2006 Pontiac Grand Prix Misfire And Chugging

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2006 Pontiac Grand Prix engine misfire. V6 3.4L. The check engine light is on. I replaced spark plugs and new wire harness, tranny flush. Question:
My check engine light is on. This started when I set my cruise one day and it started to chug immediately so I shut cruise off and engine light was flashing then stayed on. So put it in computer and it said cylinder 4 misfire so I changed all the plugs and wires and it continued to do it.

I took it to shop and got a tranny flush and had them check why engine light is on and they said it was just a malfunction so they reset it and I left there and my car chugged and light came back on.

The chugging happens when I am gradually accelerating and my RPM gauge is right above the 2. Not sure what's wrong, any ideas? I would like to fix the car myself without taking it to a mechanic.

ANSWER:

An engine misfire under load like you describe is usually caused an ignition system component. I would assume you have a code P0300 or P0302. Accelerating from speed puts a big load on the ignition system. Most common cause for this is the spark plugs or plug wires. Since you replaced them, that of course was not it.

Next you need to look closely at the ignition coil towers. If there is any corrosion, that would be the cause of your misfire. See picture below. Also look for a cracks between the towers. Those can be a little harder to see. If one is cracked, the spark will jump under heavy load. You will hear and possibly see it.

If that checks out OK, then next would be to diagnose a possible fuel injector problem. Either the #2 injector is not firing, is restricted, or has no signal. Using a test light to check for pulse signal is needed. If it has pulse, then a flow test would need to be done. A restricted injector will usually only cause a misfire at idle, but can also when accelerating.

If all this checks good, then another possibility that is not as common is a rocker arm coming loose. This can happen if the lower intake manifold gaskets were replaced in the last few months. Improper torque can lead to a rocker arm bolt coming loose from the cylinder head. This can also cause an engine ticking noise.