Sonata Misfire
I have a 1999 Sonata which just started misfiring. We brought it to the
dealer with the check engine light on and they wanted to replace all the spark plugs and wires (which requires removing the air intake plenum) and then see what the problem was. I just replaced the fuel filter and I have read on other sites about similiar problems with misfire codes and dealers performing the same repair only to find it wasn't the problem. I remove the spark plugs and they don't look that bad, neither do the wires. The car has about 50,000 miles on it. Under a load, starting from a stop the car will misfire or ping. It idles a little rough and eventually the check engine light will come on and the computer will, according to the dealer, show 3 cylinders misfiring code. I find it hard to believe that 3 spark plugs and wire failed at the same time out of 6. Anyone have any ideas? Thanks Dave |
Re: Sonata Misfire
Unfortunately, this is probably the easiest place to start. While the
plenum is off, it's probably a good idea to check out all six fuel injectors visually. I've seen the injectors on this engine cause misfires (although I've only been barely able to feel them and only when cold, with the exception of one). The fact is, the wires are usually the culprit. If you have a misfire on one of the even cylinders, you can try to address that one first, since those cyliders are in the front and the plugs, wires, and injectors are all reasonably accessible. You can try swapping plugs and injectors between misfiring and nonmisfiring cylinders to see whether the misfire changes cyliners with the change. |
Re: Sonata Misfire
Do you replace the injector or what would you recommend to do with the
wiring? From taking the spark plug wires off the coils I have narrowed it down to the back middle cylinder. When that plug wire is off it does not misfire anymore. It still runs rough, since it's running on 5 instead of 6 cylinders but it doesn't misfire with the cylinder out of the loop. |
Re: Sonata Misfire
Ok, I did more probing today. The plugs I just put in days ago are already
black. I am leaning towards a rich mixture and the backfires are from too much fuel causing detonation. So I am looking at the oxygen sensors. Then I have realized that there are 4 on this engine. The two on the top are reasonable and the two on the bottom are expensive. They are 4 wire sensors. black/white/green and yellow wires. My understanding is that two wires are the heater and the other two send the signal. Does anyone know which is which and how to test the sensors? |
Re: Sonata Misfire
If you don't have an oxygen sensor dtc, look elsewhere first. Even if you
do, you may still want to look elsewhere first. The most common problems are ignition (plugs, wires, and to a lesser extent, the coil). A misfire will cause the oxygen sensor readings to be incorrect. What do you mean by not misfiring any more but still running on five cylinders? Do you mean the car actually runs better with the #3 wire (center rear) disconnected?? What is the misfiring you describe if it's not the failure of the #3 cylinder to fire? I wouldn't do anything right off with the injectors unless one looks clogged. If it's clogged, replace it. |
Re: Sonata Misfire
I was wrong on that. This is my analysis.
I replaced the spark plugs. All black. The wires test ok. The fuel filter has been changed. The removed the front fuel injectors. All visiable ok. Look clean as a whistle. The engine backfires when starting from a dead stop or under power, accelerating. Rough idle. No power. I think that one of the oxygen sensors is bad telling the computer that the engine is running lean and the computer is running the engine rich to compensate. The back fire is the rich exhaust gas pinging on the hot convertor. The computer is interpreting this as a misfire. The dealer did state on the ticket that the computer read 3 cylinders were misfiring. Since two sets of oxygen sensors read each side of the engine 3 + 3 cylinders = V6, it appears that one of the oxygen sensors on one side is completely dead reading lean. This car has never had an oxygen sensor replaced. |
Re: Sonata Misfire
How did you test the wires? The typical mode of failure is arcing out the
side of the plastic tower that goes down into the hole in the head. Usually, you can see carbon growth, but sometimes the hole is so tiny you can't see it. I've seen cases where the plug wire had to be pulled off the plug a little with the engine running to see the arc to the head. (You cannot do this on the rear bank of the v6, of course). Which 3 were misfiring? If they were the back three (#'s 1, 3, 5), then you may be on the right track with the oxygen sensor. This should be easily discernable with a scan tool since the fuel correction will be much different between the banks. I still think you're likely to get an o2 code if you have a sensor problem. If your engine is still apart, you can move the injectors from the rear cylinders in question to a cylinder on the front bank and see if the misfire moves. This way, you won't have to pull the plenum again to replace the injector. The computer is not reading anything that happens in the converter as a misfire. The computer detects misfires by deviations in expected crankshaft speeds. Anything that happens in the converter won't be able to affect that. The backfiring you're hearing in the converter (or intake) is a result of the actual misfire. I had one car with a bad injector that I had difficulty finding. It didn't look bad, but replacing it fixed the problem. I had another car with bad fuel, and it turned the plugs black like you're describing, and the car really wouldn't run worth a crap. I had trouble just driving it up the hill to get into the shop. I went through a lot of stuff before I eventually figured out what the problem was. |
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