Misfire (P0306) on a '98 Olds LSS with L36 engine (fixed) - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 07-12-2011, 03:28 PM   #1
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Default Misfire (P0306) on a '98 Olds LSS with L36 engine (fixed)

My car has gotten a progressively worse misfire recently. Started as occasional bucking and stumbling under modest acceleration, progressed to doing it while cruising at Hwy speeds. It'* now doing it all the time, even at idle. The Check Engine light came on a couple of days ago. Had it checked at AutoZone and it threw a P0306 code. The light goes on and off. Yesterday it started blinking, then stopped. The light went off again. Came back on.

Started investigating and perused some of the threads on this forum. First, I checked to make sure the plug wires and the O2 sensor wire weren't touching. Check, although I re-routed the O2 wire for good measure.

I did the UIM/LIM job back in March, so I wondered if I did something wrong and it was starting to leak coolant into the #6 cylinder. Pulled the plug and it was dry as a bone. Re-checked the torque on the throttle body bolts, too. Everything looked good.

I pulled the plug on the MAF sensor to see if that was the problem. It idled noticeably lower, but it was still misfiring.

I used a multimeter to check the coils and spark plug wires, too. Coils look good. A couple of the wires are questionable no readings at all on the wires to cylinders 1 and 6. So I'll install a new set of wires.

Still have a couple of concerns, though. One thing I haven't checked yet is the ICM. That'* beneath the coils, right? Can you test it with a multimeter, or do I need to pulled it and take it somewhere for testing?

HOWEVER, before I do that, if leaking coolant or a faulty spark is the problem, wouldn't the spark plug be wet, not dry? Could the dry plug in combination with the misfires be a sign of a faulty fuel injector?

Should I just pull the rail and get all of the injectors cleaned? Or should I wait until I replace the SP wires? Anything else fuel injector-related, or otherwise, I should check?

Someone let me know if I'm on the right track, and any advice would be GREATLY appreciated.
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Old 07-12-2011, 04:06 PM   #2
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i wouldnt get too crazy one thing at a time. do plugs if you havent just done them and wires make sure you dont have any corrosion on the coil terminals, and use a good amount of dielectric grease then see where your at.
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Old 07-12-2011, 10:21 PM   #3
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Single cylinder misfire. Plug and plug wire. Best place to start.
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Old 07-12-2011, 11:12 PM   #4
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Wassup ScottyB, I would say you're definitely on the right track, But also like JW said don't throw so many parts at the problem at one time. Yes you will probably fix the problem but have no idea which part fixed the problem (Right). If you ohmed out the spark plug wires and got no reading on either 1 or 6 then definitely replace your wires and at least the plug in #6 cylinder unless it'* been a while since they were replaced.
Good Luck
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Old 07-19-2011, 09:12 AM   #5
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Meant to respond sooner. Label this one Fixed.

Spark plug wires. That'* all.

Still paranoid after the UIM/LIM job, and I have some other maintenance jobs looming. I'm a home mechanic by necessity (cost), not because I love it, although I do get gratification from solving problems.

I'm incredibly thankful for this forum, but it makes me overthink a little, I supposed.
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Old 07-19-2011, 09:16 AM   #6
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Good to hear it'* fixed.
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Old 09-05-2011, 05:17 PM   #7
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Default Same problem with no improvement after plugs and wires

Hey guys I have a 2003 alero gls 3.4L V6 and I am experiencing the same symptoms. Lack of power began a little while ago and has goten progressively worse and rendered the car unsafe to drive on the road. As well as repeated SES lights for Multiple Misfires code.

I have replaced the plugs and wires. Also I bought one new coil pack and one by one replaced all three with no improvements with the new coil pack in any position.

I have recently had an induction service performed (after car began to not make power under all driving conditions).

I recently changed the air, oil, and fuel filters last time i changed the oil (after car began to lose power under heavy acceleration but before it became a huge problem under all driving conditions).

Additionally I have used a fuel system treatment additive before last fillup and have used the whole tank of gas.

I do not know where to look next. Any input is welcome and appreciated. Thank you for your time and consideration.
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Old 09-06-2011, 09:33 AM   #8
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If possible, have the car scanned for live data and watch for knock retard. Also, are you only getting P0300 or are you getting some P030X codes? The ICM handles all the coils. So it'* possible the ICM is going bad.
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Old 09-07-2011, 09:05 AM   #9
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There are many here who are far more knowledgable than I, but maybe it could be a failing fuel pump? Perhaps have your fuel pressure checked, as well.
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Old 09-07-2011, 09:19 AM   #10
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The engine is throwing the 300 code for random multiple misfires.

I will have the fuel pressure test done next, I'm hoping that I can do it in the garage.

I would like to add that last night I tried unplugging the MAF to see what would happen, the car searched at idle but for the the first trip around the block the car drove flawlessly, very strong. However the next go round the car ran just as bad as before and the trans starting shifting really hard and abruptly, so i reconnected the MAF. Additionally, i changed the O2 sensor upstream of the CAT later in the night, and got the same results minus the trans shifting thing. The first time around the block with the new sensor the car drove and accelerated flawlessly but on the next lap ( after-all, I was testing for acceleration under heavy load) the problems came back and the car stumbled and slowly accelerated.

Would these symptoms lead me to a timing change caused by a knock sensor? If so would it be cheaper to just replace the sensor as opposed to having it tested and possibly replaced?

So here'* where I think I'm at. (please tell me if this is not appropriate.)

1) I need to test the fuel pressure.
2)Then I need to test or replace the knock sensor after that if the problem persists. 3)Finally I need to know whats going on with the ignition system under operating conditions to determine if i need to replace the ICM.
4)Get really frustrated and swear loudly
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