# 2002 Elantra Rough Idle and Misfire when Hot

**URL:** <https://community.cartalk.com/t/2002-elantra-rough-idle-and-misfire-when-hot/71481>\
**Category:** Maintenance/Repairs\
**Tags:** hyundai, elantra, rough, misfire\
**Created:** [March 17, 2012, 9:57pm UTC](https://community.cartalk.com/t/2002-elantra-rough-idle-and-misfire-when-hot/71481 "2012-03-17T21:57:56Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![AlanR](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@AlanR](https://community.cartalk.com/u/AlanR)\
**Post date:** [March 17, 2012, 9:57pm UTC](https://community.cartalk.com/t/2002-elantra-rough-idle-and-misfire-when-hot/71481/1 "2012-03-17T21:57:56Z")

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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?

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**Author:** ![AlanR](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@AlanR](https://community.cartalk.com/u/AlanR)\
**Post date:** [March 18, 2012, 3:32am UTC](https://community.cartalk.com/t/2002-elantra-rough-idle-and-misfire-when-hot/71481/2 "2012-03-18T03:32:05Z")

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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

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**Author:** ![AlanR](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@AlanR](https://community.cartalk.com/u/AlanR)\
**Post date:** [April 18, 2012, 9:53pm UTC](https://community.cartalk.com/t/2002-elantra-rough-idle-and-misfire-when-hot/71481/3 "2012-04-18T21:53:35Z")

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Wires and Coil fixed the problem.

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**Author:** ![friedo82](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/friedo82/32/981_2.png) [@friedo82](https://community.cartalk.com/u/friedo82)\
**Post date:** [April 18, 2012, 10:18pm UTC](https://community.cartalk.com/t/2002-elantra-rough-idle-and-misfire-when-hot/71481/4 "2012-04-18T22:18:51Z")

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That was easy !  
You answered your own question.

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**Author:** ![njm](https://avatars.discourse-cdn.com/v4/letter/n/b77776/32.png) [@njm](https://community.cartalk.com/u/njm)\
**Post date:** [April 19, 2012, 3:15am UTC](https://community.cartalk.com/t/2002-elantra-rough-idle-and-misfire-when-hot/71481/5 "2012-04-19T03:15:01Z")

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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…