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Hey guys, got a 2007 Envoy with 203k miles. Just replaced the spark plugs about 3 months ago with AC Delhi recommended plugs. No issues since but the other day as I got to about 50 mph my check engine light came on and was flashing. Got home and pulled codes, got a P0303 for cylinder 3 misfire so I replaced the ignition coil for 3 since I am assuming the plug is fine being new. Ignition coils were original so common sense. Cleared the codes and drove around town keeping it under 40mph and nothing. Runs smooth. Then I took it on the highway and as soon as it gets to 50mph plus, the check engine light flashes again. This time the codes were for cylinder 3 and also P0300 for random/multiple cylinder misfire. As soon as I lower my speed to anything less than 50mph, the light goes away completely. Other than a loss of power when the light comes on and flashes at higher speeds, it doesn’t seem to run rough at all. If I sit at idle in park and rev it to 5k-6k rpm nothing happens, it only throws the codes when I’m going at faster speeds. Any ideas?
 

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First, double check that the ignition coils are properly seated on the spark plugs. Second, make sure the electrical connectors to the ignition coils are properly seated. Third, check your fuel pump pressure to make sure it is in spec.

Now a question - where did you get your ACDelco spark plugs from? If ebay, then I would suspect them as being counterfeit plugs. If Amazon, it depends if the seller/shipper was Amazon or some other seller. If the seller was not actually Amazon, then a decent chance they are counterfeit. Also, if you got an unbelievable great price on a package deal of 6 plugs such as 6 plugs for $25 or less, then they are likely counterfeit.
 
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