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Car lurches forwards at low RPMs?

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    #1

    Car lurches forwards at low RPMs?

    Beginning about two weeks ago, my car has been acting a bit odd at lower RPMs. When accelerating from a stop, it seems to lose power, slow down for a few seconds, then pick back up. A loss of power also sometimes happens when coasting at low speeds.

    I believe I've had this happen in the past due to filling up with bad gas, as it was remedied upon refilling. But, after running down the tank once, nothing seemed to change...

    I took a quick look to see if I could find any obvious vacuum leaks, but nothing jumped out at me.

    Any suggestions on what I should be checking out?
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    #2
    Can you possibly look at the AFM that have worn carbon strips at the low-end range? You can test it. Bentley explains how.

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      #3
      I took a look at the strip, which seemed mildly worn, adjusted it a bit, and took it out for a spin.

      It doesn't seem quite as pronounced, but I did notice another jump when pulling back into the garage. It seems as if it really only happens near the 750 RPM range, which I feel would really point towards the AFM's potentiometer.

      I'll drive it around tomorrow, and see if the issue keeps coming up. Anything else I should be on the lookout for?

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        #4
        I just felt around the intake boot, and the bottom of it seems to be covered in liquid...

        Would this would suggest some sort of crack or leak in the boot?
        Last edited by MrGeekFreak; 03-05-2013, 12:24 AM.

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          #5
          Try to eliminate the problem via the following:

          1. Vacuum leak test
          2. Spark i.e. caps, rotor, wires, coil
          3. Fuel delivery i.e. clean tank, clean fuel filter, working pump, working relay, fuel pressure, clean injectors, etc.
          4. Others sensors, CTS, TPS,
          5. AFM

          There's a more extensive checklist around here from Jim Levie. The search button is your friend... All the best!

          Comment

            #6
            Thank you so much for all of the help!

            1. I found that the elbow connecting to the ICV definitely seemed to be causing a vacuum leak, and ordered a new one, along with the intake boot and breather hose to replace this week. But, the pressure is still around 15, which is right in the "acceptable" range according to the Bentley manual.

            2. The spark plugs are almost brand new, and tested fine. The distributor cap and rotor were purchased just over one year ago, so I doubt they'd be causing much of an issue.

            3. The fuel pump is around a year and a half old, but I decided I would replace some other parts before swapping that out. I also ordered an extra fuel filter.

            4. I ended up testing the AFM and CTS according to the Bentley manual, and both turned out to be working fine.

            5. The AFM flap opens and closes no problem, and resistance tested to be fine.

            I'm checking out Jim's tips now, but I've almost come to believe it might be my automatic transmission slipping? I made a 100 mile round-trip to Lynnwood/Seattle today, and it really seems to be pointing to the transmission...
            I mean, with 250,000 miles on the sucker, it would make sense.

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              #7
              I had a similar problem with terrible idle at a stop (auto tranny). I had a smoke test performed and found a vacuum leak, but it didn't fix the rough idle. I also used an ohmmeter to test the ICV, TPS and the crankshaft position sensor. But stupid me, even though the sensor was within electrical specs, I didn't visually check the CPS sensor next to the flywheel. Pulled it yesterday and found the entire sensor was coated/caked with oil/dirt. I wiped the sensor clean and reinstalled. Car runs great now. No shaking/juddering at stop lights. The oil/dirt was preventing the CPS from sending a steady electrical signal.

              Lesson learned: electrical tests are important, but visually inspecting and pulling components is a key to fixing the idle problems. Pull your crankshaft position sensor and look at the condition of the sensor head.
              1993 Alpine White II 325i convertible (last year of E30 production)
              1988 Zinnoberot 325 Super ETA coupe

              Comment

                #8
                The problem is that, I don't really seem to have any issues when idling at a stop.
                Coasting and accelerating seem to be the only problems.

                Comment

                  #9
                  I did a do-it-yourself smoke test, which is stickied somewhere on these forums, and came up with the positive results! Well, if you consider a somewhat clean sign as good news.

                  Smoke seemed to be pouring out from underneath the intake manifold. I would assume this points to the intake manifold gasket, correct? Should I get a smoke test done by a shop before moving forwards?

                  With tomorrow being one of the only days over the past weeks that I won't be working, I'll break out the ol' Bentley and check out the CPS.

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