My '03 SS is shooting a P300 and/or 305 code. I checked the #5 sparkplug and it was OK so I moved on to the coil pack. I had a spare coil pack so I plugged it in, cleared the 305 code and drove it at operating temperature. Under hard acceleration the engine doesn't want to pull past 3500-4000 RPM. If you back off to 3/4 throttle it seems to accelerate better yet still isn't pulling as hard as it should.

This said, it shot a P404 EGR code only once before this issue began and has not held it as a pending code since. Furthermore, about two years ago it held a P304 code and the coil pack and #4 sparkplug were in good condition so I replaced the upstream O2 sensor and the misfire stopped.

Do any of you members feel that the O2 sensor[s] and/or EGR valve could be the culprit[s]? The car has 236,000 miles on the clock and I exclusively use Mobil 1 Extended Performance motor oil, Wix filters throughout and use only Shell and Exxon/Mobil petrol. All maintenance is performed on schedule and I do have a deep background in automotive repair but this issue has me frustrated and stumped.

 

I've changed all six spark plugs with NGK V-Power plugs as well as the wires with a Duralast set yet I'm still getting an OBD II P300 Random Misfire code under full throttle.

 

I'm going to check the injectors. They should have a resistance of 11.4 to 12.6 Ohms. If any of them are outside this window then it's a bad injector. This all started when I changed the fuel filter. It makes me wonder if a piece of trash broke loose on the upstream side and clogged or partially clogged an injector[s]. A bad injector will cause a misfire code according to GM.

 

OK gear head friends. I've checked the resistance on all six fuel injectors on the SS and the Ohm average is 12.1.
I've eliminated the following:
1. Spark Plugs and Wires (replaced)
2. Fuel Injectors are good
3. Catalytic Converter is good
4. No burnt exhaust valves
5. No vacuum leaks
6. All connections are tight
7. Good compression
Now with that said, the last time it did this the culprit was a bad upstream O2 sensor but it set a P304 code when it went bad. That means the O2 sensor caused a misfire on cylinder #4. When I replaced it and reset the code the misfire stopped. I do know a bad O2 sensor WILL cause a misfire condition and this is shown in the GM service manual[s].
An O2 sensor isn't cheap and yes it's time for both of them to be replaced. Anyhow, Should I replace them both and the problem persists then what's the next step? Remember, this is a RANDOM misfire and not isolated to any single cylinder so I don't have a dead cylinder.

 

I came home today and couldn't just let the car set so I worked on it: again.
The O2 sensors checked out as well as the MAP, MAF and TPS. I reset all the stored codes (P300) and brought the car up to operating temperature then drove it while monitoring the ScanTool. The misfire happens at Exactly 100% throttle and not before. I then rescanned it and P305 had set (misfire on #5). I swapped out the coil from 3 & 6 hoping the misfire would move there. It didn't move there it just returned as P300/Random or Multiple Misfire.
Do any of you that have been following this issue on believe it could in fact be a faulty coil?

 

If the coil turns out to NOT be the problem could it possibly be the Crankshaft Position Sensor, one or both of the O2 sensors, or EGR which I plan on removing and checking the pintle for carbon buildup/sticking.

 

I had the same problem as you are having expect mine was 300 and 303. I changed the plugs, put a new coil on 3, changed the wires(had some old ones laying around, how could both sets be bad right), and in a last attempt I took the spark plug boot covers off. Still through the same codes. I didn't know what to do! So I got to thinking about it and I decided to put a timing lite on each of the spark plug wires while it was running and I found my problem. I watched the lite (it flashes every time your plugs fire) and on a couple of cylinders they were not firing like they should have been. Come to find out it was the wires. I dealt with this misfire for 5 months! Try this out, maybe it will work for you. I know how a misfire will dive a person crazy!!!

 

We usually recommend replacing with ACDelco parts for the ignition system, as these engines can get a little finicky with other brands.

However, if you truly don't think it's the plugs/wires, I would look into fuel filter and possible cat clogging.

Does the car feel like it is low on power when it is warmed up?

 

Crank Position Sensor maybe? I've got a few lying around if you need one on the cheap.

 

It doesn't feel low on power until you reach 100% throttle at which time the misfire code sets and it feels like you deployed your chutes after a 1/4 mile run. Also, there's not the rotten egg smell from the exhaust that would indicate a bad catalytic converter plus there's a strong exhaust flow exiting the rear pipes.
I've entertained the thought of the CKP sensor going bad yet in my experience I've never had to replace one on other cars I've worked on. This is a slow process of testing and elimination.