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My 2004 neon SXT has had several brief misfires while driving at highway speeds over the last 2 weeks. It will misfire briefly and it goes away but it will usually only happens once during my 25 mile commute. It seems to be random when it occurs. There is not a check engine light at any time and no P-code on the scanner or on the odometer. I have also noticed that the idol has been low at stoplights which causes a moderate vibration at idle. Taking it out of gear is enough to get rid of the vibration. I have 147k miles and I changed the timing belt, etc about 10K miles ago. Any ideas on what the issue is?

It is a 2004
 

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Have you tried cleaning your throttle body? Maybe you got some bad gas.
 

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I had the exact same problem on my 2003 Neon with 2.0:L 4 cylinder and 4 speed automatic transmission. At idle it would occasionally misfire but no diagnostic codes would be set.

Check spark plugs. Since the spark plug sits deep within a metal well and has a long, insulated boot, there is a tendency for arcing to occur along the ceramaic insulator of the plug and connect a ground path from the high energy electrode into the boot to the metal body of the plug. There is a fine, black line along the length of the plug insulator.

To fix this replace the spark plug wires. This will solve the problem. The arcing affects the inside surface of the spark plug boot.
 

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And while you're replacing the sparkplug wires, replace the sparkplugs. Shouldn't take long as there is only 4 of them compared to the 16 plugs in my Hemi........
 

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A misfire may happen briefly and intermittently enough not to mature fully into a fault code and light the 'ck eng' light. There may be 'pending codes' or '1 trip failures' stored in the PCM.
The OBD II misfire monitor can be read with an advanced scan tool like the DRB III (or equivalent) and show both historic and real-time misfire and in which cylinder(s) that it's occurring that could help here.
There is a TSB # 18-024-04 that addresses both idle and misfire concerns. Read it here: http://www.ptcruiserlinks.com/forum/tech-performance-forum/14658-stalling-rough-idle-running-hard-hesitation-2.html
The dealer can load the latest and greatest software into your PCM. Use the correct Champion RC9YC's.
Another possibility is an intermittant loss of fuel pressure. For that you would want to drive it with a fuel pressure gauge attached and held under a wiper blade to watch it while driving, especially when the 'sputter' occurs. There should be a screw fitting on the fuel rail for a quick attachment.
Run the engine briefly to make sure that the fuel pressure gauge hose is routed out the back of the hood safely and no leaks or sharp/moving edges can catch the hose and cause a fuel leak before closing the hood and setting out on a test drive.
 

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Discussion Starter #7
It looks like the plugs/wires replacement resolved the issues. It has picked up about 200 RPM under idle which helped the vibration. After driving over the weekend, it has not had any stumbles or misfires in the engine. Thanks for the advise, another example where KISS (Keep It Simple, Stupid) works. I was already set to start changing out sensors.
 
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