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P0306, but only appears every few months.

841 Views 10 Replies 3 Participants Last post by  mikemaj82
Every few months I seem to get a P0306 cylinder misfire code while driving. When I get home, I'll reset the code with my XTool D7, and it won't come back for a while. It seems to be in 2-3 month intervals. I don't feel any vibrating or hear anything different when the code is active, and the idle is normal. Just wondering why this would happen randomly. If it were a true misfire, the code should come right back after the reset and starting the engine again. If it happens again, I'm going to swap the coil with the one next to it, and see what happens. The car only has around 49K miles, and the spark plugs were changed about 2 years ago, and I put in Accel coils about 3 years ago. The only other thing I could think of is a possible issue with the injector or the wiring harness in that area being pinched.
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Fault codes can be intermittent. It would be easier to find if the condition were all the time. It may take 2 occurrences of P0306 to mature into an 'active' fault code & turn on the 'ck eng' light. Before that, or as a 'one-trip' failure, it may show up as a 'pending' code.

The PCM is pretty accurate about misfire faults & I'm pretty confident that it is real. The PCM can detect misfires that we can't by measuring slight changes in crankshaft speed. The crankshaft is constantly slowing down on a compression stroke & speeding up on a power stroke. It looks for that. We even out the inertia & momentum pulses with a flywheel, but they are still there.

Spark plugs can tell a story. The tip of the #6 spark plug may look different than the other five. A whiter porcelain tip may indicate a 'lean' condition. Any carbon or soot may indicate a 'rich' condition for that cylinder. Due to how intermittent this is, you may see nothing out of the ordinary.

If the XTool D7 can read stored 'freeze frame' data, copy down what the PCM sees when the misfire occurs. Is it a miss at idle or at speed? Is it during acceleration or at a cruise? Warm or cold? etc.
What conditions are present when the misfire occurs may help us to find it. See what the freeze frames have in common.
Has the top of the motor ever been apart? It is important to secure the coil/injector wiring harness so it won't wear through & short when reassembling.

Swapping the #6 plug & coil with an adjacent cylinder is a good idea. See if the fault follows the plug/coil. Look for any faint arc streaks on the porcelain body.
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Swap the #6 & #4 plug coils to see if the fault moves with the coil?
This is good information to know. Not 'stone cold', but maybe 100℉ - 150℉ (37℃ - 65℃)?
Are these the correct OEM spark plugs?
It should store misfire codes along with the P0523 code.
The 'circuit high' fault is usually caused by an open circuit, like the sensor was unplugged with the ign on 'run'? Conversely, a 'circuit low' fault is usually caused by a short-to-ground.
Maybe travel around with the scan tool plugged in? Does it record 'live' data?
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