Thread: 2001 cylinder 4 misfire only at idle

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  1. #1 2001 cylinder 4 misfire only at idle 
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    Hi folks,
    Have a 2001 Grand Prix with about 160k miles on it. It has a cylinder 4 misfire that is very consistant, but only at an idle, and when the motor is warmed up. In fact, it can even be warmed up, but the missfire will go away when the car is shut off for a few minutes. Restart car, and miss is gone for about 15 seconds until idle goes down to around 800rpm and then miss returns. Cylinder will miss a few cycles, then fire a few, in a rythmic pattern, and you can hear the miss and watch the tach climb and fall in about a 4 second pattern. This is my daughters car and I only get it a few hours on weekends. Here is what is done so far.
    New plugs and wires (AC Delco and custom Bosch wire set) They were definately due, but made no change in problem.
    Compression check, all cylinders looked good.
    switched coil pacs.
    Thought I heard a vacuum whistle on one start up so I sprayed whole top of motor with carb cleaner and no change in rpm anywhere.
    Checked fuel pressure at rail and was good and jumped up 10 psi when vac hose removed.
    Changed fuel injector cylinder 4 today.
    Getting desperate, cleaned MAF sensor today.
    Now it's gonna get weird.
    Bought a code scanner some time ago to see if problem moved to another cylinder while trouble shooting. Had scanner hooked up today to reset code after replacing fuel injector. At about the same time I unplugged what I think is the first O2 sensor to see what effect it would have and somehow blew key code out of computer. Car started once, and ran about 5 seconds, and stalled. Figured car could not run without that sensor. Plug it back in and start, and same thing, run 5 secs and stall. Takes me about 15 minutes to figure out it is key code problem, and not computer shutting down fuel because I blew up the computer by running engine with no O2 sensor connected. Whew!
    Then half an hour to reprogram key. Wild huh? Well that was the end of my trouble shooting window for today.
    It really sounds and feels to me like that cylinder is running lean after warm up. The plug is white and dry when you pull it after idling with the miss. I do not fully understand the role of the O2 sensors and wonder if one of them may be the problem. Are they just a fuel trim device after it is warmed up and ready(closed loop??) and are they not in play when engine is cold? I can read O2 voltages with my scanner and show OSS11 at .100 volt and O2S12 at .785 volt while the problem is happening. What should I be reading on these at idle, and are the O2 sensors indicating the problem, or causing it?
    Sorry about the long post, I am getting pretty frustrated.
    Anyone have any ideas or willing to explain O2 sensors role at idle when warm.
    One more thing, car runs great otherwise, plenty of power and engine is smooth off idle.
    Help!!
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  2. #2 Re: 2001 cylinder 4 misfire only at idle 
    Turbo is the way to go. Fivefingerdeathpunch's Avatar
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    The o2 sensor is for fueling, a narrowband sensor like that is will always be jumping around like that. You can reaserch more about that if you wish.

    Do you happen to know that gap you have them at? I know they say that you don't want to/need to gap the factory ac delco plugs, but if you are careful you can try to reduce the gap some and see if that helps.

    You could also maybe do a compression check and see if that cylinder is alot lower than the others.

    And lastly it possible but not likely, thats its the Ignition control module.

    SMGPFC Member #1
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  3. #3 Re: 2001 cylinder 4 misfire only at idle 
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    Hey FFDP,
    Thanks for the response. I meant to try gapping just that one plug at .050 and run a second compression test on that cylinder yesterday, and got so twisted around with the key/security thing I forgot. Will definately do that next time!

    Anybody know of a link to a good explanation of how the fuel injection system works at different times, cold, hot, idle, under acceleration, etc?
    I really want to know what determines injector pulse speed and percentage of on time, during a cycle.
    Thanks!
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  4. #4 Re: 2001 cylinder 4 misfire only at idle 
    GT Level Member Tony Mendez's Avatar
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    Not sure if this is what you are looking for but there is some decent info in this article
    HowStuffWorks "How Fuel Injection Systems Work"
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  5. #5 Re: 2001 cylinder 4 misfire only at idle 
    GT Level Member Tony Mendez's Avatar
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    When I am looking at misfire at idle conditions, I look to my scan data for clues. I have had more than a few customers who's concern was a misfire at idle, when in fact the misfire was there off idle also. As engine rpm increase, a misfire can be masked as rpm increases. Now if you are dealing with a secondary ignition misfire, you will feel it both at idle and when under load with the misfire becoming more noticeable as engine load is increased. Depending on what scan device you are using will determine the accuracy of your misfire graphing/tracking. If you have performed a compression test, I would recommend doing it again. Conduct both a wet and dry compression tests. This will help in determining if your are dealing with a valve train issue or a ring sealing problem. This is also a good time to closely inspect your spark plugs. I have seen plug insulators crack due to spark plug socket fit resulting in voltage grounding to the head before it reaches the combustion chamber.

    I would test and eliminate any engine mechanical issues as the cause of your misfire first. Next I would examine your ignition system for weak cylinder contribution...coil(s), ICM, plug wires, plugs. If you have eliminated mechanical and ignition from the equation, look at fuel delivery as the cause of your misfire. These diagnostics often require some equipment most do it yourselfers dont have. That doesnt mean you cant accurately diag your misfire, however it may be more difficult to isolate the cause of your concern. Best of luck.
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  6. #6 Re: 2001 cylinder 4 misfire only at idle 
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    Hey Tony,
    Thanks for the response. The "How stuff works" article was good and gave me a basic understanding of what does what. Will go back and do wet and dry compression test next time around. Bottom line is if O2 sensor was causing bad/lean trim condition at idle, why is it always cylinder 4 and not random cylinders. With good fuel pressure and new injector, new plugs and wires, I am starting to think mechanical/wear issue with #4 cylinder. Really don't want to take off that head. I see noid test lamps are pretty inexpensive, any chance of catching bad ICM pulses with noid lamps, or does it happen too fast? Hope to get car next weekend again, so we will see.
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  7. #7 Re: 2001 cylinder 4 misfire only at idle 
    GrandPrix Junkie
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    Mmm...idle only misfire...sounds like a certain GP I just did an engine on. Happened to have a worn lifter and cam on cylinder 4. Ended up killing the engine about 2000 miles after repaired. Cylinder leakdown test is what I would recommend doing next. If everything looks ok, you may end up finding something strange occurring internally...but I sure hope not.
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  8. #8 Re: 2001 cylinder 4 misfire only at idle 
    I live here. MoarkatsINmuhtrailer's Avatar
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    I had a miss on 4....













    Then I threw a rod...
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