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Discussion Starter #1 (Edited)
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).
 

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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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Discussion Starter #3 (Edited)
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).
 

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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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Discussion Starter #5
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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Discussion Starter #6
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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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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Discussion Starter #8 (Edited)
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).
 

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Discussion Starter #9 (Edited)
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.
 

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Discussion Starter #11 (Edited)
I finally got a diagnosis yesterday... sadly they believe the engine needs to be replaced. Cylinder #3 had compression issues and after poking around and eliminating whatever else, they think there's either a cracked/ bent valve. I'm honestly heartbroken. I've only owned it for 3 years now and this is the first and only issue it's ever had. Just hit 104k. Nothing else acting up. I hope it didn't break the timing chain before I got it. It wouldn't surprise me to have a dealer sell a car they knew could have internal damage. I was offered a used engine, installed, for around $3500 (12 month guarantee) or new for $5800 (5 year guarantee I believe). After paying the $99 diagnosis fee and telling them I couldn't afford either one (I still owe about $6k on the car)they recommend I drive it until it dies or try to part it out. I'm attempting to locate a salvage yard engine or maybe just the head in hopes I can save it. I'm at a total loss and really down over this whole mess :'(
 

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Really sorry to hear that. It could also be a piece of carbon stuck in a seat, have you tried seafoam or something similar?
 

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Really sorry to hear that. It could also be a piece of carbon stuck in a seat, have you tried seafoam or something similar?
Agreed. These things aren't known for bending valves...
 

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Discussion Starter #14 (Edited)
We've done seafoam upper engine cleaner twice (i wanted the heavy duty upper engine cleaning/de-carbonization at least, but the dealer refused to do anything but replace the engine). Oddly though, no smoke after seafoam. I'm going to use the normal seafoam introduced via vacuum hose tomorrow with a 30 minute soak and see what that does. It's just bizarre to me that it just randomly got a bent valve. Nothing else broke. It's a simple daily driver (and I drive around with my two little ones, so no crazy driving of any sort). No mods, no other parts replaced. No other damage. It's like waking up with a broken leg, it just makes no sense. My local mechanic that looked over it first thinks they are scamming me. I'm just looking at all my options since I still owe on this cat (I'm upside down in it thanks to adding an extended warranty that ran out early last year.
 

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Discussion Starter #15 (Edited)
Well, did seafoam and it did not fix the misfire or take away the code. I'm not getting a rough misfire anymore though at least. I cleared codes before seafoam and after seafoam and my misfire code on cylinder 3 came back both times. I also have a knock sensor code. One mechanic told me that it might affect camshaft position. Has anyone found any correlation between a knock sensor and misfire? I'm grasping at straws at this point. I still do not understand no voltage to the coil (tested at the harness to cylinder ).
 

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Discussion Starter #16
Well, finally have my cobalt in a shop I trust. They are checking everything since the other dealership acted so fishy and wouldn't even give me 5 seconds to list issues. The master mechanic asked if anyone had checked my wiring harness.... so, now I wait while they pick through my car. Anyone here had wiring harness issues?
 

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Well, finally have my cobalt in a shop I trust. They are checking everything since the other dealership acted so fishy and wouldn't even give me 5 seconds to list issues. The master mechanic asked if anyone had checked my wiring harness.... so, now I wait while they pick through my car. Anyone here had wiring harness issues?
I think most cobalts have had some sort of wiring harness issue :laugh:
 

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But I don't see your Cobalt mentioned in that bulletin.

---------- Post added at 02:36 PM ---------- Previous post was at 02:35 PM ----------

I think most cobalts have had some sort of wiring harness issue :laugh:
Especially if you live where there are a lot of squirrels or mice.
 

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But I don't see your Cobalt mentioned in that bulletin.

---------- Post added at 02:36 PM ---------- Previous post was at 02:35 PM ----------



Especially if you live where there are a lot of squirrels or mice.
Or if you live where there's a lot of moisture or cold winters with heavy salt (corrosion), or hot summers (destroys the cable wraps). Basically, If you live anywhere that its not 72 degrees and dry, you'll probably have an issue sooner or later :laugh: . I can't even begin to tell how many melted/chafed wires I've found in my engine bay because the plastic looms GM uses are absolute trash.
 

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The Cavalier I had with 258k miles did not have any wire corrosion issues, despite almost 20 cold, snowy/salty midwest winters and hot, humid summers.
 
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