Bella Miller skillfully navigated her BMW 735i onto the expressway, dodging a semi that was quickly barreling down on her. Darting over to the left lane she eased up on the accelerator just enough to give herself a little bit of extra room between her car and the Blazer she was now following. Traffic was fairly heavy at this hour of the afternoon, but one kilometer down the road Bella would be able to take the left fork and follow the outer beltway for seven kilometers. Within moments she would be out of rush hour traffic and on her way home.

With the fork fast approaching, Bella turned on her left turn signal and eased her car off of the expressway and onto the ramp leading to the outer beltway. Seeing that traffic was now light, she pressed on the accelerator and immediately felt a bit of hesitation followed by two loud knocks. Perplexed, Bella eased up on the accelerator and moved over to the right hand lane, reduced her speed, before pressing down on the accelerator again. This time, her BMW responded but she could still feel some sense of hesitancy, so she decided to lower her speed the rest of the way home.

Upon entering her driveway, Bella stopped her car and turned off the ignition. Immediately, the engine began to shudder, knock, ping, and shudder again, before making a noise that sounded almost like a sigh, followed by silence. Promptly, Bella grabbed her purse and cell phone and exited the car and ran into her house thinking that it might explode.

What Bella was experiencing was an engine that was in distress. Her BMW’s knock sensor, designed to permit her V8 to run efficiently on lower grade 87 octane gasoline, had failed. This, in turn, was causing her engine to vibrate as the lower grade fuel was not enough power for her car.

Before 1990, many cars did not come equipped with knock sensors. Thus, in order to prevent vibrating, high octane gas would always be a necessity in order for a car to run knock free. Eventually, knock sensors were created that allowed cars -- including Bella’s 735i -- to run on cheaper, low grade gasoline. In addition, the knock sensor had the all important benefit of stopping engine shudder which can ultimately damage a vehicle.

If you are experiencing engine vibration of any type, it could be that your knock sensor has failed. Don’t delay; have your car checked immediately to prevent possible damage. A knock sensor is a fairly cheap to replace part compared to an engine repair which can cost you thousands of dollars.

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Copyright 2005 -- Matt Keegan is a contributing writer for Auto Parts Canada, a wholesaler of fine BMW parts and BMW accessories for your BMW car or SUV.