Every misfire comes down to the same three things: spark, fuel, or compression. Your job is figuring out which one, on which cylinder, without throwing parts at it.
Here's a simple way to work through it.
Find the cylinder
If there's a misfire code, it may already tell you the cylinder. If there's just an occasional stumble and no code yet, check the misfire counters or Mode $06 on the scan tool. Those show misfires per cylinder even before there are enough to set a code.
The swap test
Once you know the cylinder, swap parts with another cylinder and see if the misfire follows.
Move the coil and plug to another cylinder. If the misfire moves with them, they're the problem. If it stays put, they're cleared. Then try swapping the injector. If the misfire moves to the other cylinder now, the injector is bad.
When it won't move
If the misfire stays with the cylinder after swapping the coil, plug, and injector, look at what stays with that cylinder: the coil connector and wiring, the injector circuit, or the engine mechanically.
Check compression. If that cylinder has low compression, the problem is inside the engine.
Misfires that only happen sometimes
- Only under heavy load: a plug gap that's too wide needs more voltage than the coil can give under load. Wrong or worn plugs are a common cause
- Only when hot: tight exhaust valve lash can keep the valve from closing once things expand
- Normal cranking compression but low running compression: a worn intake cam lobe can't open the valve enough to fill the cylinder
Several cylinders at once
If a bunch of cylinders misfire together, look for something they share. On a V6 where only the injectors fed by one splice are dead, the problem is in the wire or splice feeding that group, not the injectors.
Cheat sheet
- Every misfire is spark, fuel, or compression
- No code yet? Check misfire counters or Mode $06
- Swap the coil and plug, then the injector, and see if it follows
- Misfire stays put after swaps? Check the wiring and compression
- Misfires under load: check the plug gap
Common questions
How do I find which cylinder is misfiring?
Read the misfire code or check the misfire counters and Mode $06 on a scan tool.
What if the misfire doesn't move during a swap test?
The swapped parts are fine. Check what stays with that cylinder: the wiring, the injector circuit, or compression.
What causes a misfire only under load?
A spark plug gap that's too wide is a common cause. The coil can't put out enough voltage to fire it under load.
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