Join the Car Talk Community!

Discussion Rules

Welcome to the Car Talk Community!

Want to ask a question or join the discussion? Great! Join now.

Sign In Register

2002 Elantra Rough Idle and Misfire when Hot

edited March 2012 in Repair and Maintenance
2002 Hyundai Elantra GT 2.0L 140k miles

- 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 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.

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...
This discussion has been closed.