Misfire Only on the Long Hill
A 2010 V8 with 118,000 miles sets P0300 only during a long uphill pull at wide-open throttle. At idle and steady cruise the misfire counters stay at zero. The spark plugs are original. Measured gaps are 0.072 to 0.078 inch against a 0.044 spec. Secondary ignition on a scope shows 26 kV firing voltage and 1.4 ms burn time on the misfiring cylinders under load, versus 18 kV on the others. Coil primary resistance is 0.6 ohm on all eight, and secondary resistance is 8.9 kOhm on every coil against a 7 to 11 kOhm spec. Fuel trim at WOT is -3 percent.
1. Explain why an eroded gap causes a misfire under load but not at idle.
Read the worked answer
The gap sets the voltage the coil must produce to ionize the gap, and cylinder pressure raises that requirement.
- Required voltage rises with gap distance and with charge density in the cylinder. At idle, cylinder pressure and charge density are low, so even a 0.075 inch gap fires within the coil's reserve.
- At WOT, manifold pressure is near atmospheric, the trapped charge is dense, and the voltage needed to jump the gap climbs. With the gap already almost double spec, the required voltage can exceed what the coil can deliver under that condition, so the spark misfires or finds another path.
- The 26 kV firing voltage versus 18 kV is the tell: the ignition system is working near its ceiling on those cylinders only when cylinder pressure is highest.
This is the opposite of a rich or lean mixture misfire, which is usually most visible at idle or light load.
2. A technician argues the coils are fine because resistance is in spec on all eight. Is that a valid conclusion? What does resistance testing miss?
Read the worked answer
It is not a valid conclusion on its own. Resistance is a cold, low-voltage check of continuity, not of insulation quality.
- A coil can pass a resistance check and still break down internally under high voltage and heat, arcing through a carbon track in the boot or tower.
- It also says nothing about the secondary circuit past the coil: boot condition, plug terminal fit, and gap. Here the plugs are the known defect.
- Resistance checks are still useful for finding an open winding, but a normal reading does not clear a coil for a high-load misfire.
Given eight coils at 8.9 kOhm and a documented 0.075 inch gap, the coils are not the first suspect.
3. Give your repair order and the acceptance test. What would change your mind and send you back to the coils?
Read the worked answer
Repair order:
- Replace all eight plugs and gap them to 0.044 inch. On a waste-spark or coil-near-plug engine, do all of them; the opposite plug in a pair wears at the same rate and will be next.
- Inspect each coil boot for carbon tracking, cracking, and spring tension while the coils are off. Replace any boot that shows tracking.
- Clear adaptives and road test the same hill at WOT.
Acceptance: misfire counters stay at zero through the pull and firing voltage on the scope drops back toward 18 kV or below across all cylinders.
Change my mind if: the misfire persists on the same cylinders after new plugs and a known-good coil swap, or if firing voltage stays above roughly 25 kV with a correct gap. That would point to a coil breaking down under load, a boot arcing, or high secondary resistance in the plug wire if the engine uses wires. At that point swap a coil from a good cylinder and repeat the hill test.