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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  

Sure sounds like vapor lock if it only happens when it's hot.  That's when there's not enough insulation between the fuel line and something hot like the exhaust manifold.  But it may be a fuel filter.

My 2004 Sonata engine just started stalling and stumbling for the first minute or two after every gas fill, then it's fine again until the next gas-up.  I am clueless.  

The same happens for my 2004 sonata, but only if I let the tank get down to the point the warning low fuel light comes on.  I went to a mechanic and he said it has to do with the amount of pressure on the fuel pump and his advice was to fill the tank when it gets down to the 1/4 level.  Don't know if this helps, but I am going to try it.  

I have the same issue with our 2004 Sonata.  It started in December but seemed to be worse Today.  I even took it to Hyundai who kept it overnight and did not find any issues.  Their is no engine light to indicate a problem.  It's good to see what people have tried so far though.