5.4 Cranking Compression

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  • greasybob
    Senior Member
    • May 2008
    • 1590

    5.4 Cranking Compression

    I was going to do an engine change on a 2006 F150 5.4. Before I removed the donor motor I decided to do a quick relative compression check. The results were not good. Next I checked the compression on each cylinder . After 5 or 6 strokes the results varied from 75 PSI on the low side (cylinder 1) to 175 on the high side (cylinder 6). The vehicle was a burn so running the engine was not an option. Next I did a cranking test to compare No. 1 to No. 6. The results are below. Maybe someone could fill me in a bit at what I'm seeing. A leak down test with some smoke added on No. 1 showed leakage through both intake and exhaust valves. Maybe I try it again with a faster sweep.
    Attached Files
  • tech25
    Senior Member
    • Oct 2017
    • 172

    #2
    Hello,
    when you did the compression check did you use a mechanical gauge?
    also, number 1 cylinder is off, i dont know much about these engines but i think they use a 3 valve head with roller finger followers? and VVT, perhaps remove the valve cover and inspect the valves for cyl. 1 to make sure valve train looks ok.

    Comment

    • Witsend
      Banned
      • Nov 2012
      • 2942

      #3
      The vehicle was a burn so running the engine was not an option. Next I did a cranking test to compare No. 1 to No. 6. The results are below. Maybe someone could fill me in a bit at what I'm seeing. A leak down test with some smoke added on No. 1 showed leakage through both intake and exhaust valves. Maybe I try it again with a faster sweep.
      Have to wonder how many of the insurance covered vehicles that burned or were totalled , the owner probably was likely well aware that it was going to need some sort of major engine or transmission repair before the unthinkable happened.

      Comment

      • diesel71
        Senior Member
        • Feb 2008
        • 921

        #4
        Bob did you happen to have a battery charger hooked up when you did the relative test? . reason I asked is I always get a bad wave forum with a charger hooked up. what looks different to me is the amplitude rise on the cylinder after the bad one. I usually see a higher amplitude after the bad hole.

        Comment

        • diesel71
          Senior Member
          • Feb 2008
          • 921

          #5
          Below is a screen shot of a 7.3 diesel with one bad cylinder. you can see the next cylinder has just a little higher amplitude, this is what I usually see. bob your screen shot may be showing a bad cylinder and then then next one week?
          20171027-2048-2.jpg

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          • David Green
            Senior Member
            • Oct 2009
            • 382

            #6
            I'd of probably used an amps clamp around the battery lead, connected the scope to a CMP or CKP sensor to measure position, and used a RPM measurement for cranking speed. Using these methods you could then see individual cylinders for starter current draw, cylinder speed and each relative compression to each other. Hence if you knew the starter current draw should be a given value x, lets say 300 amps, then checking the peaks at each cylinder would show weak compressions by comparisons.

            Hope that makes sense!

            Comment

            • Witsend
              Banned
              • Nov 2012
              • 2942

              #7
              I'd of probably used an amps clamp around the battery lead, connected the scope to a CMP or CKP sensor to measure position, and used a RPM measurement for cranking speed. Using these methods you could then see individual cylinders for starter current draw, cylinder speed and each relative compression to each other. Hence if you knew the starter current draw should be a given value x, lets say 300 amps, then checking the peaks at each cylinder would show weak compressions by comparisons.

              Hope that makes sense!

              Makes sense to me, but what doesn't make sense to me at all is that an archaic ancient Counselor 2 has a wonderful cranking amps test with nice IDS like bar graph display compared to the ragged POS hump displays of the later more sophisticated vantage , modis , Verus platforms, that for this test at least makes those handhelds seem like Tinker Toys, LMAO. Why?
              Well gotta Love looking at an Iowa Class Battle ship shooting off it's big 30"guns like I shoot off my mouth, but I have to admit technology marches on, and it's a shame one of those tiny ugly Lego block looking stealth destroyers could take out an Iowa class battle ship from an internet controlled drone from 200 miles away without firing a shot.
              Attached Files
              Last edited by Witsend; 10-29-2017, 05:43 PM.

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              • GypsyR
                Senior Member
                • Jul 2017
                • 287

                #8
                Holy ****! It's like the best cranking amps test display screen ever. How the hell was THAT lost?
                I've never even seen a Counselor in person and now I want one just for that.

                Comment

                • tech25
                  Senior Member
                  • Oct 2017
                  • 172

                  #9
                  funny, I remember when i posted a nice pico scope bar graph when someone was asking about a pressure transducer and seemed no one liked the bar graph the pico scope makes, instead they wanted a wavy graph. i guess its snap on fan boys sometimes...

                  Comment

                  • Witsend
                    Banned
                    • Nov 2012
                    • 2942

                    #10
                    Holy ****! It's like the best cranking amps test display screen ever. How the hell was THAT lost?
                    I've never even seen a Counselor in person and now I want one just for that.
                    Reply With Quote
                    Bar Graph Cranking amps test seems to work reliably on it without errors if you have conventional cap rotor,and wire set up or in this case a GM-LS 1 that has the separate coils and ignition wires. Gets kinda Iffy on DIS and COP using #1 pickup around the #1 with waste spark , COP or fuel injector primary wires, but you can still set it two channel lab scope and use Hi amp probe cranking and look at the humps and just verify compression deficit with a compression gauge of the cylinder that has the high misfire counts or missfire code.

                    Comment

                    • Sir Obi Bob
                      Junior Member
                      • Jun 2009
                      • 5

                      #11
                      5.4 Relative Compression

                      Hey Bob,

                      You might want to add a synch signal from an ignition source to the relative compression and in cylinder pressure tests. On the relative compression it will help identify the weak cylinders(s) in the firing order. It will show you valve train timing issues on the in cylinder pressure test. Good friend of mine, Brandon Steckler, has done a lot of work with this type of testing. I'm pretty sure he's posted some of it on iatn. If not let me know and I'll get some info to you.

                      Later, Bob

                      Comment

                      • Witsend
                        Banned
                        • Nov 2012
                        • 2942

                        #12
                        I'm not sure how that COP #1 trigger reference will work off the primary circuit if each coil on a Ford has that goofy multi strike spark discharge setup. I think I would suspect the cylinder with the miss fire code , high misfire counts, power balance test deficit, and confirm the low compression hump from relative compression test without a #1 trigger reference . Confirm that suspect cylinder with a low compression by testing that particular cylinder with the High misfire count or power balance test is indeed low with a compression gauge.

                        Comment

                        • Sir Obi Bob
                          Junior Member
                          • Jun 2009
                          • 5

                          #13
                          Originally posted by Witsend
                          I'm not sure how that COP #1 trigger reference will work off the primary circuit if each coil on a Ford has that goofy multi strike spark discharge setup. I think I would suspect the cylinder with the miss fire code , high misfire counts, power balance test deficit, and confirm the low compression hump from relative compression test without a #1 trigger reference . Confirm that suspect cylinder with a low compression by testing that particular cylinder with the High misfire count or power balance test is indeed low with a compression gauge.


                          Agree that there are easier and faster ways to identify problem cylinders. Was leaning more to using an ignition synch with the pressure waveforms.

                          Bob

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