Using The Map Sensor To Check Cylinder Balance

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  • greasybob
    Senior Member
    • May 2008
    • 1590

    Using The Map Sensor To Check Cylinder Balance

    I recently had two vehicles where I back probed the MAP sensor to diagnose the problems. The first was a Dodge with a 4.7 with code P0306. The coil was OK, the injector was good and it even had good compression on no. 6 cylinder. I back probed the MAP sensor and triggered off the no.6 injector and disabled all the coils to get a longer cranking time. The results below speak for them selves. You can see a sharp rise in manifold pressure after the no.6 injector is fired. Removed the valve cover and found the no.6 cylinder exhaust rocker arm had fallen out from under the cam.
    The second vehicle was an 01 cavalier 2.2 no start with a backfire during crank, no. 2 and 3 plugs would not come out so I did the same crank test, disabled the ignition and back probed the MAP and the no. 1 injector first. Got a similar result, but when I did a leak down test found the intake valve on no.1 leaking badly. I plan on adding this simple test to more vehicles that have misfires or rough idles in the future, I think it could help find things like plugged exhaust and jumped timing.
    Attached Files
  • eddiesverus
    Senior Member
    • Dec 2008
    • 414

    #2
    Very Good Information, Thank You For Sharing This. I've Also Ran Test But I Use A Vetronix Mt5200 Engine Analyzer, But Using Any Labscope Is Just As Good. Thanks
    Hard Work, commitment, Honesty and not giving up is what Makes us better Tech's

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    • Nickb
      DSD
      • Aug 2007
      • 206

      #3
      You can also use a low amps probe. If you set it a 40 amp scale and place it around the positive battery cable going to the starter. Set the scope at 2 volts and 1 second to start. Set it at no trigger at first, just to see if there is a problem with the engine. Then you can trigger after if needed. Also disable the engine, fuel pump relay or ignition, your choice. Hope this also helps. If you have a diesel engine you will need the 40 - 400 amps probe. This is so you can get it around the larger battery cables. Disable the crank sensor on the diesel engines.
      DSD

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      • greasybob
        Senior Member
        • May 2008
        • 1590

        #4
        I recently did an experiment with an 05 neon where I removed the no.4 cylinder spark plug and disabled the ignition coil. Then I did 3 no start cranking tests to see which method of cylinder contribution testing worked the best. I triggered off of the no.4 injector each time. I used a high amp probe on the battery cable, back probed the MAP sensor and connected a vacuum pressure transducer. The amp probe is the easiest to identify the dead hole, but with experience I think the pressure transducer will give the most information on what the likely cause of the low compression would be. I chose to use the high amp probe instead of a low amp probe coupled to AC for clarity reasons.
        Attached Files

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        • just2damfast
          Junior Member
          • Mar 2010
          • 8

          #5
          I've heard of cylinder contribution tests from the battery cable before (I think) how exactly does that work? What is the theory behind it that changes the amperage depending on cylinder output?

          Thanks
          Brian

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          • hefaus
            Senior Member
            • Dec 2007
            • 435

            #6
            amperage

            Originally posted by just2damfast
            I've heard of cylinder contribution tests from the battery cable before (I think) how exactly does that work? What is the theory behind it that changes the amperage depending on cylinder output?

            Thanks
            Brian
            Hi. If the cylinder has good compression then the amperage would be higher to turn that cylinder.If a cylinder was of lower compression then so would the amperage. Pull a spark plug, Disable the injectors and try it. You will see the low cylinder in the amperage wave form. Hope this helps you.
            Attached Files

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            • goop
              Junior Member
              • Jun 2009
              • 7

              #7
              where did you get that scope file viewer on your verus?

              Comment

              • greasybob
                Senior Member
                • May 2008
                • 1590

                #8
                I used the camera on the virtual keyboard to save a frame into paint where I inverted the colors, added the text, and then saved as a JPEG file into pictures. It prints better this way too.

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