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post #1 of 23 (permalink) Old 10-09-2017, 06:27 PM Thread Starter
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Hi everyone, having weird issues with my 2010 cobalt LT. Having a bad misfire and CEL code for only cylinder 3 misfire recently. Replaced all coils and plugs just to discover that cylinder 3 is receiving no electricity. Anyone else had this issue? I was advised I may have to do a reflash.

Other symptoms: rough idle with rpm at 600 or below, often misfiring after about 1 minute of idling with flashing CEL unless I throttle up to 1000 rpm or more. Low gears often shift roughly (P to D, P to R and Drive as it shirts into first and sometimes second-auto transmission).

Last edited by Stray; 10-09-2017 at 06:40 PM. Reason: Adding additional info.
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post #2 of 23 (permalink) Old 10-09-2017, 07:29 PM
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We've covered P0303 lately, do a search. P0303 usually has nothing to do with cylinder 3.

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post #3 of 23 (permalink) Old 10-09-2017, 09:54 PM Thread Starter
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Looking through these, the closest I see sounds like o2 sensor issues, though i keep seeing that p0303 is a ghost code for up to '08, do the '10s have it too? I'm baffled by the lack of voltage though. The coil for cylinder 3 also had discoloration where it meets the plug (brass tone with splashing). I didn't see anything like that or the literal lack of voltage. I don't have access to anything but a basic scanner to test anything else though. The car was violently misfiring though and now sounds like a cartoon jalopy chugging along at a limp. I'm on a tiny budget, so what would you suggest I start with? This is our only vehicle, so I'm a bit stressed over it, apologies if I sound rude at all).

( so far up to this point I have replaced all coils and plugs, run seafoam through the fuel system as well as upper engine cleaner through the through throttle body and cleaned throttle body and cleaned MAF sensor).

Last edited by Stray; 10-09-2017 at 10:27 PM.
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post #4 of 23 (permalink) Old 10-10-2017, 04:30 AM
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You may have the first actual good P0303 then, unless you assumed that because you had a P0303 that everything looked bad at cylinder 3. It is easy to go off track with these. Maybe your cat is plugged..... Sometimes we have to bite the bullet and visit an actual mechanic. Not the dealer, they'll rob you blind but a reputable corner garage.

By the way I didn't find you sounded rude, frustrated maybe...never fun.

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post #5 of 23 (permalink) Old 10-10-2017, 05:51 AM Thread Starter
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Sadly the fun part is finding a decent shop in my area. If I can find one that will listen to me and check the right things instead of go on a wild goose chase, I'll be happy. My father's recommended 'shade tree mechanic ' wanted to replace the throttle body and the MAF sensor without really looking at the car. Time to do some research on my local shops.
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post #6 of 23 (permalink) Old 10-13-2017, 12:39 PM Thread Starter
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Update! Car is in the shop, actual cylinder #3 misfire confirmed. Also weak injector on #3. Before they did anything, they found a GM service bulletin for excessive carbon build-up in the engine. So, we're headed of the the dealer for a second diagnosis... I'll update with what they find. Has anyone else heard about this bulletin?
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post #7 of 23 (permalink) Old 10-13-2017, 11:05 PM
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Can't say I've heard of that one, to be honest. If you have a weak injector, that could definitely be affecting how cylinder 3 is working.

I'm assuming if you ground the cylinder 3 spark plug to the block you won't get any kind of spark?

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post #8 of 23 (permalink) Old 10-14-2017, 06:44 AM Thread Starter
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I'm not super knowledgeable myself, though my part-time mechanic simply used a multimeter and found the lack of voltage at the electrical connection for the #3 cylinder (trying to narrow down where the issue came in since the whole car shook with the misfire). I'm hoping it's simply the carbon build-up (the throttle body was filthy even after two seafoam upper engine treatments - burned on carbon on the edges of the butterfly) and clogged/ failing injectors. Honestly really curious why it lacked electrical power though...i was warned I may need a reflash as well. I hope to hear back by Monday from the shop either way (first mechanic looked over the car and ran scans verifying the misfire and recommended following the bulletin with the dealer first). I'll post the bulletin below (for reference, it is bulletin PIP5029).

Last edited by Stray; 10-14-2017 at 06:54 AM.
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post #9 of 23 (permalink) Old 10-14-2017, 06:47 AM Thread Starter
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PIP5029: Engine Misfires Due To Major Carbon Deposits On The Intake And/Or Exhaust Valves - (May 29, 2012) Subject: Engine Misfires Due To Major Carbon Deposits On The Intake And/Or Exhaust Valves Models:
2008 - 2012 Cadillac CTS, STS 2008 - 2010 Chevrolet Cobalt SS, HHR SS 2007 - 2010 Pontiac Solstice GXP 2007 - 2010 Saturn Sky Redline 2009 - 2012 Buick Enclave 2009 - 2012 Buick Lacrosse 2009 - 2012
Chevrolet Traverse 2009 - 2012
GMC Acadia 2009 Saturn Outlook 2010 - 2012 Cadillac SRX 2010 - 2012 Chevrolet Camaro, Equinox 2010 - 2012 GMC Terrain With any of the Following Direct Injected Gasoline Engines: 2.0 (RPO LNF) 2.4L (RPO LAF, LEA, or LUK) 2.8L (RPO LAU) 3.0L (RPO LF1) 3.6L (RPO LFX or LLT) The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.
Condition/Concern: Some customers may complain of a MIL and engine misfire. In some cases, the misfire may be more apparent on a cold start, may count on a single cylinder or several cylinders, and may or may not be felt by the driver. Upon inspection, the technician will find one or more misfire codes (DTC P0300-P0306) stored in the ECM and SI diagnosis may or may not isolate the cause of the misfire depending on whether the intake/exhaust valves are sticking at the time of the diagnosis. This may be the result of major carbon build up on the intake and/or exhaust valves as shown below so the misfires should not have appeared until the engine has accumulated around 5,000 miles or more.
Recommendation/Instructions: If this concern is encountered, perform SI diagnosis. If SI diagnosis isolates a valve sealing concern and/or eliminates everything else external to the engine, decarbon the engine with Upper Engine and Fuel Injector Cleaner by following the guidelines below:
Important Extreme care must be taken not to hydrolock the engine when inducing the cleaner, especially if it is induced without Kent Moore Tool # J-35800-A. If too much cleaner is induced at too low of a RPM, or if you force the engine to stall by inducing too much cleaner at once, the engine may hydrolock and bend a connecting rod(s).

1. In a well-ventilated area with the engine at operating temperature, slowly/carefully induce a bottle of GM Upper Engine and Fuel Injection Cleaner into the engine with RPM off of idle enough to prevent it from stalling (typically around 2,000 RPM or so). Depending on the engine configuration, induce the cleaner through the throttle body or an engine vacuum hose/pipe. For best results, it is suggested to induce the cleaner with Kent Moore Tool # J-35800-A (shown below).
2. Turn the engine off after inducing the cleaner and allow the cleaner to soak with the engine off for 2.5 to 3 hours (Do not let cleaner soak for more than 3 hours as remaining deposits may start to harden back up again).
3. Add a bottle of GM Fuel System Treatment Plus to the fuel tank and fill the vehicle with one of the Top Tier gasolines listed at http://www.toptiergas.com and/or in the latest version of 04-06-04-047 (USA) or 05-06-04022 (Canada). See Bulletin 05-00-89-078 for more details on GM Fuel System Treatment Plus.
4. Test drive the vehicle extensively to circulate the GM Fuel System Treatment Plus, which will help to eliminate/reduce any remaining intake valve deposits.
5. Re-evaluate the concern to determine if it is repaired or improved at all. If the concern is improved but not repaired, it may be necessary to perform the above decarboning process a 2nd time.
6. To complete the repairs, advise the customer to only use one of the Top Tier Gasolines listed at http://www.toptiergas.com and/or in the latest version of 04-06-04-047 (USA) or 05-06-04-022 (Canada) to minimize future deposits. It can also be recommended to add a bottle of GM Fuel System Treatment Plus at every oil change as mentioned in the latest version of 04-06-04-051. Kent Moore Tool # J-35800-A Upper Engine and Fuel Injector Cleaner Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed. GM bulletins are intended for use by professional technicians, NOT a "do-it-yourselfer". They are written to inform these technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions, and know-how to do a job properly and safely. If a condition is described, DO NOT assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See your GM dealer for information on whether your vehicle may benefit from the information. WE SUPPORT VOLUNTARY TECHNICIAN CERTIFICATION 2012 General Motors. All rights reserved.

Last edited by Stray; 10-14-2017 at 06:50 AM.
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post #10 of 23 (permalink) Old 10-18-2017, 05:09 AM
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