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Question on Kia Optima Idle

Hi, I have a 2004 Optima 4-cyl with 160k on it. When we got it, it was idling rough, with arcing on one of the ignition coils, but no CEL. Once we replaced both coils, it smoothed out a little, but was still pretty rough, to the point where the car was shaking and surging, and the idle would fluctuate a few hundred rpm.

The CEL then came on, and showed a P0401 “Insufficient EGR Flow”. We went and replaced the EGR valve immediately, then reset the CEL and reset the ECU.

Afterwards, the car runs much smoother, almost bearable, the idle is stable, no fluctuation, but it is still idling rough, and the car shakes a little, compared to before it is much better.

Any ideas on how to make the car idle smoothly again?

Thanks in advance!

Try replacing the plugs and wires.

Thanks for the input! I’ve replaced the ignition coils, and I actually bought a set of ngks for it, no plug wires though. I’ll try installing the plugs first and see if that helps.

I just replaced the plugs, and no change, idle is still not smooth, it fluctuates a little, and not unbearable, but annoying. I am thinking it might be the egr lines are a little clogged, and not allowing the egr valve to do its job properly; what do you guys think?

Have you tried cleaning the throttle body with throttle body cleaner?


Go start that puppy up and shut off the lights. I’ll bet there is arcing of the plug wires.


Did you “read” the plugs? Any clues there?
If you still have them, but didn’t look at them, pull them out of the barrel and take a look. Sometimes they can provide the answer.

Considering what you’ve already done, another good possibility is a clogged injector. You can test them, but considering the mileage I’d probably just change them and see what happens. Cleaning the throttle body can’t hurt, as already suggested, and a compression check to see if there’s a weak cylinder can’t hurt either.

Just replaced the plug wires. No change in idle smoothness. Will check throttle body next.

I compared the plugs to the NGK plugs condition photos on their website. And they look like the ones which are good. Will bad injector make the plugs look any different?

At the gas station, just bought some fuel injector cleaner, and put it in with a half tank of fuel. Will drive it around and see if that helps. Buying new injectors now sounds expensive! Same with getting a compression test. Budget is tight for this car

The plugs could look good and an injector still not be. Often when an injector is bad it’ll be a worn or clogged orifice or pintle, causing a poor spray. The cylinder will still fire, but it’ll be a splurt rather than a mist and the flamefront won’t propogate properly. The cylinder will operate weakly and it’ll be like having uneven compression.

Do a search on the internet and you may find that injectors for your car aren’t that expensive.
Note too that you can remove, clean, and reinstall injectors too. It does a better job than using additives. You’ll need to apply 12VDC to open the injector, but I’m sure you an figure out a way to do that.

Just got some new info, after driving around to run the fuel injector through the system, it ran better for a bit, then worse as I drove.

I went through the drive thru just now, and noticed that the headlights were flickering slightly. I plugged in my wifi scanner with the dashcommand app, and it gives me real time stats on the car.

The car was running very low voltage, the app logged the lowest at 12.8 and the highest at 13.7.

Would low voltage cause a rough idle?

Would this be battery or alternator? I’m hoping battery!

That is proper voltage.
12.8 …engine off…13.7 engine running!!!

Headlights flickering…a poor connection at the battery of somewhere.


Clean your battery terminals. Is the battery old? If the lights are flickering, it’s probably a bad alternator. Advance or Autozone will test your charging system for free. Get sensor cleaner and clean the MAF sensor. Low voltage will play havok on your sensors and ECM and cause it run bad.

+1 for the bad alternator. Specifically the regulator portion.
The headlights flickering are a dead giveaway.

Sorry, it was late last night and I wasn’t clear on the voltage. While driving it averaged about 13.3-4 volts, with a low of 13.1 and a high of 13.7.

When off, it ranged from 12.3-12.8 as I ran the radio or turned on the fan.

We actually checked the map sensor couple days ago and it was clean.

The battery terminals are clean, I don’t know how old the battery is. I’ll run it down to autozone to get that checked real quick.

Just started it up and drove it a little; voltage started off good, 14.1-14.3, but then I turned on the a/c, and dropped way down to 12.9 , then up to 13.7.

But when the idle is still rough no matter if the voltage is low or high. If the voltage was on the high side, it would run smooth for the time that it was high am I correct?

Get the charging system tested. I think you have a charging problem. When it fluctuates it causes the voltage outputs on the sensors to change. It may not be the only problem, but they can scan your car for free at the same time. Post any codes and it will help. OBD11 Codes look like P0400

Dude at autozone checked it, says batt and alt and charging sys are good, but couldn’t explain why the voltage would fluctuate so much.

I’m still at the store, going to try throttle body cleaner next.

Tried the throttle body cleaner, no change. There are no CELs or pending CELs. Any other suggestions?

I think I’m going to buy a new battery just to make sure. I don’t trust the autozone testers as they’ve told me a batt was bad when it was still good.