers. I recently saw the result of an improperly-installed valve cover gasket on my brother's 530. It's a long story -- and no, I didn't have anything to do with it aside from getting my tech to fix it the right way -- but it was a $275 job. If I do the valve cover gasket myself, that job will pay for the torque wrench and then some.

Mileage: 138450, Parts: $0 ($45 part covered under warranty), Labor Saved: $110, Tools: $210