Idle-Only Misfire with Lean Fuel Trims on a Four-Cylinder

The trims are telling you where the extra air is coming from.

• LTFT +19% at idle means the PCM is adding a lot of fuel to correct a lean exhaust at low airflow.
• LTFT +4% at 2,500 rpm means the same leak is now a tiny fraction of total airflow, so the correction nearly disappears.

That rpm-dependent pattern is the fingerprint of an unmetered air leak - a vacuum or intake leak - because a leak's mass flow is roughly constant while the engine's airflow rises steeply with rpm.

What it rules out:

• A leaking injector or rich-running fault would drive trims negative, not positive.
• A weak fuel pump or restricted filter would be lean at high load and high rpm, not worst at idle. Pressure held at 400 kPa, so the supply side is not the primary fault.
• Mechanical condition is sound: compression is even and leak-down is 8–12% with no coolant or crankcase breach. A burnt valve or blown head gasket would show an outlier cylinder, not a uniform set.
• Ignition is largely excluded by the coil swap and new plugs.

So: a lean condition present mainly at idle, concentrated on the #2 cylinder.

Leading hypothesis: a leak localised to the #2 intake runner - most often the intake manifold gasket at that port, a cracked or porous manifold runner, an injector O-ring that is not sealing, or a loose manifold bolt. It affects one cylinder at idle and costs almost nothing at higher airflow, which matches both the misfire pattern and the trim behaviour.

Confirmation - do not guess at a part:

1. Smoke test the intake with the throttle closed. Watch for smoke at the #2 runner flange, injector bosses and any vacuum tee. This is the cleanest confirmation and finds multiple leaks at once.
2. If no smoke machine is available, propane or water/carb-cleaner directed at the #2 flange with the engine idling - a propane increase raises rpm and drives STFT negative; water causes a stumble. Keep water away from electrics.
3. Cylinder balance / injector kill test to quantify the miss before and after the repair.
4. Verify injector spray pattern and balance if the leak test is inconclusive - a partially clogged #2 injector can mimic this, but it would lean out under load, not at idle.

Record the before/after LTFT at idle and the misfire counter. A correct repair should pull idle LTFT to within roughly ±10% and stop the counter.

A misfiring cylinder dumps unburned fuel and oxygen into the exhaust. The catalyst burns that mixture and can overheat internally - a melted or contaminated brick is a four-figure repair that the customer did not come in for.

Protect the vehicle while you finish diagnosis:

• Keep run time at idle short where possible; idle is where the misfire and the trim error are worst.
• Watch catalyst temperature if the scan tool reports it, and look for a glowing converter after a road test.
• Do not clear the code and send the car out to 'see if it comes back' with an unresolved lean cylinder.

Customer message: explain that the misfire is being caused by extra air entering at cylinder 2, that driving with it risks the catalytic converter, and that the repair is a gasket/seal job rather than a major engine repair - which is true now and may not stay true if the converter is cooked. Get authorization for the leak repair plus a post-repair verification of trims and misfire counters.