So, this has been quite an adventure. If anyone can provide any input, I would appreciate it. I purchased a 997 turbo with an evt700 kit on the car. Very soon afterwards, there was CEL and it was misfiring code on cylinder 3 and 4. The plugs and coils hadn't been replaced for quite some time and so we went ahead and changed out the plugs and coils.

The headers were cracked and we got new headers in and replaced them as well. The cats had a little unraveling in the center but overall looked find. Everything went back on and we started the car. We had the CEL still on. P0300 and P0304. Misfiring and cylinder 4. When we actually went in to look at the values and the etc, cylinder 4 looked find. There were no misfirings in the log, but the values were up and down on cylinder 1 and 5...it would vary...sometimes -48, 0, and 3-400. But there would be NO misfires in the log. Cylinder 6 maintained 1400-1800 values BUT there were no misfiring documented. We tried to reset the codes, but for some reason, it would not let us reset the codes.

Our gameplan is that we are going to try another diagnostic tool Monday to reset the code, if that doesn't work, we are going to replace the two coils AGAIN. If that doesn't work, it may be the ECU? If anyone has any ideas or has seen this before, please let me know. I am thinking it is the diagnostic tool and that the codes are from the old CEL and that maybe cylinder 6 coil was bad from manufacturer. Fingers crossed....

 

If you cleared the misfire codes the tool no matter how poorly implemented would not be able to read the erased codes.

If the tool is reading misfire codes after they have been cleared and the CEL went dark the codes are fresh codes, not stale codes.

Persistent misfires can be serious way beyond replacing plugs and coils serious.

Within the last year or so I have encountered two vehicles -- not Porsches -- with persistent misfires. One proved to be due to a burned exhaust valve. Another arose from the owner for reasons I can't fathom reving the engine with the tranny in neutral and apparently the engine has no rev limiter. As a result the engine over speeded and at least one valve was bent.

You can replace the coils if you believe or want to believe the coils are bad. They may be bad so this is not necessarily a futile effort. It doesn't have to be two coils. It may only be one, which actually works in some way in favor of a bad coil.

The misfire in the other cylinder may be due to the misfire in the cylinder with the bad coil. A misfiring cylinder can negatively affect the filling and combustion of one or more cylinders and as a result these can misfire too.

Be sure you are connecting the coils properly, securely. This means at the plug but also at the wiring harness connector. Before you connect the coil at the harness you should check the coil's electrical connector for any signs of damage and the connector at the harness for the same. A bent or broken pin or damaged pin socket can cause the coil to not operate properly.

If the new coil or coils doesn't help there can be other explanations for the misfires that don't involve burned or bent valves. But these possible explanations have one messing with injectors.

If an injector is suspected viewing the short term fuel trims as the engine is running can sometimes offer some illumination. A leaking injector can have the fuel trim on that bank lean as the DME seeks to increase oxygen level in the exhaust. Or a blocked or weak injector can have the fuel trim on that bank rich as the DME adds more fuel. In some cases the bad injector -- if that is what it is and I'm not yet ready to make that diagnosis -- can be on another cylinder. The cylinder or cylinders that are misfiring are the good cylinders they are just being several misfueled as the DME attempts to compensate for the bad cylinder.

There is too the crankshaft position sensor. I know in one case the tabs on the flywheel the sensor, well, the sensor senses in its function to measure flywheel speed changes for each cylinder's power stroke were bent and as a result the sensor was not sensing all the tabs and their gaps.

 

The CEL just needed to be reset. The diagnostic tool they were using was acting up and when they plugged in another, they were able to clear the codes that were showing PRIOR to changing the coils and plugs. After a drive cycle, no codes or values. Thanks for your help....