2010 LLT P0303 - Cyl 3 misfire

[gpskinzhut]

What's the diagnostic tree on this? I'm about 900 miles from home and the car has decided this is as good a time as any to decide to shit the bed.

There's been a minor skip/miss in the idle and cruising without CEL. I took it in to a dealer up here to have them go through the system and see if there's anything glaring. THey didn't find anything and suggested taking it to a performance shop because it has a C.A.I. and catch can on it. With no CEL they didn't want to take it very far.

Anyway, while cruising back to my mom's house with my daughter, I was snapping the throttle as she was dozing to mess with her (like any good day would). When doing so, the car stumbled, shudder and would then pick up as RPM's came up. Did this 3 times wondering what the hell that was and CEL kicked. P0303, C3 misfire. Driveability went to shit, CEL stayed on for a bit but eventually went off and driveability went back to normal. I switched the coils on 1 & 3 and test drove, same thing. Pulled #3 plug and it looked fine, replaced anyway, no change. Feels like it's getting worse but is typically at part throttle or at onset of heavy load at low RPM's. Car isn't cold either.

I'm not sure what else to look at. Maybe plop it back at the dealer with the CEL history now there?

[qcman]

Not sure on the miles but the LLT motors are known to have Timing Chain issues so I wouldn't be surprised if it were something like that. Especially when the miles get high on them. Though I hope not for your sake.

[gpskinzhut]

Originally Posted by qcman

Not sure on the miles but the LLT motors are known to have Timing Chain issues so I wouldn't be surprised if it were something like that. Especially when the miles get high on them. Though I hope not for your sake.

Aware of those, yeah. I would imagine that would trigger more than a single cylinder though, so fingers crossed on that one.

[qcman]

Originally Posted by gpskinzhut

Aware of those, yeah. I would imagine that would trigger more than a single cylinder though, so fingers crossed on that one.

Thats a good point I would think it would affect more than one cylinder too. Maybe thats a good sign in the big scheme of things....

[RichSS2006]

Pull the spark plugs. Does #3 look different? Any other symptoms? Smell of gas? I ask because wehad one. Recently that had a miss fire on #1. Ran like you're explaining. Plug was wet with fuel. It ended up being a stuck open fuel injector. Cleared itself and has been running normal since. Let us know.

[gpskinzhut]

Originally Posted by RichSS2006

Pull the spark plugs. Does #3 look different? Any other symptoms? Smell of gas? I ask because wehad one. Recently that had a miss fire on #1. Ran like you're explaining. Plug was wet with fuel. It ended up being a stuck open fuel injector. Cleared itself and has been running normal since. Let us know.

Nope, plug actually looks fine, and I swapped it out with a brand new one just in case. I was expecting fouling of some kind and nada. No fuel smell at all. No improvement with the new plug.

[RichSS2006]

Check the connections at the coil(s).

[RichSS2006]

Here are the pages from the GM manual related to this issue. Hope this helps.


Sent from my iPhone using Tapatalk

[gpskinzhut]

Thanks for those, Rich. I'll have a read through.

[gpskinzhut]

Still has a hesitation and noticeable miss at idle and cruise but the hard misfire seems to have gone. Changed all the plugs, didn't smooth out the idle but 'seems' to have fixed that. Of course though, I was working off of a SBC V8 firing order and didn't think to look up the LLT firing order when I was doing diags. So I was swapping coils on the wrong cylinders. N00b. Still could be a coil, I just can't get it to CEL to confirm. Yay. At least it made it home from MA to TN.

[chevyracingfan]

Originally Posted by gpskinzhut

What's the diagnostic tree on this? I'm about 900 miles from home and the car has decided this is as good a time as any to decide to shit the bed.

There's been a minor skip/miss in the idle and cruising without CEL. I took it in to a dealer up here to have them go through the system and see if there's anything glaring. THey didn't find anything and suggested taking it to a performance shop because it has a C.A.I. and catch can on it. With no CEL they didn't want to take it very far.

Anyway, while cruising back to my mom's house with my daughter, I was snapping the throttle as she was dozing to mess with her (like any good day would). When doing so, the car stumbled, shudder and would then pick up as RPM's came up. Did this 3 times wondering what the hell that was and CEL kicked. P0303, C3 misfire. Driveability went to shit, CEL stayed on for a bit but eventually went off and driveability went back to normal. I switched the coils on 1 & 3 and test drove, same thing. Pulled #3 plug and it looked fine, replaced anyway, no change. Feels like it's getting worse but is typically at part throttle or at onset of heavy load at low RPM's. Car isn't cold either.

I'm not sure what else to look at. Maybe plop it back at the dealer with the CEL history now there?

had same thing last week opened up plug gap to .045. and cleaned mass air sensor idle became smooth and seam to have better top end pull.

[EsteSS]

I would definitely check to see if you have spark. Or at least "full" spark. They make cheap spark testers to put in line with the coil/plug. Don't forget, your spark plug wire could be fouled. The smallest nick can cause issues. I'm a big believer in compression checks just to prove that good. Then work your way up. Doesn't take long and you prove that your cylinder is getting compression. All cylinders need to match within 15%. Then you can work your way up, Plugs, Wires, Injectors, Coil Pack, etc.