2004 Kia Optima computer problem?

  • 2004 KIA OPTIMA
  • 2.4L
  • 4 CYL
  • FWD
  • 77,000 MILES
Hello. I changed spark plugs, but got the wires wrong. Tried one afternoon to drive a little and something was very wrong.

Next day, realized the problem, and got the wires right. Car was running alright.

Made a drive for about 30 miles and car worked great. Then started misfiring, after engine was hot.
I don't think head gasket got blown when wires were wrong.

Just drove it a little to Walmart, etc. Car was not running consistently. Would run perfect, then I would start it again and would misfire badly.

For example, I took the car for a short drive to one local store. It was running great - and idling fine for this little shopping trip. I was amazed how solid the engine sounded. Before going in the store, I sat there for 1 or 2 minutes and gassed it gently, a few times, to make sure engine was not misfiring at all. Went in the store and was very happy! BUT. When I came out of store, and restarted it, it was misfiring badly, and could barely get it home. That is when I decided to disconnect the battery and reset the whole computer back to factory settings. I think for the past 11 years, the computer had never been reset back to factory settings.

Decided the computer was very confused.

Took the battery cable off for about an hour. Computer reset.

Made 2 short trips of about.2 miles each, and was misfiring badly.
Also, at this point, when I would try to start it, would start very sluggish, like timing was not right.

But. I wanted to take it for longer drive to let computer learn. Drove the car 30 miles on the Interstate highway to give the computer a chance to "learn." This drive was fine! Got the car home and engine was running perfect! Even better than it did before I changed the plugs.

BUT. Next time I started the car (a few days later and engine was cold), car immediately was misfiring.
But the problem with sluggish cranking speed was gone; it started easily and quickly.

Before I originally changed the plugs, car had always ran perfectly! I hate myself for getting the plug wires mixed up, but would be happy again, if I can solve this funky problem.

It seems to me that the computer is not remembering the Perfect Settings from when I took the initial "relearn" test drive. What are your thoughts?

Thanks a lot. Bob
Do you
have the same problem?
Saturday, October 31st, 2015 AT 1:43 AM

1 Reply

You're expecting way too much of a computer. It adjusts short and long-term fuel trims and it learns the characteristics of the various sensors by comparing their readings and engine operating conditions to each other. There's factory pre-programmed values for fuel trim starting points, but not for sensors. The factory starting points are fine, but after that, there's no reason to wipe that all out by disconnecting the battery. Doing that is a throwback to the days when GM had a lot of trouble with their Engine Computers and people thought disconnecting the battery would magically fix something.

Putting spark plug wires on wrong would cause spark to occur at the wrong time in those cylinders. Head gaskets are designed to contain maximum pressure when the spark fires the mixture just as the piston is reaching its maximum compression. Anything you do to mess that timing up is going to result in severely reduced head pressure. That will reduce the stress on the head gasket.

The most common cause of what you're describing is one or more spark plug wires are or were not fully seated. If there's a gap between the mating terminals, a spark will jump that gap and the spark plug's gap, as long as the two together don't require more voltage than the ignition coil can develop. Arcing like that leaves a carbon track behind. Carbon tracks will conduct spark voltage and cause the current to take an alternate route rather than go through the spark plug. Current will go whichever way it finds easiest, and that can change.

If a boot on the spark plug wire didn't seal well on the plug, arcing to ground alongside the spark plug will also cause carbon-tracking there. Since that carbon conducts current, the more carbon-tracking occurs, the more likely current will bypass the spark plug, and the more it arcs outside the plug, the more carbon-tracking builds up. When that gets real bad, you'll find the boot is burned away and brittle. In those instances you won't solve the problem by just putting the wire on tight. At a minimum the carbon has to be cleaned off the spark plug and the boot, but if it's really bad and has been occurring over many weeks or months, you might have to resort to installing a new spark plug and wire.

Humidity plays a role in determining when the spark will occur outside the spark plug. Higher humidity helps carbon conduct current better. When accelerating or under load, the increased cylinder pressure makes it harder for the spark to jump the spark plug's gap. A higher voltage will be developed, and that can allow the voltage to be reached at which the spark will travel down the outside of the spark plug causing a misfire.

To find if this is what's happening, while the engine is idling okay, use a spray bottle with water to mist the spark plug wires and boots. You'll hear and feel the misfire develop, and you may see or hear the spark occurring by the spark plug.

This arcing across a gap can occur where the other end of the plug wires are plugged into a distributor cap or coil pack, but it's less likely to cause a running problem because there's nothing metal in the area to entice the spark to occur outside that connection. What's more likely to happen there is the arcing will burn away the terminals more and more over time until the gap is just too big for the spark to jump, then you'll have the misfire again.
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Saturday, October 31st, 2015 AT 2:51 AM

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