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Inconsistent spark rear bank

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  • #581579
    ChrisChris
    Participant

      1990 Buick Century 3.3L V6. When I start the car for the first time of the day it will start fine and run fine until the engine warms up and then the rear 3 cylinders develop an irregular/erratic spark while the front 3 cylinders remain steady; I tested with a timing light with an ignition wire adapter. The car was parted out before I got my hands on it so a lot of sensors and other components have been replaced.

      The ignition parts that have been replaced are spark plugs, wires, coil packs, ICM, and crank position sensor and all sensors have been tested and they checked out fine. I also unplugged the O2 sensor (also new) and checked to see if that had an effect and it didn’t. It did set a check engine light after a while.

      Since this only happens when the engine is warmed up I don’t think it would have anything to do with wiring. The coolant temp sensor was replaced so I don’t think that’s the problem either. I tested the resistance of the injectors and they all checked out fine and they all sound fine (screwdriver method). Maybe the ECM is on it’s way out? Any help would be greatly appreciated.

    Viewing 9 replies - 1 through 9 (of 9 total)
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    • #581588
      EricTheCarGuy 1EricTheCarGuy
      Keymaster

        That’s a strange problem. I would isolate my search to the ignition system however. I don’t think the O2 or any other sensors would have anything to do with that. Don’t rule out the wiring. Warm or not it can still effect ignition output. If I were to guess I’d say you have a bad coil pack or ignition module, especially since the problem shows up when warm. That’s when most electrical components break down. I know you say those parts are new, but there could still be an issue. Especially if they aren’t name brand parts.

        Lastly, be sure it’s an ignition problem. There could be some other issue. You mentioned the car was ‘parted out’. Perhaps something could have happened during that.

        http://www.ericthecarguy.com/faq/solving-automotive-performance-issues

        Good luck and keep us posted.

        #581590
        ChrisChris
        Participant

          Thanks for the quick reply! I have tried 3 different sets of coil packs and 3 different ICM’s and the problem still exists (all are AC Delco). A friend of mine owns a salvage yard so I got lucky with testing out those parts. I also tested for vacuum leaks and voltage leaks and didn’t find any. I should also mention that the car has a waste spark setup. Each coil fires 2 cylinders simultaneously (one front bank cylinder and one rear bank cylinder). I did a power balance test by pulling the injector connectors and each cylinder had a similar drop in RPM so I can assume that the fuel is being delivered properly. I checked the wiring and the harnesses and they all looked fine.

          I have replaced just about every sensor on the engine with the exception of the knock sensor so could a knock sensor cause a problem like this? I’m not sure on how to test one.

          #581636
          BillBill
          Participant

            That’s a strange problem. One thing I do know about that engine combination is that the ICM has to ground to the bracket it’s bolted to. The battery ground cable fastens to a stud on the bracket. I have seen because of corrosion where the stud bolts through the bracket, the ICM does not get a good ground. I don’t know if this applies to your problem but it might be worth checking.

            #581638
            ChrisChris
            Participant

              I did sand down the bracket that the ICM sits on but I did not check that stud to see if it was corroded, I probably didn’t see it. I’ll check it tomorrow morning and post back here with the results. Thanks!

              #581694
              ChrisChris
              Participant

                I went out this morning and took a look at the bolt that you were referring to and it was a little rusted up but not too bad. I sanded it down and cleaned it up real good, put everything back together and started the car and the spark issue was still there.

                Just to be sure that the coil packs or the ICM aren’t the problem I hooked up the old ones and the problem still existed so I know that they aren’t at fault.

                I also removed the ECM and looked it over and I noticed that it had a slight electrical burning smell. I took it apart and everything appeared to be fine, there were no shorts on the circuit board and the wires and harnesses looked fine but that burning smell is a little suspicious.

                Could that smell be an indication that the ECM is bad or is it normal?

                #581829
                EricTheCarGuy 1EricTheCarGuy
                Keymaster

                  To be honest it may be beyond you at this point. If it were me I’d want to hook a lab scope up and look at the ignition waveform pattern. That can tell you what’s going on with the ignition system in a very detailed way.

                  #584644
                  ChrisChris
                  Participant

                    Well after a lot of troubleshooting I was finally able to figure out what was causing the erratic/inconsistent spark problem.

                    For those that have the GM 3.3L engine make sure that you have the most up to date MEMCAL because it fixes a bunch of problems that existed with the original MEMCAL the car came with. GM listed a service bulletin for the updated MEMCAL but it was aimed towards Oldsmobile’s with the 3300 since it was made in an Oldsmobile factory.

                    The updated MEMCAL broadcast code is BLYB 8289.

                    Hope that info helps!

                    #584698
                    BillBill
                    Participant

                      Hey, that’s a cool fix. I had a Buick one time that the torque converter would not go into lockup and it had the wrong prom in it. It’s very hard to obtain that info from anywhere in my experience. I would have to say that the mem cal was faulty rather than needing an updated one.

                      #584919
                      EricTheCarGuy 1EricTheCarGuy
                      Keymaster

                        Thank you very much for that update. It’s always great to have a conclusion to something like this. Your posts will go on to help others as a result.

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