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Engine Missing . . . . .And I'm Stumped!

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  • #1

    Engine Missing . . . . .And I'm Stumped!

    Hey everyone! I really need your help because I'm stumped - I just can't figue this one out! My Z28 has got a rough idle and the engine misses while accelerating. It's got about 130K miles. In the last couple of months, I've replaced the wires, plugs, opti cap & rotor. Then, when the problem started last week, I replaced the coil and ignition control module but no improvement. The head gasket is fine and there isn't a intake manifold leak - I changed the oil and everything looks normal. I got a solid Service Engine Soon light for a couple of minutes this morning but it went away after driving for a while. I took it to Autozone for a scan and they told me that the codes show a "random miss" and a rear O2 sensor is bad but they don't know if it's the one before the cat or after the cat.

    I'm lost on this one! Would an O2 sensor cause symptoms as severe as this?

    What else do you think it is?? Thanks for your help!
    Darrin C
    '97 Z28 LT1 157K (((S O L D ))) A4, C/I Cold Air Induction, Flomaster Exhaust, SLP Fan Control Mod, Eibach Springs w/1" Drop, Racing Dynamics Shock Tower Brace, Lakewood LCA's.
    07 Ford F150 - Daily Driver. I went from f-body to f-series. I think I'm out of my f'in mind.....
    Tags: None
  • #2
    It could be the opti sensor.
    Richard Harvey Jr.
    '94 T/A LT1 (stock) - SOLD

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    • #3
      A rear O2 sensor would not cause the missing you describe, but a front o2 could for sure. Its very common for o2 sensors to do this on our cars. You really need to get it on a scanner and see what its doing while its stumbling. If youre sure the ignition system is up to snuff, than youve got to move on to fuel.....it may be a control issue (like o2's or other sensors) or a delivery problem (like fuel pump or injectors).
      96 WS6 Formula: Ram Air, 383 Stroker, Ported LT4 Heads and Manifold, 1.6 Crane Rollers, 58MM T.B., AS&M Headers, Borla Exhaust, Meziere Elec. H2O Pump, Canton Deep Sump Oil Pan, 100 HP OF TNT N2O!! , T56 Conversion w/ Pro 5.0 shifter, SPEC Stage 3 Clutch, Hotchkiss Subframe Conn., Lakewood Adj. Panhard Bar, Spohn Adj. LCA's, BMR Adj. T.A., Custom 12 bolt w/ 3:73's, Moser Axles, Eaton Posi, Moser Girdle
      11.6 @ 123mph (1.6 60' - getting there )

      Comment

      • #4
        Check to make sure that all of your plug wires are firmly snapped into place. Also check to make sure that you don't have any damaged wires that are routed close to the accesories. Those are a few of the many causes of a random misfire. Also there have many posts about people having a problem with their ICM getting heat soaked and causing problems with misses. They spaced the ICM away from the head and in most cases it seemed to help.

        Basically the PCM sees a "misfire" as a varation in the expected crank angle in relation to firing order, cam position, and engine speed. So it is possible it is a loose or malfunctioning crank sensor as well.

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        • #5
          I put new plug wires on mine and it had a severe miss. Turned out it was the stupid metal heat shields. I took them off and it ran perfect. My neighbor had run into that same problem before. I may still owe him a 6 pack for that one.
          2002 Electron Blue Vette, 1SC, FE3/Z51, G92 3.15 gears, 308.9 RWHP 321.7 RWTQ (before any mods), SLP headers, Z06 exhaust, MSD Ignition Wires, AC Delco Iridium Spark Plugs, 160 t-stat, lots of ECM tuning

          1995 Z28, many mods, SOLD

          A proud member of the "F-Body Dirty Dozen"

          Comment

          • #6
            Originally posted by Jeff 95 Z28
            I put new plug wires on mine and it had a severe miss. Turned out it was the stupid metal heat shields. I took them off and it ran perfect. My neighbor had run into that same problem before. I may still owe him a 6 pack for that one.
            Were the heat shields cutting into the wires?

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            • #7
              Well, after a spark plugs & front oxygen sensors replacement I got a P0300: cylinder 4

              Spark plugs are fine, spark plug wires...I don't know exactly (the mechanic said are fine???), fuel injector maybe faulty, cranck position sensor faulty?????, oxygen sensors faulty???, even a DNS module (opticrap) faulty???
              I would like to have enough time to check each posibility by myself, but I work in 2 hospitals... my daily car is parked in the mechanic shop without a reliable clue... in the meantime I have to wide the budget in order to purchase the "faulty"-guessing-sensors-wires-etc etc etc...

              Dochidalgo
              97 A4 Pontiac Firebird Trans am with $everal mod$

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              • #8
                A couple of you are referring to a "crank sensor." Is this the same as the Opti Sensor?
                Darrin C
                '97 Z28 LT1 157K (((S O L D ))) A4, C/I Cold Air Induction, Flomaster Exhaust, SLP Fan Control Mod, Eibach Springs w/1" Drop, Racing Dynamics Shock Tower Brace, Lakewood LCA's.
                07 Ford F150 - Daily Driver. I went from f-body to f-series. I think I'm out of my f'in mind.....

                Comment

                • #9
                  Nope, they are different devices.
                  The cranckshaft position sensor
                  The DNS module or optispark or "distribuitor"

                  Dochidalgo
                  97 A4 Pontiac Firebird Trans am with $everal mod$

                  Comment

                  • #10
                    Definitely check those spark plug wires first...make sure they are all on nice and snug. Check them on your opti as well. I'm not too sure about the heat shield boots because I've never encountered that problem, but it's definitely worth a shot.

                    After that, move on to your opti sensor. If that isn't it, check your ICM and coil. Just like FastTA said, it could be getting heat soaked. I don't know the term here, but check the piece on your coil that hooks up to your opti with that spark plug like wire. Make sure it isn't corroded. Did you replace that wire that hooks up to your opti from your coil as well?

                    Those are the easy things to get out of the way. If I wasn't successful after that, I would move on to my front O2's and fuel system. Check my injectors...etc...

                    Good luck bro!
                    black 95 t/a, a4, beefed up tranny w/ higher stall converter, transgo shift kit, trans temp gauge, trans cooler, richmond 3.73's, loudmouth, hypertech programmer, 160 thermo, descreened maf, TB bypass and airfoil, trick flow intake elbow, underdrive pulleys, moroso cai, edelbrock panhard rod, bmr stb, slp sfc's, fiberglass firehawk hood, hawk pads, taylor wires, ngk plugs, royal purple fluids,...and hopefully more to come

                    Comment

                    • #11
                      Originally posted by fastTA
                      Were the heat shields cutting into the wires?
                      No. It was the strangest thing I have ever seen. I've been dealing with electronics for 25 years and I have never seen anything like it. It appeared to be going around the end of the boot which makes no sense considering the shields are not grounded or anything. They are just sitting there. But I'm telling you I took them off and it fixed the skip.

                      My neighbor builds performance engines for a living and had run into this before. That's how he knew to remove them. I was skeptical but said what do I have to loose. Funny thing is he knew what it was from across the street. Dang I hate it when people do that.

                      If they put the heat shields on the wires, take them off. It won't cost you a dime.
                      2002 Electron Blue Vette, 1SC, FE3/Z51, G92 3.15 gears, 308.9 RWHP 321.7 RWTQ (before any mods), SLP headers, Z06 exhaust, MSD Ignition Wires, AC Delco Iridium Spark Plugs, 160 t-stat, lots of ECM tuning

                      1995 Z28, many mods, SOLD

                      A proud member of the "F-Body Dirty Dozen"

                      Comment

                      • #12
                        "Engine Missing . . . . .And I'm Stumped!"

                        psst.... it's under the hood.

                        Seriously......


                        Originally posted by Jeff 95 Z28
                        No. It was the strangest thing I have ever seen. I've been dealing with electronics for 25 years and I have never seen anything like it. It appeared to be going around the end of the boot which makes no sense considering the shields are not grounded or anything. They are just sitting there. But I'm telling you I took them off and it fixed the skip.

                        My neighbor builds performance engines for a living and had run into this before. That's how he knew to remove them. I was skeptical but said what do I have to loose. Funny thing is he knew what it was from across the street. Dang I hate it when people do that.

                        If they put the heat shields on the wires, take them off. It won't cost you a dime.
                        Mine did the same thing. I used taylor firesleeves in place of the metal boot covers and no problems since. Never could figure out why this happened. The plugs and wires were ACDelco too.

                        Comment

                        • #13
                          Originally posted by 97LT1Z28
                          A couple of you are referring to a "crank sensor." Is this the same as the Opti Sensor?
                          96-97 LT1's have a crank position sensor located at the bottom right of the timing cover. It detects the crank's angle by way of a reluctor wheel. This is an important factor used to determine the timing map or the firing of the plugs. It is separate but not isolated from the data from the "front" O2 sensors which provide a voltage reading in which the PCM adjusts the fuel trim by way of injector pulsewidth and hence a/f ratio.

                          The PCM has the ability to monitor the integrity of the opti but remember the ICM and Coil have to receive a good signal from the opti in order to correctly fire the plugs. So you may want to check the opti short harness as well. It is not uncommon for it to become corroded.

                          Comment

                          • #14
                            Originally posted by Jeff 95 Z28
                            No. It was the strangest thing I have ever seen. I've been dealing with electronics for 25 years and I have never seen anything like it. It appeared to be going around the end of the boot which makes no sense considering the shields are not grounded or anything. They are just sitting there. But I'm telling you I took them off and it fixed the skip.

                            My neighbor builds performance engines for a living and had run into this before. That's how he knew to remove them. I was skeptical but said what do I have to loose. Funny thing is he knew what it was from across the street. Dang I hate it when people do that.

                            If they put the heat shields on the wires, take them off. It won't cost you a dime.
                            Well the reason I asked is because I recently replaced the Vortec motor in my 1998 Tahoe and I had the exact same problem. No visible ground discharge from the wires in the dark but I had a definite ignition miss. We could not figure it out. Brand new Delco Platinum plugs and new wires and a big time miss perceived by driving the truck and scanner info. The scanner showed a random misfire in alternating cylinders so I was confused being that it was essentially a brand new ignition system. Checked all of the usual suspects including all sensors. I noticed that most of the plug wires were physically touching the manifold heat shields. So I removed them and walla, no more miss. Very strange....I'm not sure why the plug wires would ground through their insulation to the heat shields, but apparently they were.

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                            • #15
                              But like Joe mentioned, you have to use firesleeves in place of the heat shields or else...

                              Comment

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