Cracked Plug on a Scope

Collapse
X
 
  • Time
  • Show
Clear All
new posts
  • greasybob
    Senior Member
    • May 2008
    • 1590

    Cracked Plug on a Scope

    It was an 05 F150 5.4 with a misfire on number 8. Some people would have thrown a coil at it then go to Sure track when that didn't fix it. I'm not most people. I gotta know it, see it and figure it out. I back probed the control side of the coil to see. There it was on the secondary side if the firing, Too many lines. Something happening, it could still be the coil I guess but I pulled the plug and there it was, a crack in the ceramic. I replaced the plug and the coil boot. All is well again. I put a screen shot in here too of a normal coil firing with current ramp, well as normal as 200,000 mile 5.4 can get. Now you know what a cracked plug will look like on a scope.
    Attached Files
  • Crusty
    Senior Member
    • Dec 2007
    • 2450

    #2
    Bob, is that a triple "multi-strike" at idle on that Ford-??

    The three in a row makes me think so, but I don't know that for sure.

    Comment

    • greasybob
      Senior Member
      • May 2008
      • 1590

      #3
      Yes, at idle it fires 3X per. The screen shot is one coil firing.

      Comment

      • greasybob
        Senior Member
        • May 2008
        • 1590

        #4
        The current ramp wasn't really affected by the secondary problem. You can see a difference in the steepness of the ramp on the 2nd and 3rd firing. Not sure if that's a coil difference or if there is left over energy in the coil on the normal firing.
        Attached Files

        Comment

        • Witsend
          Banned
          • Nov 2012
          • 2942

          #5
          Got me thinking Bob, maybe i could scope out my old furnace's erratic igniter with my counselor 2 and see something similar .

          Comment

          • Crusty
            Senior Member
            • Dec 2007
            • 2450

            #6
            Originally posted by Witsend
            Got me thinking Bob, maybe i could scope out my old furnace's erratic igniter with my counselor 2 and see something similar .
            Don't have that problem up in the Great White North with a WOOD stove

            Comment

            • Crusty
              Senior Member
              • Dec 2007
              • 2450

              #7
              Originally posted by greasybob
              The current ramp wasn't really affected by the secondary problem. You can see a difference in the steepness of the ramp on the 2nd and 3rd firing. Not sure if that's a coil difference or if there is left over energy in the coil on the normal firing.
              Bob, looking at the two examples of current ramps, the amplitude of the two appear different. The shape looks close but without the "graph" feature turned on in your scope its difficult to see the amplitude of the three sequential current patterns.
              One appears to be around 4 and the other appears to be around 8 in the middle strikes.
              A quick "glance" might miss that difference.
              What do you think-??

              Comment

              • Crusty
                Senior Member
                • Dec 2007
                • 2450

                #8
                Your first post indicated that many would toss a coil. That's probably because of the common failure with Ford coils. The "pattern failure" that many rely on is the road to hell AFAIC.
                Sure, use those pattern failures as an INDICATOR, but not as a definitive diagnosis.
                Too many times, the "fundamentals" are overlooked, and with vehicle manufacturers and parts makers "claiming" the mileage that plugs "can go" up to, the basics are easily jumped past.

                Good work on your part. But then again, I EXPECT that from ya...

                Comment

                • greasybob
                  Senior Member
                  • May 2008
                  • 1590

                  #9
                  Ok, here goes. For a current ramp to look the way it does wouldn't you have to say that the resistance in the coil is changing as it charges ? So the resistance starts high and goes lower thus accounting for the increase in current draw, until the point of saturation when it levels out (or the case where there is a current limiting ignition system) or the circuit is opened and there is the discharge (spark). So for the 2nd and third current ramps to be different, either having higher current starting points or not obtaining the same high current draw something about how the current is discharged in the shorted plug is at cause. The thing you notice about the cracked plug is a longer inductive kick and a shorter burn time. In the third strike there is no burn time at all. I would almost want to think that in the proper coil cycle that there is not a total discharge, accounting for the lower resistance on the 2nd and 3rd charge cycles. Not sure ? But my best guess at this time.
                  Last edited by greasybob; 03-25-2016, 04:33 AM.

                  Comment

                  • Crusty
                    Senior Member
                    • Dec 2007
                    • 2450

                    #10
                    I think you're misunderstanding.
                    There are two examples you put up showing the pink current traces.
                    In the picture just under the cracked plug, the AMPLITUDE of the first strike goes up to almost 12.
                    In the second picture, lower down, the pink trace on the first strike, the AMPLITUDE only goes up to 7 or 8.

                    That's a significant current difference.

                    Whether you look at the first or second or third strike, the amplitudes are lower under one condition compared to the other.

                    Look at the pink scale on the left.

                    Comment

                    • Witsend
                      Banned
                      • Nov 2012
                      • 2942

                      #11
                      The Kicker to me is the lack of spark line at the 3rd. It goes from a firing spike right into coil oscillation, which tells me to go look at the plug.
                      I don't care to see Champion spark plug in a Fords. Eventually they run like a F'ed Over Rebuilt Dodge?

                      Comment

                      • BMW JEDI
                        Senior Member
                        • Feb 2012
                        • 422

                        #12
                        You just replaced the ONE coil boot and spark plug?

                        Comment

                        • Rich Shepherd
                          Snap-on D&I
                          • Nov 2006
                          • 553

                          #13
                          Here is some information based on my understanding of this system:
                          Ford has been using the Multiple spark strategy for many years.
                          They will make from 2 to 4 or maybe 5 sparks (at least as far as I have seen).
                          The specific strategy as far as the number of sparks and duration has varied over the years.
                          All but the last spark are limited in duration. They will typically be .3 or .5 ms in duration. At least that is what I have observed in the past.
                          With all but the last spark, the coil is turned off, the field collapses, voltage builds until a spark jumps the gap, then the spark voltage is sustained for a period until the coil is turned back on at which time the spark goes out and the field in the coil starts to build again.
                          Under normal circumstances the coil is not completely discharged until the last spark on that cylinder at which time the spark is sustained across the plug for as long as the energy in the coil can sustain it.
                          In the case of the bad plug it appears all of the energy was used up before the coil was turned back on.
                          Maybe that is because it was taking excessive voltage to start a spark due to the plug being bad and it was used up more quickly as the arc was sustained across wherever it was going. Maybe the plug was going open which caused the crack and eventual short.
                          On the bad one, the coil ramp goes up gradually each time it is charged.
                          On the good, it jumps up when the coil is turned back on.
                          This is because on those events, the coil energy had not been completely used up before the coil was turned back on. It takes current over time to build the field in the coil. If the coil still has energy left from the previous charge, the current must immediately be able to jump to a level based on the field strength the coil still has left at that moment.
                          I am sure there are people that are more knowledgeable and can explain it better than I can so hopefully this makes sense.

                          Comment

                          • greasybob
                            Senior Member
                            • May 2008
                            • 1590

                            #14
                            Cool !!!

                            Comment

                            • crackerclicker
                              Senior Member
                              • May 2008
                              • 400

                              #15
                              Originally posted by Crusty
                              In the picture just under the cracked plug, the AMPLITUDE of the first strike goes up to almost 12.
                              In the second picture, lower down, the pink trace on the first strike, the AMPLITUDE only goes up to 7 or 8.
                              The trace that shows "12" amps is just the scope picking up RFI/EMI from the bad plug. Sparking outside the cylinder tends to encourage interference on those big antennas we call current probes.

                              Comment

                              Working...