2013 Hyundai Elantra Limited, with 164k miles, 1.8 L 4-cyl engine.
I was told by the previous car owner that it was misfiring, and needed replacement spark plugs. Without really checking, I got four new plugs. I pulled out the old plugs, and they were all iridium. All looked fine except 1 which was covered in black carbon build-up. I replaced them anyway.
10 miles later, the car is misfiring again, especially near idle. Especially as the car warms up or idles a while.
It will idle at 900 on cold-start, down to 750rpms, then once it warms up more, start dropping as low as 400rpms, up to 1100 and down and up again, repeatedly, while misfiring. Also misfires (putt-putt effect) from around 750rpms up to ~2,000 rpms until it gets above 2,000 and then seems to smooth out.
The engine seems to shake and the car vibrates pretty strongly, but it comes and goes and doesn’t always vibrate.
A final possible factor: The car was sitting for around 6 to 8 weeks, until the owner sold it to me. It had 4 gallons of old gas in it. I filled it with 8 more gallons of new gas.
I have also recently replaced oil with 5W20 synthetic, and done a brake job on all four tires.
Hope this isn’t too much information. Thanks in advance for any help. 
To start with, never buy a car that just needs a cheap and easy fix. If there was a cheap and easy fix, the previous owner would have done it before he sold it.
Is the check engine light on? If it isn’t, does it light up when you turn the ignition of without starting the engine. The check engine light is designed to tell you what system is having abnormal readings.
Change the ignition coil from the sooty cylinder to a different cylinder. Does the misfire move with it. If yes, replace that coil. If not run a dry and wet compression check.
6 Likes
First, I appreciate the fast answer.
“Never buy a car that needs a cheap and easy fix”
Understood. In this case, it was a friend letting me have the car, so I can start working, and I will pay him back once I have income. He said he expected repairs to be around $1000 and lowered his price to $2000.
“Is the check engine light on?”
Yes. I haven’t gotten up to Autozone yet to get the codes.
“Change the ignition coil from the sooty cylinder to a different cylinder. Does the misfire move with it. If yes, replace that coil. If not run a dry and wet compression check.”
Can move the ignition coil by myself, I believe. The dry and wet compression check… Can I borrow a compression test kit from a shop? I watched a video on Youtube. Near the edge of what I can do from watching a video, I’m guessing.
See if Advance or Autozone have a compression tester in their free tool loaner program. Don’t forget to prop the throttle open. Running a compression check is a piece of cake compared to replacing a head.
1 Like
Will try swapping the coil first. If that doesn’t work, compression tester I will move to next. Will update with results. Thanks, sir.
Update:
Used OBD scan tool. Fault codes detected before swapping ignition coils #2 and 3:
P0303 - Cylinder 3 Misfire Detected
P0106 - Manifold Absolute Pressure/Barometric Pressure Circuit Range/Performance Problem
After swapping ignition coil #2 and 3, car didn’t start on first turn of the key. Turned again and held a little and after 2 or so seconds it started, but was not running well. The fault codes detected after restarting the engine (I did not clear the codes):
P0303 - Cylinder 3 Misfire Detected
P0106 - Manifold Absolute Pressure/Barometric Pressure Circuit Range/Performance Problem
P0302 - Cylinder 2 Misfire Detected
B2aaa - Unknown code - More information may be available on web
So it sounds like bad ignition coil. Ordering a Denso 673-8305 Ignition coil, which is the OEM part.
What about the other codes? How can I clean out cylinder 3 effectively but also hopefully inexpensively?
P0106 could be a vacuum leak or a bad MAF sensor. Do a visual inspection for possible vacuum leaks.
1 Like
I will have a friend who knows more help me look for vacuum leaks, but the engine bay looks in very good shape.
Thanks for info.
The misfire is causing low engine manifold vacuum, the MAP input to the computer is abnormal and considered out of range.
After repairing the misfire, P0106 should not return.
2 Likes
Clean or replace the plug in cylinder 3 also.
So, Update 2:
The bad ignition coil was replaced. I pulled the autolite plug from cylinder 3. Was clean as a whistle. I put the NGK plug in anyway, per recommendations.
Car started up fine, but idled up at 1500 rpms, and had a slight tick at around ~5 Hz, but on a friend with some experience’s recommendation, I drove it around for about 45 minutes on the highway.
Tick gone, misfire gone, idling at 600rpms, smooth, and check engine light off.
My friend thought he had a compression tester kit to bring with him, but he couldn’t find it. Is it very important to have that done still? I plan on replacing with the new NGK plugs within a week or so, per everyone’s recommendations, but can’t do that immediately.
If the car is running normally I wouldn’t worry about a compression test. That was only if the coil wasn’t the problem. Glad to hear you got the issue solved.
Old school a miss fire ment spark plug, or coil now days with fuel injection a miss fire most likely a injector is leaking or not working properly, switching coil packs may help ID which cylinder, injectors testing is best I found one company out of AZ which make sure your new injectors have bal pressure by matching OMS reading.
I’ve never worked on a Hyundai, but on my 2002 Mazda Miata I had a similar experience with a misfire. After going through all the diagnostics you have been discussing, and more, I finally was told to check the valve lash (the maximum space between the cam lobes and the valves). The lash was tight on every exhaust valve. I fixed that and when I buttoned up the engine and started it the first time, I knew that the problem was solved. The engine started fast and run so much better, it was shocking. The check engine light never went on again, fuel mileage went up, and happiness filled the garage. So that’s my suggestion.
With my 4 previous Hondas ('75 Civic, '81, '85, '88 Accords) adjusting valve lash was THE most satisfying DIY maintenance for me.
1 Like
American designed engines (with a couple exceptions) have been using hydraulic valve lash adjustment since world war 2. Why do Honda and some other companies persist in using an antiquated system requiring periodic adjustment?
1 Like
Nissan does it, too. I suspect there’s some performance reason, but maybe it’s just cheap. Anyway, it’s not a fun job but it sure made a difference.