2009 Silverado Misfire

[CarbonToast]

I'm diagnosing a Multiple Cylinder Misfire on 2009 Silverado 5.3 with 220k miles. It the threw a P300 code and is running rough at idle.

I took a data log using Torque Pro, cylinders 1,2,4,5 all have misfire history. Not wanting to go full parts cannon, we first replaced the spark plugs. (Cheapest option). Next I took a data log of fuel trims, O2 sensor voltages, and MAF sensor at idle after warmup. Catalyst temp is normal.

From what I was able to research, the o2 sensor readings are typical. Sensor 1's should cycle high - low, sensor 2's should be steady around 0.7v

There are 3 big dips in the o2 sensor 2 readings, is this a sign of bad sensor?

The LT fuel trims all indicate a rich condition (pulling fuel out). Is that normal?

The spot where ST fuel trims went to 10, does correspond with a valley in the upstream o2 sensor voltage.

Next steps is to throw coils and wires at it, but that's $300 proposition. So, I'm looking for a 2nd opinion.

 

[clinebarger]

1 coil failing is uncommon on these....More than one is highly unlikely!

What does the live/current misfire data show?

 

[CarbonToast]

I don't have a log of current.(didn't take one, but can at a later date) Here are 2 logs of the misfire history.

1st is while driving
2nd is idle

Driving

IDLE

 

[iCurious]

I would do a leaking down test to check if the valves are sealing. And if it does have a fuel pressure regular that is operated by vacuum, I would pull off the vacuum line and observe the fuel pressure regulator if it's spewing out gasoline.

 

[zrxkawboy]

It’s not likely to be the coils, could possibly be wires, I suppose. Both are easy to swap around and see if the misfires follow.

 

[Oliveoil2]

I’d throw a new set of OEM wires on it and see what happens. At 16 yrs old you’ve gotten your use out of them. If you don’t get any change I’d do a compression check. If you’ve go a few with low compression then Go for a leak-down test. What’s your normal OCI? Does it use any oil between changes?