GMC Troubleshooting Questions? Ask a Mechanic for Answers
have you removed the spark plug to inspect it? also was your check engine light flashing?
coils are a known issue. and a flashing light means it is an electrical misfire. you replaced just one coil?
what size engine do you have?
just to be clear the coil you replaced was the one closest to the front of the engine against the firewall of the vehicle?
i would start with removing the plugs to inspect them to ensure they are not bad.
Caution: Wear safety glasses when using compressed air in order to prevent eye injury.Notice: Clean the spark plug recess area before removing the spark plug. Failure to do so could result in engine damage because of dirt or foreign material entering the cylinder head, or by the contamination of the cylinder head threads. The contaminated threads may prevent the proper seating of the new plug. Use a thread chaser to clean the threads of any contamination.
Notice: Allow the engine to cool before removing the spark plugs. Attempting to remove the spark plugs from a hot engine may cause the plug threads to seize, causing damage to cylinder head threads.
do you have a scanner to watch misfire counters to see what cylinders are being affected? if it is more than just number one or if it is isolated to only that cylinder since you have a 300 i am thinking you have more than one cylinder having an issue?
also are you able to do a compression check on cylinder one since low compression will cause a cat damaging misfire.