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Intermittant Misfire

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Good evening all...

I recently purchased a 1997 Boxster with 5 speed transmission, with 85k miles. I have an issue involving intermittent misfire condition. The misfires occur generally when the air temp is cooler; maybe < 75 deg.; doesn't seem any worse in rainy conditions. The code reader spouts out P0300 (Random multiple misfire), P0304 (Cylinder 4 misfire), P0305(Cylinder 5 misfire), P0306 (Cylinder 6 misfire). The seriousness of the issue differs; at its worst the CEL light flashes and the car has very little power. Generally, after the engines heats up, if I shut off the ignition and restart the car, the condition disappears. Most times when I leave work in the evening the car runs as poorly as it did that morning, but if I recycle the ignition, it runs fine. When its running seemingly OK, sometimes I will still get a very occasional, very short misfire in the higher RPM ranges.

The first O2 sensor on that bank was also defective when I purchased the car; I have replaced that already. The car definitely has a smoother power range, and more power after replacing the sensor (when its not misfiring).

I have seen some posts concerning cracked ignition coils; all six of my coil packs are cracked. It seems to me that moisture would make the misfires worse if it were due to the cracked coil packs. It also seems odd to me that all the cylinders on one side are showing the misfire, and none on the other bank.

Any suggestions? Thanks!

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I for one would start with replacing the cracked coils.

You could swap the coils from cylinders 4-6 over to 1-3 and see if the misfires follow.

It is possible that the coils on the 4-6 side have seen more moisture or some of them are on their way out.

Here is the coil pack diagnosis flow chart if you want to diagnose the coil packs using multimeter/etc..


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