[Alan]

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? Are there any other ways I can test the coil before replacing it?

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 pretty good for 140k

Compression:
Specification: 145-218psi, <15psi between cylinders
Actual: (dry/wet) cyl1: 190/220, cyl2: 190/225, cyl3: 180/200, cyl4: 190/235

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?

Any ideas? Thanks!

 

[Racecarkid321go]

Just for fun. Test to see if the coolant has a charge... I'm not pulling your leg. I have heard that the coolant in chevy trucks have had the problem where it had 12 volts or so. They would change it out and it would run fine again. I guess it has something in it to do that... Idk exactly.

 

[CaptainSlow]

Just because your coils and wires are in spec for a resistance measurement doesn't mean they won't short to ground. Try this, let the vehicle warm-up and use a spray bottle with water in it to lightly mist all of the ignition components. Load the engine by either power braking or snapping the throttle, you will likely see the secondary ignition is shorting to ground under load. It's pretty common to have a waste spark ignition coil misfire on only one cylinder, this is due to the resistance being higher in one direction than the other, the voltage usually just takes the easier path...

Good luck!

 

[Alan]

Thank you for that information! I will try the spray bottle trick. Or, I may just get a new coil and put it in.

Update: I ordered a new coil and will put it in this weekend.

 

[Alan]

The new coil and wires fixed the problem

Thanks all