Why Your 2025 Toyota 4Runner is Misfiring and How to Fix It

How to Fix Misfire

A misfire in your 2025 Toyota 4Runner is a disconcerting issue that can shake your confidence in the vehicle's legendary reliability. While the new generation represents a more complex machine, understanding the root cause is the first step to a fix. The primary culprit, as identified by owners, is related to the ignition system's boots, which are the rubber insulators covering the spark plugs. As one owner reflected on the changing nature of Toyota's modern vehicles, "They straight up said these new gens are not the old ones, they are offering a more technically complex and environmentally friendly product line" (source). This complexity makes precise diagnosis critical.

Symptoms

The symptoms of a misfire in this truck are often described as nerve-racking and can manifest in several distinct ways. The most common report is a noticeable hesitation or stumble during acceleration, particularly when the engine is under load, such as climbing a hill or merging onto a highway. This isn't a smooth power delivery; it feels messy and inconsistent, as if the engine is momentarily choking.

Owners also describe a general sense of body bloat in performance—the truck feels sluggish and unresponsive, lacking its typical eager power. This can be accompanied by a rough idle where the vehicle shakes more than usual while stopped. In some conditions, particularly high humidity, the problem can worsen. Moisture in the air can find its way into compromised ignition components, exacerbating the misfire and making the issue more pronounced on damp mornings or during rainy weather.

The overall experience is one of diminished drivability and anxiety. What should be a dependable and robust powertrain begins to feel fragile. This is especially troubling for owners who chose this model for its reputation. One owner, in a discussion about reliability concerns, argued the point passionately: "We argued that a premium price was paid... because we were buying a truck with Toyota’s reputation for reliability" (source).

Most Likely Cause

The most likely cause of a misfire in the 2025 4Runner, based on direct owner reports and part mentions, is failing or damaged spark plug boots. These boots are critical components of the ignition system. They are long, flexible rubber tubes that fit over the top of the spark plug and connect to the ignition coil. Their primary job is to insulate the high-voltage current (often exceeding 40,000 volts) traveling from the coil to the spark plug, ensuring it arcs precisely at the plug gap inside the cylinder and nowhere else.

When these boots age, crack, or become damaged, they lose their insulating properties. This allows the high-voltage spark to "leak" or arc to the nearest metal ground—typically the engine's cylinder head—instead of jumping the spark plug gap. This is called a "spark leak" or "flashover." When this happens, the air/fuel mixture in that cylinder does not ignite, resulting in a misfire. The compromised boot can also allow moisture and contaminants to reach the spark plug terminal, causing corrosion and further disrupting the electrical connection. This failure directly leads to the hesitation, rough running, and power loss owners experience.

How to Diagnose

Diagnosing a boot-related misfire requires a systematic approach to isolate the faulty cylinder. You will need a basic OBD2 scanner capable of reading manufacturer-specific codes. A simple code reader that only shows generic P0300 (random misfire) codes is not sufficient; you need one that can display specific cylinder misfire codes like P0301 (Cylinder 1), P0302, etc.

First, connect your scanner to the OBD2 port under the dashboard on the driver's side. Turn the ignition to the "ON" position (engine off) and read any stored codes. A specific cylinder code is your best starting point. If you only have a generic P0300 code, the process becomes more hands-on.

With the engine cold, start the truck and let it idle. Carefully listen and feel for the rough running. Using a mechanic's stethoscope or even a long screwdriver (handle to your ear, tip carefully placed on each ignition coil), listen for a consistent ticking sound from each coil. A coil with a leaking boot may produce a slightly different, erratic sound or even a visible snapping spark in a dark environment. Extreme caution is needed here due to high voltage.

A more definitive test is the "coil swap" method. If you have a code for Cylinder 3, for example, swap the ignition coil and boot assembly from Cylinder 3 with the assembly from Cylinder 1. Clear the codes with your scanner, run the engine, and re-scan. If the misfire code moves to Cylinder 1 (P0301), you have confirmed the coil/boot assembly is faulty. If the code stays at Cylinder 3 (P0303), the problem is likely the spark plug itself or an issue within that specific cylinder (like low compression).

Step-by-Step Fix

Replacing a faulty spark plug boot is a straightforward DIY job that can resolve the misfire. Always work on a cold engine.

Step 1: Safety and Preparation. Disconnect the negative terminal of the battery to prevent any accidental electrical shorts. Identify the misfiring cylinder based on your diagnostic code.

Step 2: Access the Ignition Coil. Remove any engine cover or plastic shrouds that block access to the ignition coils on the top of the engine's valve covers. You may need a basic set of sockets and a trim removal tool.

Step 3: Disconnect the Electrical Connector. Each coil has a small electrical connector. Press down on the locking tab and pull the connector straight off. Do not pull by the wires.

Step 4: Remove the Ignition Coil. The coil is held in place by a single 10mm bolt (size may vary; confirm visually). Remove this bolt and then firmly but gently pull the entire coil and boot assembly straight up and out of the spark plug well. It may require a slight rocking motion but avoid excessive force.

Step 5: Inspect and Replace the Boot. Once the coil is out, inspect the rubber boot for any visible cracks, tears, carbon tracking (thin black lines), or a hardened, brittle texture. The boot is often a separate part that can be pulled off the bottom of the metal coil. Install the new boot by pressing it firmly onto the coil until it seats completely. Apply a small, thin coat of dielectric grease inside the new boot to aid future removal and improve insulation.

Step 6: Install the New Spark Plug (Recommended). While the coil is out, it is highly advisable to replace the spark plug. Use a spark plug socket with a rubber insert and a long extension to remove the old plug. Gap the new plug to the factory specification (check your owner's manual) before carefully threading it in by hand to avoid cross-threading. Tighten to the proper torque specification.

Step 7: Reinstall the Coil Assembly. Carefully guide the coil and new boot down into the spark plug well, ensuring it sits squarely over the spark plug. Push down firmly until it is fully seated. Reinstall and tighten the 10mm hold-down bolt. Reconnect the electrical connector until it clicks.

Step 8: Repeat and Reassemble. Repeat the process for any other coils you are servicing. Reinstall any engine covers and reconnect the battery negative terminal.

Step 9: Clear Codes and Test Start. Use your OBD2 scanner to clear all diagnostic trouble codes. Start the engine. It may run roughly for a few seconds as the computer relearns, but it should smooth out. Take the truck for a test drive, paying close attention to acceleration under load to confirm the misfire is resolved.

As one owner shared after receiving help from the community, "Y'all cant imagine the year I just had, so some unsolicited kindness from an internet stranger is greatly appreciated" (source). Fixing your own truck can bring that same sense of relief and accomplishment.

Parts and Tools Needed

• Parts:
  • Ignition Coil Boot Kit (Set of 4 or 6, depending on your engine). Toyota part numbers can vary; a common aftermarket equivalent is Denso 671-8234.
  • Spark Plugs (Full set is recommended). For the 2025 4Runner, likely Denso FXE22HR11 or equivalent Iridium plug. Confirm exact part number via VIN.
  • Dielectric Grease (Small tube).
• Tools:
  • OBD2 Scanner (capable of reading specific cylinder misfire codes)
  • Basic Socket Set (including 10mm socket for coil bolt)
  • Spark Plug Socket (with magnetic or rubber insert)
  • Long Extension (3-inch or 6-inch)
  • Torque Wrench (for accurate spark plug installation)
  • Trim Removal Tool (optional, for engine covers)
  • Mechanic's Stethoscope (optional, for diagnosis)

Real Owner Costs

The cost to fix a misfire caused by a boot issue varies dramatically between DIY and professional service.

DIY Cost Example: A full set of high-quality aftermarket ignition coil boots can cost between $30 and $60. A set of OEM-equivalent iridium spark plugs adds another $60 to $100. If you need to purchase a basic OBD2 scanner, add $40. Your total investment in parts and a new tool would be $130 to $200, and your labor is free. This is the most economical path.

Professional Repair Cost Example: At a dealership or independent shop, you are paying for parts at retail markup and labor. Diagnosis typically incurs a 1-hour labor charge ($150-$200). Replacing all spark plugs and boots/coils on a V6 engine can be 2-3 hours of labor. Total costs can range widely:

• Dealer (Spark Plugs & Boots/Coils): $600 - $1,200+
• Independent Shop (Spark Plugs & Boots): $400 - $800

One owner, frustrated with recurring issues on a newer model Toyota, expressed a common sentiment: "In my case, when the engine recall was issued I went to dealership and said I was over the recalls and wanted a trade in so I could by something used" (source). A simple DIY fix can often prevent such drastic and costly decisions.

Prevention

Preventing future misfire issues revolves around proactive maintenance and awareness. Adhere strictly to the factory-recommended spark plug replacement interval specified in your 2025 4Runner's maintenance schedule. Replacing plugs before they are completely worn out reduces the stress on the ignition coils and boots.

During every major service (like when replacing spark plugs or air filters), perform a visual inspection of the ignition coil boots. Look for any cracking, hardening, or carbon tracking. Applying a small amount of fresh dielectric grease to the inside of the boot during reassembly helps maintain the seal and insulation.

Avoid washing the engine bay with high-pressure water, as forcing moisture into the spark plug wells can accelerate boot degradation and cause immediate misfires. Finally, address any minor drivability concerns immediately. A small hesitation today could be a full misfire tomorrow; early diagnosis is always cheaper and easier.

What Owners Say

Real experiences from TOYOTA owners:

Owner Experiences

"I talked to a Toyota rep during an arbitration hearing (I wanted a buyback after 200+ days without Tundra). We argued that a premium price was paid for 23 Tundra because we were buying a truck with Toyota’s reputation for reliability." — ExploreHomerAK (source)

"They straight up said these new gens are not the old ones, they are offering a more technically complex and environmentally friendly product line. In my case, when the engine recall was issued I went to dealership and said I was over the recalls and wanted a trade in so I could by something used." — ExploreHomerAK (source)

"Just wanted to thank the reddit bro who sent me a replacement taillight for my 4runner free of charge. Y'all cant imagine the year I just had, so some unsolicited kindness from an internet stranger is greatly appreciated." — MickStash (source)

Lessons Learned

⚠️ "Half way their the whole dash lit up. Check engine light, stability control malfunction, emergency break malfunction, traction control disabled and maybe some other warning I didn’t take note of." — Top-Advantage-1248 (source)

⚠️ "Check engine light, stability control malfunction, emergency break malfunction, traction control disabled and maybe some other warning I didn’t take note of." — Top-Advantage-1248 (source)

FAQ

Q: How long does it take to fix a misfire caused by a bad boot? A: For a competent DIYer, diagnosing which cylinder is misfiring takes about 30 minutes. The physical repair—replacing the boots and spark plugs on all cylinders—typically takes 1 to 2 hours for a first-timer, assuming you have the correct tools. A professional mechanic can often complete the full job in under an hour.

Q: Can I drive my 4Runner with a misfire? A: Driving with an active misfire is not recommended. It can cause immediate damage to the catalytic converter due to unburned fuel entering the exhaust system, leading to a repair that costs thousands. It also puts extra stress on the remaining cylinders and can lead to further engine damage. If the check engine light is flashing, this indicates a severe misfire, and you should stop driving immediately.

Q: Is this a common issue on the 2025 4Runner? A: While the 2025 model is new, the failure of spark plug boots is a common wear item on all gasoline engines over time and mileage. It is not a design flaw specific to this model year but a standard maintenance concern. The complexity of newer engines, however, makes proper diagnosis essential. As noted by an owner, the new generations are "more technically complex," which means components must be maintained precisely (source).

Q: DIY vs mechanic - what's recommended for this fix? A: This repair is highly recommended for the DIY owner. The process is largely mechanical, requires no advanced programming, and the risk of causing damage is low if you follow basic safety rules (disconnect battery, work on a cold engine, use a torque wrench). The cost savings are substantial—often $500 or more compared to a shop. The satisfaction of fixing a key drivability issue yourself is significant, echoing the community spirit where owners help each other out.

Q: Will replacing the boots fix a P0300 random misfire code? A: It can, especially if the boots are old and leaking. A P0300 code means the misfire is jumping between cylinders, which is characteristic of a problem affecting multiple cylinders, like bad fuel or a systemic ignition issue. Replacing all the boots and spark plugs is a logical first step in resolving a P0300 code, as it eliminates the most common wear items from the equation.

Q: Do I need to replace the entire ignition coil, or just the boot? A: In most cases where the boot is the diagnosed culprit, replacing just the boot is sufficient and cost-effective. The boot is a separate, replaceable part on most Toyota ignition coils. However, if the coil itself has failed (confirmed by the "swap test"), you will need to replace the entire coil assembly. It's common to replace the boots and spark plugs first, as they are cheaper and wear out sooner than the coils themselves.