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Diagnosing a Noise

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Mike Falconer

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Yes

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Maintenance

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Question

Hi Gil. I own a beautiful 2002 BMW Z3 3.0. Great running and well maintained. My question please is the knocking clicking noise (sounds like it’s from the bottom back of my car) . It only appears (the noise) when I am at a stop, and then i begin to accelerate hard off the stop while turning left or right into a corner……the clicking and knocking sound happens for only a short time as I am accelerating hard.

Otherwise, she continues to run perfectly and no mechanical issues. Thanks. Mike

Answer

Hello Mr. Falconer, I have a question for you first, I’m not clear as to when the noise happens, is it only under acceleration or does it happen when the vehicle is stopped as well ? If it only happens when accelerating I would suspect the driveshaft, axles or possibly suspension, but they would not be there if the vehicle is stopped. If it happens when the vehicle is stopped I would suspect the catalytic converters. Of course the only way to know for sure is to have a look at the bottom of the vehicle. Let me know if you need a referral please. Thank you for your question.

Gil Neves

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This Post Has 4 Comments

  1. HI Gil – it is only for 2 seconds while I am accelerating. My local bmw guy told me this weekend…its probably the DSC system, which this model has installed. Its as you know, a sophisticated computerized braking system, to proactively prevent your car from losing traction. It typically applies the brakes as needed when wheels are going into a skid, or wheels might spin out due to heavy acceleration or the car steering wheel is too far left or right… when heavy acceleration is happening…..this keeps car from over or under steering. It all made sense to me….as the car underneath looked great he said.

    I tend to agree…but appreciate your comments below regarding a few other ideas or possibilities. Do you agree with his interpretation?? Just curious. J

    “the clicking and knocking sound happens for only a short time as I am accelerating hard. “

    It only happens during the hard acceleration, only, for a few seconds only…..NOT when the car is at a stop or idle.

    Have a great day. Thanks. Mike

  2. Hi Mr. Falconer, it could very well be the DSC, however, if it is the DSC light on your dash should flash. The light flashes whenever the system is active. Thank you.

    Gil Neves

  3. Mike, I happen to be very intimate with the Z3 platform, having a ’99 M Rdstr coming up on 20 years (CPO purchase in November 2000), also an ’01 M Rdstr since it was new, and six years ago I bought a former customer’s ’99 M Coupe (I fitted a Eurosport Twinscrew supercharger to it back in 2005).

    That “clicking” might be sheetmetal edges rubbing against each other. The spot welds that hold the rear crossmember to the trunk floor are a well known weakness of that platform. The rearmost mount of the differential (aka rear axle) is attached to that xmbr, and the forces of acceleration pull it downward, conversely, slamming on the brakes forces the xmbr upward. We all know what happens when you keep folding a piece of metal back and forth; it “work hardens” until it fractures!

    With the convertible versions of the Z3, it is very easy to check the integrity of those spot welds, simply remove the false -floor of the trunk, and look in the area ahead of the tool tray, particularly on the left hand side (peek under the felt trim panel too__that’s where the damage starts).

    Lets not jump to conclusions, but if you do find the area damaged, it is not the end of the car’s life. I have fixed, or prevented, and reinforced that area of over a hundred (104-105?) cars myself, and have sold about a thousand of the kits worldwide since 2004. Unfortunately, the host for my server is down, or I could point you to the work albums of all the cars I’ve done, so you’d know exactly what to look for.
    It’s seldom down for long, so maybe the admin is doing some maintenance…? http://spcarsplus.com/piwigo/

  4. Hi Gil. It ended being worn out wheel bearings in driver side rear wheel. When I accelerated hard from a stop, turning right, the noise came from my back wheel on my driver side. I guess all the weight of the car transfers to that wheel on a right turn. Just thought I would share :).

    Mike.

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