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04 Kia Rio misfire and rough idle issues


I have a 04 Kia Rio 5 speed manual transmission. Oh boy, this car is a doozy. Have replaced fuel pump, coil, spark plugs, spark plug wires, oxygen sensor, fuel filter, air filter, and a few other things not to mention the suspension parts replaced. So in 4th gear now, it starts to have some really bad misfires it starts shaking and acting haywire, in 5th gear if I let off the gas just a smidge it misfires. AC of course adds to this craziness, but have tested with it on and off. Same results. When it starts to get really bad, it will shake crazily while it idles, can tell it’s having a hard time even staying running so I pour in a bottle of Lucas injector cleaner into gas tank, only Lucas works though. And within half a mile it starts acting a little more normal. Only misfires occasionally. Getting to be smaller gaps between feeding it Lucas though, about every week and I only use about a tank and a half a week. I grew up around cars, have a family of mechanics, they all hate my car btw, and everyone at their shop has scratched their heads of this car. It’s gotta be a fuel problem since Lucas helps, right? Btw, I replaced the fuel pump because the filter is in the fuel pump and it was completely clogged. Would start but pressing on gas did nothing, just shook and acted crazy. New fuel pump only fixed it after pouring bottle of Lucas in tank, and it finally ran smooth for like, 2 weeks lol. It also has a new radiator, and it has a small oil leak at the head gaskets. Anyone have any ideas?

No. This one is 15 years old and has needed some repairs and maintenance. Needs good diagnosis (not head scratching and hate) now.


Back then KIA’s were a POS . It is just in the last 8 years or so they have really upped the quality end of their products .

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No, not a lemon, 15 year old car with ??? miles. (you don’t say)

Everything you replaced is normal for a car this old.

Is the check engine light on? (you don’t say) If it is, what are the codes? Surely someone in your family of auto mechanics can read the codes so you can post them. If not, the local auto parts store can read them for free.


Many of the parts you have replaced would be considered maintenance items so not part of a lemon issue.

Sounds like your gas tank is polluted with some foreign matter or the gas is breaking down and varnishing up the filter sock. What kind of usage do you have (e.g. miles per year, trip lengths, etc) and are you buying gas from a reputable station? Any reason to suspect someone put something into your tank? I would assume whoever changed the pump inspected for this but perhaps not…

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Not to mention that most of what OP has replaced is just ordinary service and maintenance items which should be replaced at relevant intervals anyway just as well as the oil changes. I wonder if those were done.

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How about telling if you bought it used and how long you have had it .

By definition, a NEW or nearly-new car with many defects/problems can legitimately be called a lemon. By the time that the warranty expires (which, in this case was many years ago), the term “lemon” does not apply.

Maintenance items, and items that typically have to be replaced after 8-10 years of use, are to be expected with all makes of vehicles.

Many years ago, I worked with a woman who was fond of saying, “Your car is being repaired AGAIN?”. However, in almost every instance, my car was merely in the shop for routine, scheduled maintenance.

Back in those days, I had a hard time understanding how people could confuse maintenance and actual repairs, and I still have a hard time understanding how people can confuse those two categories.


Check engine light is on. It has codes for misfire 1, misfire 3, and misfire unknown, misfire unknown. Yes we have a code reader. And oxygen sensor code. Have been cleared out after every pull in to shop for new parts. It has 130,000 miles. I don’t think it was taken care of before I purchased it about 6 months ago. The reason I say lemon is because I’ve been researching these misfire issues with Kia Rio, older models of course, think newest I’ve seen with same issues was 2011. There’s so many stories of people adding part after part after part, same part multiple times usually, and misfire always comes back within atleast a month or less. Yes I realize this is an older car, divorced and single mom who’s ex took car then voluntarily repoed it before court date. So had to start completely over and this was in my price range of buying car, plus getting a home for me and my kids. This is my first experience with foreign cars. I’ve never had a vehicle that is so complicated to diagnose. I understand maintenance, it’s been pounded into my brain by mechanic family, but nothing so far has fixed this misfire issue. And yes, I buy fuel from same reputable station every time, nothing was suspicious during fuel pump replacement. Although, at idle, when it’s shuddering and acting a mess, it smells very rich. Sometimes even inside car, I can smell very rich fuel. I’ve been told electric issues but it just doesn’t make sense, has gotta be a fuel issue. But then again, maybe not, nothing would surprise me with this car. I want to do more upgrading to this car and keep it but if I can’t the reason for these misfire issues, gonna have to get something different. But would rather put money into this, which I’ve already put so much into already. More than the car is worth. So has anyone out there has these issues personally and found the ultimate fix? Obviously mostly anyone who has had one of these cars in a later model has dealt with this, judging from so many complaints of exact same issue. Surely someone has figured it out.

One of the early Kia introduction to the US market I presume. So it may well have had some fledgling problems which have since been rectified. However I’m not seeing anything particularly unusual in terms of known problems. By the symptoms you describe I’m thinking you have some kind of easily corrected fuel system problem there.

I’m seeing recalls on the fuel rail & fuel distributor, so make sure those have been done. What else ? hmmm … There’s been some electrical system problems due to pin tension and pins being twisted. there’s a computer software upgrade recommended, associated with check engine light and p0101.

If I had that problem I’d start with a fuel pressure and vacuum system check, followed testing cylinder compression, and the crank position sensor.

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Thank you for your advice, I know the crank position sensor has been tested and all the wiring. Will for sure take your other advice and test vacuum and compression. According to the car lot where I bought the car, all the recalls have been done already. Thank you.

Is a 15-year-old car really a “late model?”

Everything you’ve done to your car is something I’ve done to my ‘98 Civic over the years, and I still consider it a great car, so I don’t think your issue is the car. Your issues are:

  1. You’re driving a car you hate.

  2. You need to keep shopping for a better mechanic who can diagnose the problem.

Maligning the model of a car as old as this one isn’t going to help.


It’s a 15 year old car that was about as cheap a car as you could get when it was new. Your expectations can only be but so high.


Suggest to double check at the link below.

I was working at a Chrysler dealer in 1996 when they brought in a truck load of Kia Sephias from the auction. Dealers at that time wouldn’t take a Kia in trade.Would a 2004 Kia still be considered an early model introduced into the US?

Had the OP traded this car ten years ago this cheap car wouldn’t be a problem today.

The OP just bought this thing about 6 months ago . The question is why the family mechanics did not go shopping with her.

Hmm, this one is interestingly odd. You didn’t mention if you replaced the fuel injectors though, or the fuel lines.

Also, where are you getting your parts? It’s very likely you got a bad part.