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what is causing my rumble?

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  • #16
    I'd remove the belts and rotate the PS pump, alternator, waterpump and AC compressor pullies by hand so see if they rotate smoothly.
    I see this as an issue that is related to an ancillary component.
    If the engine driven accessories are good, your motor mounts are good then start listening with a hearing aid at the flywheel for a worn T/O bearing or pilot bearing.
    Also take a good listen to the timing chain...for a worn guide as a remote cause.
    If your motor mounts are bad, the bellhousing might be touching the swaybar.
    m

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    • #17
      Re: ...rumble...

      Originally posted by t13vom
      If you would like to have a second set of ears listen to it I am across town from you... I think I saw you occationally when I lived behind the Safeway off of Fairview...
      "Stickers add 20HP"?

      I was planning to cut the A/C out at some point anyway, so I will rip that belt out and see if anything changes. If I'm still stuck, I might take you up on that offer.. Thanks.

      Nice to know there's another S14er in the area.

      Would a weak alternator cause something like this but still maintain sufficient voltage?

      Thanks much guys.
      Bry

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      • #18
        Since the alternator has small diameter bearings, you would expect the sound to scream and to rise and fall with RPM.
        The alternator will charge ok for a while but seizure or disintergration is likely eminent.

        m

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        • #19
          Re: Re: ...rumble...

          Originally posted by sirrus
          Would a weak alternator cause something like this but still maintain sufficient voltage?
          if you want to eliminate the alternator, check the voltages at the rear terminals. if you see ~13.7 at idle (after blipping the throttle) it is good. it's also worth checking the voltage across the battery terminals. but if you're able to run it through the entire RPM range, i think it's not a low voltage problem.

          you sure none of your plug wires are connected wrong?

          if you lived closer, i'd stop by
          James
          sigpic

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          • #20
            Re: Re: Re: ...rumble...

            Originally posted by jht3
            if you want to eliminate the alternator, check the voltages at the rear terminals. if you see ~13.7 at idle (after blipping the throttle) it is good. it's also worth checking the voltage across the battery terminals. but if you're able to run it through the entire RPM range, i think it's not a low voltage problem.

            you sure none of your plug wires are connected wrong?

            if you lived closer, i'd stop by
            Yep, that's why I asked. Voltage at the terminals is 13.6 or so and it is a brand new battery (for that exact reason).

            Plug wires all show resistances in spec and are in the right sockets and are connected appropriately at the distributor. I know cause I've taken all of the ignition components apart and back again trying to figure this out...
            Bry

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            • #21
              Do you have a steady idle or is the engine 'hunting' or trying to fall over itself at idle ?

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              • #22
                Originally posted by oscarcab0
                Do you have a steady idle or is the engine 'hunting' or trying to fall over itself at idle ?
                Idle is steady. It's not stumbling or "randomly" misfiring or surging or anything; it's a constant rumble. With the reverb in the garage it might as well be a 5.0 at a stoplight.

                Would an exhaust leak at the mating of the cat/header cause something like this?
                Bry

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                • #23
                  So when you throttle on and run through the rev range does the engine feel 'normal' ( smooth if you like ) apart from the noise ?

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                  • #24
                    Originally posted by oscarcab0
                    So when you throttle on and run through the rev range does the engine feel 'normal' ( smooth if you like ) apart from the noise ?
                    I would say yes, at least sitting still. But I have driven it twice to try to diagnose the issue and there is a significant drop in power. Part of it may be that I am hyper-sensitive to the sound and so I tried not to 'push it' too hard, but I recall it really struggled anywhere above 40mph.
                    Bry

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                    • #25
                      Hmmm........I've been thinking fuel pressure regulator but that noise issue is odd. Does it sound like an exhaust leak to you ? (sorry, but I'm still trying to understand just what sort of a noise we're talking about here i.e. how loud, where it's coming from etc.).

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                      • #26
                        Originally posted by oscarcab0
                        Hmmm........I've been thinking fuel pressure regulator but that noise issue is odd. Does it sound like an exhaust leak to you ? (sorry, but I'm still trying to understand just what sort of a noise we're talking about here i.e. how loud, where it's coming from etc.).
                        I'm sorry I can't explain the sound well enough.

                        I would only think exhaust leak if it was at the manifold because of how dramatic it is when driving and I've read on the SIG that people have experienced a "knocking" sound when it turned out to be header studs/nuts.

                        But I certainly didn't tear arse or anything in the car before I put it away for the winter, so I dunno how or why the manifold nuts could be loose.

                        The noise (of course) is most noticeable out of the tailpipe and if I hadn't driven the car or known what she sounded like before the winter, I wouldn't think anything of it at idle.

                        Maybe when I get home I'll see if I can find some video/audio of something similar, but I don't want to keep wasting people's time. I appreciate all the help so far.
                        Bry

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                        • #27
                          Hmmm...odd. I had actually typed into my last reply have you checked the header studs but then I deleted it because I remembered that you said you had. Don't worry about whether you are wasting anyone's time. We're here to help each other if possible. Have another look at those studs. I do agree with you though that it is strange that this has occurred given the circumstances.

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                          • #28
                            My M3 had a similiar loud grumbling and this was due to a broken exhaust hanger.

                            Try shaking your exhaust and see if there is any movement.
                            October 1987 build date.

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                            • #29
                              Post winter storage?

                              Most likely is a leakage on your valves. Valves seats that were open during the storage could have been corroded. Might help also if you check you valve clearance.

                              HTH

                              audi

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                              • #30
                                Originally posted by audibm
                                Post winter storage?

                                Most likely is a leakage on your valves. Valves seats that were open during the storage could have been corroded. Might help also if you check you valve clearance.

                                audi
                                Really? Would valve seats corrode in a very mild 2 month garage storage?

                                Valve clearances are all in spec; triple checked since the valve adjustment.
                                Bry

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