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Random misfire on Acura – and seems mysterious

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  • #989572

      1999 Integra LS with 144k miles and Auto, B18B1 – no mods
      Well maintained. Due to the random misfire at idle (car shakes too), did the following still an issue:
      P0300, P0304 and at times P0303 – now all cylinders
      – sparks (OEM), wires (after market)
      – front O2 sensor (OEM)
      – cap & rotors
      – valves adjusted 22k miles ago
      – air filter has 8k
      – cleaned IACV
      – Compression were from 175 to 190
      – Spark plugs did not show any abnormality
      – Swapped ignition coil but no change
      – Batt and Alternator tested fine
      – cold start gave gas smell through exhaust
      – at times smelt gas under the hood but no sign of leaks
      – engine compartment is mostly dry – needed to put new O-ring on Dizzy – the issue started before this repair
      – Lately idle misfire goes away when warms up but happens at 2500rom – then misfires on all Cyl
      – only known issue is the car would need jump start if you don’t drive for 2 weeks & high NOx

      Fuel Trim:
      1500 rpm 2500 rpm
      LTFT 6.2 6.2
      STFT 6.2 8.6

      After I disconnected the battery and let all the Capacitors discharge at idle:
      LTFT – 4.6 4.6
      STFT – 7.8 – 12.5

      Had the car for more than 10 yrs.
      Any help? How can I get Eric to reply?

      Can 87 vs 91 gas or additives cause misfire?

      • This topic was modified 3 years, 1 month ago by ms2000.
      • This topic was modified 3 years, 1 month ago by ms2000.
    Viewing 15 replies - 1 through 15 (of 31 total)
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    • #989577
      Nightflyr *Richard Kirshy

        Fuel trims seem a bit high.
        Have you checked for vacuum leak(s)?
        Might also check flow rates on the fuel injectors.


          Thank you for being spot on.

          There was air leak around the IACV – M/c fixed it. Now its on Rich condition.

          Last yr, AC would not work on idle – it turned out to be IACV was clogged. It was cleaned and everything worked fine.

          O2 sensor, Plugs, are new.

          Nightflyr *Richard Kirshy

            First try resetting the ECU and let it relearn the fuel mapping.
            Next would be to check the flow rate of the injectors.


              So I asked the shop He said rich condition was there by looking at LTFT/STFT.

              Fast forward, he found P0304 was caused by a bad injectors. And Fuel pressure/Regulator were fine.
              So used a reman 4 injectors and misfire is gone.

              After coming home, I noticed LTFT was quite high at 14.
              Today I changed engine oil and within about 3-4 miles of driving, it gave P0171. It happened around 2900rpm.
              Coolant at 172, MAP 10.15psi, LTFT = 19, STFT= -0.78, 26.7mph
              Load = 69%

              Could PCV be the culprit? The PCV hose is also stiff. But shop found issue on IACV gasket – so they would have checked for leaks. Only reason I asked because of oil change.

              Car runs fine – no hesitation even with the code.
              Any advice?

              Nightflyr *Richard Kirshy

                My suggestion would be to smoke test the engine and see if there are any vacuum leaks.
                If the smoke test doesn’t show any leaks.
                You could have a bad MAF sensor ( if so equipped ) that is miss reading.
                Issue with the replacement fuel injectors.


                  It has a MAP sensor
                  No EGR


                    These are more accurate desc of the issue:
                    – When I am not accelerating while driving, the STFT shoots up
                    – When the car is idling, the STFT slowly creeps up

                    So the car earns the P0171 CEL at idle
                    When driving steadily, CEL goes off.

                    Does this tel anything?


                      Further to my last post, it seems that the (new NTK) O2 sensor is not seen to be responsive enough as per the Scanner.
                      Could this be ECM?

                      Nightflyr *Richard Kirshy

                        Have you done a smoke test yet?


                          Today I did the pinch test on EVAP and Brake booster cables when the engine is on closed loop. No change in fuel trim. I am going to build a Smoke m/c and test it or find another shop with smoke machine.


                            I did a smoke test using this mtd.
                            I didn’t find a leak. I might have to re-do and use 2 Cigars instead of one.

                            Is there any special procedure to do to the car before smoking?


                              At 80k, I upgraded to Platinum NGK spark. No issues. At 144k, replaced with new Platinum NGK spark.
                              Could this be causing it?

                              Also, this time, I put 5 gals of 91 gas + 7 gal of 87 gas. I have always used 87 gas only as recommended by the Honda. Hope this is not causing it.

                              Nightflyr *Richard Kirshy

                                Did you seriously try that smoke test by yourself???????
                                Here is a little science for you…
                                While doing a smoke test, it is required that a minimum of positive pressure be maintained into the system to allow the smoke to escape from any possible leaking area.( keep that bit of information in mind )
                                Now you start pumping up your homemade smoke machine to begin filling the engine intake ( BTW, did you remember to fully open the throttle body butterfly during the testing?)
                                Now what happens to the positive pressure your applying while pumping when you stop pumping to begin inspecting for any possible leaks in the engine compartment, under the vehicle, back around under the fuel tank and by the filler neck?

                                Concerning your spark plugs ….
                                If NGK’s worked and were the correct fitment for your engine.
                                A new set of replacements should cause any issue(s) UNLESS…
                                They were improperly gaped
                                Or if they were counterfeit NGK plugs. Yes that is a thing these days, you can look up counterfeit NGK spark plugs.
                                Mixing grades of gas, though not a great thing to do should not cause any major running issues.


                                  I tried with the throttle body butterfly open!

                                  I might need to use two Cigars – some smoke escaped through my improvised connection.

                                  Spark plug was upgraded to Platinum at 80k and I maintained it at 144k too. I bought it at OReilly store not online. It is the correct fitment with upgrade.


                                    Does this condition has anything to do with idle Air Control Valve (IACV):
                                    – the car earns the P0171 CEL at idle
                                    – When driving steadily, CEL goes off.

                                    The car last yr gave P0505 – also AC would shuts off at idle.
                                    So I cleaned the IACV and AC started working fine.
                                    I didn’t change the IACV gasket that it introduced a leak which shop fixed – Ever since shop fixed it, the P0171 came.

                                  Viewing 15 replies - 1 through 15 (of 31 total)
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