June 5, 2004
Oil Service, Rattling Exhaust Fix
You know your car needs a lot of maintenance when, after a mere month or two, you start to "miss" seeing and chatting with your mechanic. But, given my 22000 mile per annum driving habit, it didn't take long to rack up another 4500 miles on the odometer and schedule a visit for another mid-cycle synthetic oil service.
Over the last few weeks, I'd noted an occasional rattling in the exhaust while idling. The sound was rather faint -- so faint, in fact, that I had to be outside of the car with my ear positioned "just so" to hear it coming from the front half of the car. I figured either a heat shield bracket had broken, or -- worst case -- the catalytic converter core had started to disintegrate.
Fortunately, as I pulled up to my mechanic's bay to say hello, the car was kind enough to demonstrate the malady for him. As usual, his experienced ear didn't take long to confirm my suspicions. An hour later, my mechanic found me and told me that he'd found (and fixed) the problem.
BMW apparently uses a steel wool type of insulation packed between the catalytic converter and the associated heat shield. This insulation is also used at the attachment points of the heat shield to reduce heat transfer into the shield. In the case of my car, some of this insulation had fallen out due to simple wear and tear. This caused the shield attachment point to vibrate against the exhaust pipe and create that distinctive sound.
He outlined three typical solutions for this:
- Remove the heat shield. Not recommended because it really does serve a purpose; it results in a several hundred degree differential between the exhaust components and the external face of the heat shield. If removed, the risk of fire when parking over grass or other combustables is greatly increased (remember, it's not just about direct contact, but radiation as well). My mechanic said that most people choose this option, but since I hate the smell of a BMW toasting over dry grass on a warm summer's night, I'll pass on this "solution".
- Replace the catalytic converter assembly (which actually consists of two cats sandwiched together combined with the forward half of the exhaust piping). Just for giggles, I'd asked for a price, and was told $1395. But, my mechanic was quick to point out that the internals are still doing their job, so there was no real need to replace the cat assembly. All I could think and say was "uh, you got that right!".
- Fix the rattle at the source. My mechanic chose this solution. He said he drove some screws between the exhaust pipe and the shield mount to take the place of the missing insulation. I should point out that this did NOT, in any way, involve driving holes into the exhaust system. The screws simply tighten the gap between the shield and pipe and eliminate the source of the rattle.
Parts $36, Labor, $36, Total $86. Mileage: 97317.