Another P0302: Misfire on Cylinder Issue - Chevy Cobalt Forum / Cobalt Reviews / Cobalt SS / Cobalt Parts
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post #1 of 3 (permalink) Old 01-29-2018, 03:20 PM Thread Starter
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Another P0302: Misfire on Cylinder 2 Issue

Hello, everyone. I've extensively read through as many topics as I can find on this issue already and I'm hoping with fresh context, maybe a resolution can be found.

My Cobalt runs beautifully under load. Smooth, powerful, even. At idle, though, it simply will not stop tripping a P0302: misfire on cylinder 2 code.

Step 1, for me - I changed the plugs and swapped the coil packs around, to see if this eliminated the issue or moved it to another cylinder. No joy.

I was at 115k miles, it was almost due for a tuneup, and I hate that ethanol is in gas nowadays, so I added a cleaner to the gas.

The light went off and stayed off for about 2 months.

It came back. Same code. Naturally, I tried the same thing and gave it a tune-up minus the brand new plugs (oil, filters, checked sensor operation, really worked it up).

Didn't work. Well, I routinely use Seafoam in my lawnmower to remove the ethanol residue with beautiful results, so gave it a shot, in proper amounts, in the gas and crankcase. This worked.

Less than a week later, P0302: Misfire on cylinder 2 again.

At this point, I brought it to my mechanic. They checked compression, checked the job I did gapping the plugs and reseating the coils, tried swapping the coils, ran it under full diagnostics, ultimately could find absolutely nothing wrong - except cylinder 2 continued to misfire. They concluded there must be sludge, they cleaned it out and cleared the code.

The engine light came on 2 days later. Same code.

This weekend, we did a full court press on it, following probable causes in as logical order as we could come up with (it's ridiculous how many emissions sensors and devices can potentially generate this code).

I replaced the coil though it checked out fine for both me and the mechanics. All 4 coils generate great voltage, they're operating fine. Code returned.

We pulled off and checked the rail, replaced the injector at the same time to see if that made a difference. It didn't.

We pulled off the intake, meticulously cleaned it, replaced every gasket on the intake and ignition sections of the engine. We replaced the throttle-body (because the stupid IAC and TPS are built into it and can't be checked independently). Didn't work.

While we had it open, even though everything in there LOOKED clean and perfect, no sludge whatsoever, we cleaned it all out anyway. No difference.

We pulled off the valve cover to confirm that the timing chain is tight and just fine. While that was off, we confirmed that the cams are in beautiful shape and all ports open. And we cleaned everything anyway for the sake of being thorough. Still didn't work.

We checked the fuel pressure (irritating that the pressure regulator is built into the pump).

We replaced the lesser MAP sensor on the intake. We replaced the secondary-injection control (EGR valve analog). We replaced the camshaft sensor. Code still returned.

We even replaced the serpentine belt in case that being bad was somehow causing things to run slightly out of sync. Did nothing.

After all the tinkering and the mileage put on since the problem began, I returned to step one and, in case the plugs were damaged along the way, replaced them again, this time with iridium plugs, meticulously gapped to factory 0.043.

Still, P0302: Misfire on cylinder 2.

I'm at a bit of a loss now. We followed through all that trying to follow a logical order based on old-school auto-knowledge, but there doesn't really appear to actually be any, since so many different faults can reportedly cause this problem.

I'm wondering... While watching real-time diagnostics display from a computer tap, the O2 sensors were super slow to report. At 85 miles of driving (thruway, no stops so no idle to cause the code), they still hadn't reported as confirmed, so emissions were 35 miles beyond where it should have registered as ready for emissions inspection. But I understand from multiple sources that they're notoriously slow to report when they're not new anyway, it's not failure just a function of their age.

Could this indeed be a bad O2 sensor? If so... I'm baffled as to why the code is P0302 and not a code saying an O2 sensor is bad.

I considered a plugged cat, but she never stalls and has a beautifully smooth power stroke under load, with no code generation. That just doesn't jive.

I thought maybe a bad secondary injection pump, but that's checked and operating just fine.


*hands in the air*

Suggestions, please?

Oh! One more thing... Over time, on a long trip, the tranny gradually becomes more and more stubborn, slightly slamming into drive from a full stop. Something's just simply not right somewhere and overcompensations by the computer are obviously hitting the limits of potential adjustments, but I can't figure out what.

Last edited by Ralph Abramo; 01-29-2018 at 03:44 PM. Reason: Minor typos. Additional final thought.
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post #2 of 3 (permalink) Old 01-29-2018, 04:30 PM
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Have you tried cleaning the ECM connector with spray alcohol and re-seating? Have you check the oil sending unit to see if it's leaking oil? and last but not leased and this is for your irratic transmission shifting re-seating the BCM connectors several times to clean possible corrosion. Transmission problem is probably separate from your P0302.

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post #3 of 3 (permalink) Old 01-29-2018, 05:48 PM Thread Starter
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I did check the oil sending unit, yeah,it's good and everywhere through the engine everything's being very well lubricated.

I removed the ECM and checked all the pins/sockets, seeing no sign of damage, but was wary of using any kind of cleaner on it (had NOT thought of alcohol, though...) for fear of ruining it and necessitating an extremely expensive potential secondary repair which may or not have solved the original problem. Tomorrow, I'll try the alcohol, though, that does seem harmless enough.

And yes, the BCM connections were checked just like that, being reseated a few times and visually inspected.

I can't help but continue to suspect the O2 sensor. If it's slow to respond, it may be good but possibly reading incorrectly due to build-up.

Seems to make sense to me (except... why always cylinder 2 remains fairly unexplained), but then again, I've had similar suspicions about each thing I've pulled off and there's got to be some line in the sand somewhere where just replacing things one after another to root out the cause gives way to some kind of reasoned proof of failure. :/

Last edited by Ralph Abramo; 01-29-2018 at 06:13 PM.
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2008 , cobalt , cylinder 2 , misfire , p0302

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