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2005 Chevy Impala with misfire

Hi. I’ve got a 2005 Chevy Impala SS 3.8 Supercharger that has a misfire that I can’t find. I’ve replaced the wires and plugs, crank sensor, and one of the ignition coils. I know it’s not the gas because I’ve gotten complete fills multiple times since this started happening. The only code thrown is a 420 - catalytic converter. The guy at my muffler shop says that I need to figure out the misfire before he fixes that as it will just burn up the new catalytic converter and that a 420 means that it’s acting like I straight piped it.

Anyone have some other ideas of what I can look at? I’m at a bit of a loss here. Thanks!

the guy at the muffler shop is 100% correct

It would be a darn shame if you replaced the catalytic converter, only to ruin it because the misfire wasn’t resolved

There are many potential causes for a misfire, such as spark, fuel, compression and so forth

Is the misfire on a particular cylinder?

And even if you happen to have a rather vague code, such as P0300, doesn’t mean the misfire isn’t primarily on one cylinder

Hook up a noid lite . . . make sure the injector(s) are being pulsed correctly

Use a spark tester to make sure you have an extremely bright blue spark

Perform compression test on the cylinder(s) in question. Post the results. Don’t assume that a particular value is okay. it might be too low for your particular engine.

As for that P0420, with the scanner, you need to look at upstream and downstream oxygen sensor readings with the engine warmed up. The upstream sensor should be fluctuating quite a bit, from below 200mv to above 800mv, with an average of 450mv. The downstream sensor should be very stable, not wildly fluctuating.

Exhaust leaks can also have an effect, as to generating a P0420 code. You want to make sure you don’t have any

Is the engine burning a lot of oil?

any previous blown cylinder head gaskets or intake gaskets?

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I had my O2s checked at the muffler shop. He said that it’s because of the burn out catalytic converter and not to worry about that one just yet (because of the misfire.) I’ve not been able to pinpoint any cylinder in particular. The engine doesn’t throw any kind of code to give me the slightest hint. I can just feel the misfire as my car jerks down the highway.

I did have the valve cover gasket and intake replaced just before I did the tune up. We noticed some smoke coming from under the motor and were able to determine it was the gasket. The intake was cracked, so I had him replace that at the same time. The fluid from the crack shorted out the ignition coil for cylinders 3 and 4 (which I replaced) and the crank sensor (which I replaced). I’ve tried throwing some heat into my gas tank as well as letting it get to fumes and refilling completely to try to rule out bad gas. I’ll check the spark and see if that gives me any indication.