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Misfire after extreme load

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    #1

    Misfire after extreme load

    I have a 1994 MGM, and it runs and drives great, no drivability problems EXCEPT for one thing. Consider the following scenario
    1. Engine is completely up to operating temp (to prevent damage)
    2. Engine is floored from any gear other than first
    3. Transmission drops a gear and engine revs high
    4. Engine shifts after reaching maximum RPM (because floored)
    5. Let off the gas
    6. Engine settles down fine
    7. Apply a light amount of gas
    8. An extreme misfire will occur SOMETIMES

    So what I'm saying, is after flooring it and letting off the gas, trying to apply a small amount of gas after the pedal-to-the-metal-action causes a pretty extreme misfire, but I'm not sure in which cylinder, the car will jerk and stutter violently, and if I let off the gas again and let it calm down, the misfire will go away. The misfire does not occur under any other driving conditions. The misfire doesn't even always occur after flooring it. Just sometimes.

    Spark plugs and wires done 2 years ago

    Have not done coil packs

    Oil changes every 3 months (odometer is broken)

    Transmission has fresh fluid from a flush about 8 months ago but it was transmission fluid from a shop, not Mercon V. No transmission issues so far.

    New serpentine belt

    Muffler removed, both catalytic converters inoperable (blown out).

    Check engine light does not occur when misfire does, and running diagnostics through the under hood diagnostics port comes up with a clean bill of health.

    What's going on here?
    Tags: None
    #2
    Probably a dead spot in the throttle position sensor.

    Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
    rides: 93 Crown Vic LX (The Red Velvet Cake), 2000 Crown Vic base model (Sandy), 2003 Expedition (the vacation beast)

    Originally posted by gadget73
    ... and it should all work like magic and unicorns and stuff.

    Originally posted by dmccaig
    Overhead, some poor bastards are flying in airplanes.

    Comment

      #3
      It almost feels like the transmission is seeking violently. But I'm not sure.

      Comment

        #4
        The throttle position sensor is just a variable resistor. You can test it with a multimeter (preferably analog - needle style). If you sweep the sensor slowly and the needle jumps at any point and it's not due to the probes coming loose/losing connection, then replace it. If it's all good, then it may be the ECM or the ignition module. The transmission just can't do that kind of stuff and still manage to actually function afterwards.

        Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
        rides: 93 Crown Vic LX (The Red Velvet Cake), 2000 Crown Vic base model (Sandy), 2003 Expedition (the vacation beast)

        Originally posted by gadget73
        ... and it should all work like magic and unicorns and stuff.

        Originally posted by dmccaig
        Overhead, some poor bastards are flying in airplanes.

        Comment

          #5
          Unless an injector is sticking when at full throttle and is flooding the cylinder, and/or coating the spark plug with fuel, causing the misfire?

          Comment

            #6
            I guess that's entirely possible..I should test the TPS and see what I find. I don't have an analog meter, but I can try to find one. Uh, I know for a fact that I can't remove the TPS, the screws are loctited or something. I would ruin the TPS before I got it off. Maybe I should just replace it...and as for the injector sticking, I suppose that's entirely possible as well, but why would an injector get stuck at full throttle? Wouldn't that be an ECM/ignition module command error?

            Comment

              #7
              No idea about the injector, but you could pull the spark plugs and see if any are coated in fuel.

              For removing the TPS, are the heads phillips style screws?

              Use your 1/4" ratchet with the 1/4" socket that will fit a phillips head screw tip and remove them that way. I have learned that this method works PERFECTLY for removing the loc-tite phillips screws for TPS removal, that would normally get stripped out when using a normal screwdriver. The 1/4" ratchet provides the extra torque necessary to break it free, while not rounding out the phillips head.

              Comment

                #8
                no need to remove the TPS to test it, just turn the car on but don't start it and probe the green wire with the black lead from the meter on a good ground point somewhere.

                it should sweep between 0.9 and 4.5V ish. http://www.grandmarq.net/oldfuelinjection/page30.html
                If there's dead/short spots (0V or 5V) at any point, even a digital will show you that, but you've gotta go much slower on your sweep to catch it.
                Last edited by sly; 11-25-2021, 03:48 PM.

                Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
                rides: 93 Crown Vic LX (The Red Velvet Cake), 2000 Crown Vic base model (Sandy), 2003 Expedition (the vacation beast)

                Originally posted by gadget73
                ... and it should all work like magic and unicorns and stuff.

                Originally posted by dmccaig
                Overhead, some poor bastards are flying in airplanes.

                Comment

                  #9
                  So I know this has been a long time since I've replied to this thread, but the problem still occurs. I haven't had a chance to check on the TPS, but I do not believe it is the problem, because I have found something out about this miss, after going full throttle, when the miss starts to occur (full throttle > let go of throttle at high rpm > try to apply light throttle > bad miss), if I continue to apply light throttle, the miss will never go away. On the other hand, if I push to half throttle, it acts like it "clears" the miss out.

                  I have brand new spark plugs and wires as of a couple weeks ago as well, and it doesn't help. I just can't understand why it does this.
                  Last edited by AmuletOfNight; 03-01-2015, 10:54 AM.

                  Comment

                    #10
                    could still be a dead spot in the TPS as that sensor is a major player in fuel mix.

                    Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
                    rides: 93 Crown Vic LX (The Red Velvet Cake), 2000 Crown Vic base model (Sandy), 2003 Expedition (the vacation beast)

                    Originally posted by gadget73
                    ... and it should all work like magic and unicorns and stuff.

                    Originally posted by dmccaig
                    Overhead, some poor bastards are flying in airplanes.

                    Comment

                      #11
                      I tested the TPS, and I got .30 volts no matter what the throttle position.

                      Comment

                        #12
                        Probing the green wire with the car on but not running?

                        Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
                        rides: 93 Crown Vic LX (The Red Velvet Cake), 2000 Crown Vic base model (Sandy), 2003 Expedition (the vacation beast)

                        Originally posted by gadget73
                        ... and it should all work like magic and unicorns and stuff.

                        Originally posted by dmccaig
                        Overhead, some poor bastards are flying in airplanes.

                        Comment

                          #13
                          I tried a different voltmeter since I thought that it was odd, the other voltmeter read correct. It read 1.21 volts closed throttle and a steady climb to almost 5 volts when open all the way.

                          Comment

                            #14
                            I have an analog meter I use specifically for this test to find dead spots... a digital might read a steady climb all the way... but an analog will show a drop during sweep if there a bad spot. If you have access to an analog meter, use it. If that shows a smooth sweep, the TPS can be considered good.

                            Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
                            rides: 93 Crown Vic LX (The Red Velvet Cake), 2000 Crown Vic base model (Sandy), 2003 Expedition (the vacation beast)

                            Originally posted by gadget73
                            ... and it should all work like magic and unicorns and stuff.

                            Originally posted by dmccaig
                            Overhead, some poor bastards are flying in airplanes.

                            Comment

                              #15
                              Misfire after extreme load

                              Well unfortunately I don't exactly have a analog meter, but I do think that the TPS is slightly out of spec, at 1.21 volts at closed throttle. I do suppose that maybe going full throttle and then throwing the throttle shut could cause the computer to think the throttle is still open slightly, over injecting gas based on the TPS voltage, flooding the cylinder next in firing line. Could that be it?

                              Comment

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