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Misfire under load when hot

6217 Views 4 Replies 3 Participants Last post by  Alan
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? 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

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

- 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?

Any ideas? Thanks!
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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.
The new coil and wires fixed the problem

Thanks all
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