Relentless P0300 code

[gofast]

Hope someone can help. 2004 Monte Carlo SS supercharged. Rough idle, runs perfect from off idle to any rpm as long as it's not boosting. When it gets to boost it goes right back to its random misfiring. And sometimes shows a too rich code but only if it tries to boost for more than a couple seconds. It has a new cat, resonator, MAF, IAC, PCV, ICM, plugs and wires, and has updated PCM from the dealership today and still has the same problem. New head gaskets and intake gaskets less than a yr ago. Could it be a bad intake gasket or a warped lower intake manifold?

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[Fivefingerdeathpunch]

Done a compression test yet??

And what brand of plugs??

[gofast]

No compression test yet because it runs great without boost. Don't know brand of plugs, just bought it with this problem and was told he tried new plugs and wires and they do look brand new. Wires are actually Delco so I'm assuming the plugs are too.

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[gofast]

Wire are AC Delco. All the other stuff is the new stuff I have done to it.

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[Scottydoggs]

pull a plug read the # on it, google it, see if its a platinum plug, if it is buy iridium plugs.

[gofast]

I will do that but would that cause the rough idle and rough under boost?

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[gofast]

And also it has a new EGR valve too.

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[Scottydoggs]

I will do that but would that cause the rough idle and rough under boost?

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they cause misfires. your engine hate that kind of plug. if you didnt install them, see whats in there.

[gofast]

I will look. I was told the car ran great till it just started doin that one day and he put new plugs and wires on it and it still does the same thing.

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[Scottydoggs]

icm could be bad too. the part under the coils.

[gofast]

Already replaced that new too.

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[buickman104]

I had that exact same issue. Turned out to be a crank sensor.

[Turbocharged400sbc]

i had one with a wiped exhaust lobe cause this issue, then a few hundred miles later it was a hard mis as the cam lobe and lifter ceased to exist

[gofast]

I hope that's not the problem. It runs strong and smooth as long as it's not idling or boosting.

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[gofast]

Found the problem, cracked head between #4 & #6 cylinders.

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[J57ltr]

Cracked where and how did you end up figuring that out?

[gofast]

The machine shop just said it was cracked between the #4 & #6 cylinders. I haven't seen it yet.

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[buickman104]

Don't most of these heads have cracks and run perfectly fine like that?

[J57ltr]

Not between cylinders, that's usually between the valves. I would be interested in seeing what happened.

Jeff