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2011 5 sp. bucks intermittently at low RPM on light ~ moderate throttle

Hyundai Accent Offered in a 2 or 3 door hatchback, or 4 door sedan ,this compact offers room to seat 4 people and excellent economy.

2011 5 sp. bucks intermittently at low RPM on light ~ moderate throttle

Old 01-09-2019, 08:54 PM
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Default UPDATE-SOLVED 2011 5 sp. bucks intermittently at low RPM on light ~ moderate throttle

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.
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.


Last edited by slmjim; 03-20-2019 at 01:51 PM. Reason: Clarity & additional symptom info
Old 01-09-2019, 11:00 PM
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Join Date: Aug 2015
Location: Iowa
Posts: 158

That's a GDI engine? - Check for crud on the intake valves.
Old 01-11-2019, 06:18 AM
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Originally Posted by allegretto View Post
That's a GDI engine? - Check for crud on the intake valves.
I haven't looked close enough to know if it's port, GDI or CRDI. The online research I've done is inconclusive.

What would the mechanism be that would cause the symptoms described if intake valve deposits are present?

Also, see the edit re: idle symptom.

Old 01-11-2019, 10:42 AM
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Join Date: Aug 2015
Location: Iowa
Posts: 158

The deposits accumulate on the back of the intake valve and on the walls of the intake channel. They are restricting the airflow. That causes rough idling , hesitation , misfires, increased fuel consumption. In sequential port fuel injection engines the valves and the intake channels get cleaned by the fuel and this problem does not occur.
Meanwhile there are cleaners available that do a reasonable job of removing the deposits. I don't have experience with them because I don't have a GDI engine. CRC products tend to be trustworthy.
This guy gives you a good idea of what to expect:

Good luck.
Old 03-20-2019, 11:48 AM
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Join Date: Jan 2019
Posts: 5
Default Update - solved

It was coils.
I replaced the remaining three original Hyundai coils with the same Delphi coils that I used for the replacement of the first one that failed & coded out a few thou. mi. ago.

I no longer have that first, original failed coil for reference.

Some of the clues were;
The one coil code a few thou before the bucking symptoms appeared. One failed coil may lead to suspect the others failing from a common cause (age, design or materials flaw?) (see OP).
Only happened when hot (leaner mix/harder to fire, more likely to lead to spark leak/blowout?).
Just "felt" like spark blowout under load conditions that would lead to increased chamber pressure.

It's run perfectly for the last ~1K or so mi. since I replaced the remaining three coils. I've made numerous, unsuccessful attempts to operate in such manner as to encourage the bucking symptom. Lugged it down pretty low a couple of times. If it didn't do it then, it's not gonna. The intermittent idle glitch is gone too.

Close examination leads me to believe there was a spark leak on the coils themselves, from the boot area to the coil wells in the valve cover. Why no code was generated I don't understand. Pics below are what leads me to believe it was spark leak.

In addition to what appears to be a model or part #, there is an engraved number that may be a serial or batch # that is different on each coil. I'll refer to the coils in the images below by the engraved #'s.

Not all the pics were good enough to post. These are representative of all three coils. Wish I still had the first one that failed months ago...

31664. Crack developing. The real curiosity though, is the white ash at the arrow:

31664(a). A better look at the ash. Subtle, but it's there:

31649. Different crack, ash in different location:

31673. Major crack, wide ash trail:

I removed the rubber boots to get a better look. These are representative of what I saw, The inside of the boot is oriented same as it came off the coil in each pic:


So there we have it. I'll post updates if anything changes.

Hopefully this info will assist another owner in performance diagnosis.


Last edited by slmjim; 03-20-2019 at 11:53 AM.
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