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2002 Elantra Rough Idle and Misfire when Hot

2002 Hyundai Elantra GT 2.0L 140k miles

Symptoms:

  • Intermittent rough idle when hot, slight shutter, surges between 700-800rmp
  • Intermittent hesitation when accelerating, cylinder 2 misfire when hot and under load
  • Misfires in all gears under load, mostly when under 2000rpm
  • Code P0302 when scanned, no other codes
  • Symptoms appeared suddenly, not gradually over time

What’s been changed:

  • Replaced spark plugs, little or no improvement. Old plugs looked worn, but generally in good shape with no significant deposits.
  • Cleared code and swapped injectors between cylinders 1 and 2, after test drive P0302 code came back, so it seems that the injectors are good. If the injector were bad I would have expected the misfire to change from cylinder 2 to cylinder 1.

Inspection and Testing:
Ignition System Testing
Spark plug wires and primary coil are within specification. Secondary coil resistance is high on both packs. Could this be causing a problem with just cylinder 2? Is it worth replacing?

Spark Plug Wires:
Specification: 5.6 Kohm/meter ± 20%
Cylinder 1: 2.49 Kohm (Longest)
Cylinder 2: 2.03 Kohm
Cylinder 3: 1.64 Kohm
Cylinder 4:1.36 Kohm (Shortest)
Primary Coil:
Specification : 0.58ohms ± 10%
Actual: .6 and .6
Secondary Coil:
Specification: 12.2 Kohms ± 15%
Actual (Cold): 16.03K for Cylinder 2-3 pack and 15.94K for Cylinder 1-4 pack
Actual (Hot): 18.54K for Cylinder 2-3 pack and 18.44K for Cylinder 1-4 pack

Air/Fuel System Testing:
Fuel system seems to check out. I do not believe the problem is fuel related. There are no obvious cracks or leaks on the intake manifold. A good spray of Carb cleaner all around the manifold causes no change in idle. I might try propane later. No obvious vacuum leaks.

Fuel Injector:
Specification: 13.1-14.5 ohms
Actual: Cyl 1: 14.9, Cyl 2: 15.3, Cyl 3: 15.3, Cyl 4: 15.7

Compression Testing:
Compression is not great, but within specification. Could the slightly lower compression in Cylinders 2 and 3 indicate a head gasket leak between those cylinders? Could this cause a misfire in Cylinder 2?

Dry Compression:
Specification: 145-218psi, <15psi between cylinders
Actual: Cyl 1: 165, Cyl 2: 155, Cyl 3: 155, Cyl 4: 160

Oil/Coolant/Tailpipe:

  • Oil looks normal, not brown or milky
  • Coolant looks normal and is full, no signs of leaking (coolant has not been changed in a very long time, could a coolant flush help?)
  • Exhaust looks and smells normal from tailpipe.

Thoughts
Sticking valve? How would I check this?
Bad sensor somewhere? Doesn’t a bad sensor usually throw a code?

I did the compression test again, realizing that I did not have the trottle open the first time and got much better results: (dry/wet) cyl1: 190/220, cyl2: 190/225, cyl3: 180/200, cyl4: 190/235

Wires and Coil fixed the problem.

That was easy !
You answered your own question.

I “liked” your original post because it was so thorough. As we say in our office, half the time when you think through explain a problem in detail, you come to the solution…