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Does your IS 250 crank a long time on cold mornings, then idle rough for half a minute? That pattern points at coked fuel injectors, not a weak battery. A coked injector has hard carbon baked onto its nozzle tip.

The carbon makes it spray a poor stream instead of a fine mist, and cold cylinders give a bad spray the least margin. The fix is a matched set of six — the 6x fuel injector set 23250-31020 for the 2006-2015 Lexus IS 250 2.5L V6.

Why these injectors coke up

The 4GR-FSE is a direct-injection engine: the injector sprays fuel straight into the cylinder, not into the intake port. A cam-driven pump feeds it at several hundred psi.

The nozzle tip lives inside the cylinder, in combustion heat on every power stroke. Leftover fuel bakes onto it as hard varnish at shutdown. Injectors pulled after years of trouble come out visibly coked.

Gasoline never washes the intake valves on this engine, so oil vapor leaves carbon there too. That is a separate walnut-blasting job. It mimics injector symptoms, so check it before you order.

What it feels like from the driver's seat

A coked injector still fires. It just sprays badly, so that cylinder burns slowly. Watch for:

• Long cranking on cold starts; warm restarts stay normal.
• Rough idle for the first 20-30 seconds, then it smooths out.
• Hesitation on light throttle from a cold engine.
• High long-term fuel trims — the small corrections the ECU adds to its fuel math.
• P0171, P0300, or a single-cylinder code such as P0303.
• A slowly rising oil level, from a dripping injector thinning the oil.

Do not sit on the repair. A misfiring cylinder overheats the catalytic converter — a far pricier fix than six injectors.

The rest of the fuel path is grouped in the air and fuel delivery collection. Browse it while the rail is off.

What the codes mean

Both codes are generic SAE J2012 definitions, not Lexus-specific. Read them precisely to avoid chasing the wrong system.

• P0171 — "System Too Lean (Bank 1)": the oxygen sensor still reads lean after the ECU added fuel. An injector that under-delivers produces exactly that.
• P0300 — "Random/Multiple Cylinder Misfire Detected": more than one cylinder failed to contribute power. A lean mixture is harder to ignite, so weak injectors scatter misfires.

A cylinder-specific code (P0301-P0306) is even more useful. It convicts or clears one injector for free:

1. Swap the suspect injector with a neighbor.
2. Clear the code and drive the car.
3. Misfire follows the injector → replace it. Code stays → check the coil, plug, or compression.

For the misfire logic itself, the P0300 misfire diagnostic guide explains how an ECU counts misfires.

The numbers, and what each one fits

Row four is the trap: two numbers one digit apart, not interchangeable.

The 2.5L unit works alone at high rail pressure. The 3.5L unit is the low-pressure half of a dual-injection system. Cross-installing one means leaks or fuel sprayed in the wrong place.

23250-31020 versus 23250-31070: different jobs

The 3.5L 2GR-FSE runs D4-S dual injection. Each cylinder gets one direct injector and one port injector, and the ECU blends them by load.

The 23250-31070 is the port side of that pair — a two-pin, twelve-hole Denso unit (FJ786). It sprays into the intake port and washes the valves with fuel.

Your 2.5L never got that second system. Its six direct injectors do all the work, all the time. Shopping by price across the two listings puts the wrong hardware on your car.

Confirm it before you spend

Three checks, cheapest first, separate injectors from other causes.

1. Read every code and write them down. P0171 plus P0300 tells a different story than P0304 alone.
2. Run the injector swap test on any cylinder-specific code.
3. Check long-term fuel trims at a warm idle and at 2,500 rpm. High at idle only → vacuum leak, not injectors.
4. Pull the spark plugs. One fouled plug is a symptom; six fouled plugs are months of misfires.
5. Verify the engine under your hood is the 2.5L. The IS 350 shares the body, not the injectors.

A compression test rules out mechanical misfires. A borescope look at the intake valves tells you whether carbon needs its own appointment.

What new injectors will not fix

Hard starting after a fresh set points elsewhere. Suspect the high-pressure pump, the in-tank pump, the battery, or the crank sensor.

A rough idle caused by a tired engine mount stays rough after the swap. Owners refreshing a fifteen-year-old chassis often book the coilover and spring kit for the 2006-2013 IS 250 / IS 350 RWD during the same shop visit.

Replacing the set of six

This is a rail-out job with hand tools. No programming is required — the ECU relearns its trims over a few drives.

1. Relieve fuel pressure and disconnect the battery.
2. Unplug the injector connectors and the high-pressure line.
3. Unbolt the rail and draw the injectors out with their seals.
4. Fit new O-rings and seals. Reusing a hardened seal is the most common cause of a fuel smell after the job.
5. Reassemble, cycle the key to prime, and watch every injector bore for leaks before the engine warms up.

Replace all six, not just the worst one or two. They coked at the same rate, and the labor covers all six at once.

Comparing injectors across brands? The fuel injector comparison guide for performance engines covers flow matching and why sets matter on V6s.

Frequently asked questions