I bought the car used with 21K mi. on the clock from CarMax. History unknown, except it came from NJ, and was very clean. Obviously well cared for. No particular history in online VIN searches. Currently at 35K mi.
Prior to purchase I had it evaluated by a local Hyundai dealer. They found no problems other than old firmware, which they updated. Motor & running gear is box-stock AFIK.

It's run fine until recently.

Code history:
~31K mi. P0302 - examined and swapped coils #1 & #2. Code never returned.
~33K mi. P0303 - close examination of #3 coil showed a very faint hairline crack on the outer surface at the plug end. Not a die line. An actual crooked crack. Not sure it even went all the way through. Replaced that coil with a new Delphi coil. Code never returned.
~33K mi. - replaced plugs along with the #3 coil above. The old plugs looked fine, just normal spark erosion.
No other codes have been seen during my ownership.

Let's define some parameters:
Buck - the feeling that spark or fuel is cut for a fraction of a second, then resumes. If you've ever experienced a single-coil vehicle that is way overdue new plugs, under load (climbing a hill), where the spark is simply "blowing out", that's what it kind of reminds me of.
Intermittently - doesn't always do it, although the longer it runs, the more likely it is to buck. When it bucks, it usually only bucks once.
Low RPM = 1,500 ~ 2,000 RPM.
Low ~ moderate throttle = that which would be applied in, say, second gear after making a right turn at an intersection, accelerating to a 2-3 upshift in a 25 MPH zone.
Or,
Coasting toward a red light. Light turns green. Downshift 4-3 @ 30 MPH (~1,700 RPM in 3rd.) and accelerating in a 35 MPH zone.
It'll buck in any gear; it seems RPM-dependent.

Cold starts and the first 5 ~ 10 miles or so miles it runs fine regardless of throttle or load.. The longer it runs, the more likely it is to exhibit the "bucking" symptom at low RPM. By the time it's run for 20 ~ 25 mi. it tends to buck more & harder under the same throttle and load.

Fully warm and just off idle after 20 ~ 25 mi. there is a noticeable stumble just off-idle when applying throttle from a standing start. Again, low RPM at low ~ moderate throttle.

EDIT: After running long enough to exhibit the problem, there is a very random, subtle, instantaneous stumble at idle. It's not a cyclical miss; happens maybe once every 5 ~ 10 seconds. It's barely felt, and deflects the tach needle by only the tiniest amount, maybe the width of the needle tip if that.

Higher RPM (2,500 and up) and greater throttle up to WFO loads do not exhibit any bucking or performance problems.

The fact that it begins to misbehave only after running for a time leads me to believe it may be heat related. I just don't want to throw parts at it hoping something will stick.

What say the forum motorheads?

Thanks for your help.

slmjim

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