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2004 Hyundai Sonata - 4 cyl. - Automatic Transmission

I am now completely puzzled. This problem started over a month ago – spark knocking, lurching, minimal response to accelerator positioning, occasionally stalls…almost like it is running out of gas. But, it would not do this all the time…sometimes it would run for a few days just fine, then out of nowhere, it would start giving me this problem.

At first I thought it was water in the gas since the very first time it happened was 20 minutes after filling the tank. So, I tried a gas additive to get rid of any potential water in the fuel tank over the course of 3 tank fills…didn’t help.

Found out I needed a special adaptor just to check fuel pressure.  Since I didn't have one, I decided to just replace the fuel pump and spark plugs…didn’t help.

Started to notice it was happening more frequently (now daily), but always after about 20 – 30 minutes of driving. Let it sit and cool down for a while, it would be fine until it heated up again. The engine temperature never overheated…stayed around 205 – 210 degrees.

During the course of all this we found codes for the cam sensor, cylinder 1 misfire, cylinder 4 misfire, random misfires (I don't remember the actual code numbers now). Then, one night, I cleared all of the codes and restarted the engine. Revved it up until it started acting-up again, so I could recheck the codes…did this 3 times. Each of the 3 times it only returned a cam sensor code, nothing else. So, I replaced the cam sensor (which was only 8 months old…it was replaced back in November)…didn’t help

Most recently, the engine "stumbled" and eventually stalled, after about 20 minutes of driving. Checked the codes, and it returned only 2 codes:
P0421 - Warm up catalyst below threshold bank 1
P0340 – Cam position sensor A bank 1 ckt malfunction

Out all of the times it has been acting-up and returning codes, this is the very first time we have seen the catalyst code. Although, the Cam sensor code seems to appear every time.

Checked ignition coil and plug wire resistance...nothing odd with them.

To summarize:
Runs rough and stalls after 20 – 30 minutes of driving
Tried gas additive to get rid of water
Replaced fuel pump and spark plugs
Replaced cam sensor
Nothing has solved the problem

Any suggestions…  

I have the same problem. It had gotten really bad a month ago. I checked and replaced the PCV valve, which made an immediate improvement, but did not fix the problem. I am going to clean the throttle body next.
I have a 2000 hyundai with 133,000 miles on it.  

Any luck with getting this resolved? My Sonata is starting to act up as well. I have not replaced any sensors or other parts. But before i plunk down some 90$ for a diagnosis that may or may not replicate the problem...
I was wondering if you found out anything?
~Todd