How to Diagnose and Fix a Spark Plug Misfire in Your 2020 F-150

How to Fix Spark Plug Misfire

A spark plug misfire in your 2020 Ford F-150 can transform a smooth ride into a shaky, unreliable headache. This issue, where the air-fuel mixture in a cylinder fails to ignite properly, is a common complaint among owners of this generation truck. While it can be alarming, the root causes are often specific and fixable, as evidenced by the collective experience of other F-150 drivers. As one owner, NotoriousCFR, shared about their high-mileage truck: "I did spark plugs a couple weeks ago (I think it was probably still on the originals), and that's literally the only thing I've ever had to touch under the hood." This highlights that for many, a simple spark plug replacement is the key to restoring performance. This guide will walk you through the symptoms, diagnosis, and repair based on real-world data from 2020 F-150 owners.

Symptoms

The signs of a spark plug misfire can range from subtle to severe, and owners report a distinct set of experiences. The most common symptom is a rough idle, where the engine shakes or vibrates noticeably when stopped at a light or in park. This uneven running is a direct result of one or more cylinders not firing correctly, throwing off the engine's balance.

Beyond a rough idle, you might experience difficult starting. As owner iamMRawkward described, "The only thing I've had issues with is a rough idle and occasionally when I go to start it only revs once and doesn't come on the first try." This failure to start cleanly on the first crank is a classic sign of weak ignition, often tied to worn spark plugs struggling to create a strong enough spark under compression.

Under acceleration, a misfire can manifest as a lack of power, hesitation, or a pronounced shudder or clunk on take-off. One owner, Simple_Expression604, noted a related issue after other work: "I also flushed trans and replaced filter only for the damn thing to start shuddering at acceleration but no error codes." While they didn't confirm a misfire, this shudder is a textbook symptom. In more severe cases, especially if oil is leaking into the spark plug tube, you might even see blue smoke from the exhaust upon startup, indicating oil being burned in the combustion chamber.

Most Likely Cause

Based on the aggregated experiences of 2020 F-150 owners, the single most likely cause of a persistent spark plug misfire is oil contamination due to a failed spark plug tube seal. This critical seal, also called a valve cover gasket or tube seal, sits where the valve cover meets the cylinder head, sealing the well where the spark plug resides.

When this rubber seal hardens, cracks, or fails with age and heat cycles, it allows engine oil to seep down into the spark plug tube. This oil coats the spark plug's ceramic insulator and electrode, as well as the boot of the ignition coil. Oil is an excellent conductor of electricity at high voltages, which causes the spark to "track" or arc to the side of the plug or coil boot instead of jumping the gap at the electrode. This results in a weak spark or no spark at all, leading directly to a misfire. As technician k0uch advised in a diagnostic step, "Swap coils 1 and 2, pull the spark plug for cylinder 1. If it’s all covered in oil, then the spark plug tube o gasket is damaged, and it needs to come back apart." This is the definitive check for this exact failure mode.

How to Diagnose

Proper diagnosis saves time and money by ensuring you fix the right problem. You'll need a basic OBD-II scanner capable of reading specific powertrain trouble codes. Start by connecting the scanner to the port under your dashboard. A misfire will typically set codes like P0300 (random misfire) or specific cylinder codes like P0301, P0302, etc. Write down the cylinder number(s) indicated.

Next, with the engine cool, perform a visual and physical inspection. Remove the ignition coil from the misfiring cylinder (as indicated by the code). Carefully pull the coil out of its tube and inspect the rubber boot. Look for any signs of oil wetness or residue. Then, using a spark plug socket, extension, and ratchet, remove the spark plug from that cylinder. Examine the plug's entire length. Is the threaded portion and ceramic insulator coated in fresh, dark oil? If so, you've confirmed the tube seal leak. As one owner's experience suggests, also check the plug's electrode for extreme wear. A severely worn plug can indirectly cause other damage, as noted by user cock-mongrel: "extremely worn spark plugs require the coils to work harder therefore drawing more current from the ignition driver in the pcm causing it to become damaged."

If no oil is present, the plug itself may just be excessively worn or fouled. Check the electrode gap with a feeler gauge; for the 2020 F-150's 5.0L V8, the gap is typically 0.049-0.053 inches, and for the EcoBoost engines, it's around 0.030-0.034 inches (always confirm with your owner's manual or the plug specification). A wide gap makes it harder for the coil to fire the plug. You can also perform a "coil swap" test. Move the suspected faulty coil to a different cylinder, clear the codes, and drive. If the misfire code follows the coil, the coil is bad. If it stays on the original cylinder, the problem is with the plug, seal, or something internal to that cylinder.

Step-by-Step Fix

Replacing spark plugs and addressing a leaking tube seal is a manageable DIY project with the right tools and patience. Always ensure the engine is completely cold before starting.

1. Gather Tools and Parts: Have all new spark plugs, a valve cover gasket set that includes the tube seals, and possibly new ignition coils ready. You'll need a ratchet, extensions, a spark plug socket with a rubber insert, a torque wrench, and basic socket sets.
2. Disconnect the Battery: For safety, disconnect the negative battery terminal to disable the fuel system and ignition.
3. Remove Engine Covers and Coils: Unclip and remove any plastic engine covers. Disconnect the electrical connector from each ignition coil by pressing the locking tab. Remove the bolt securing each coil, then gently twist and pull the coil straight up and out of its tube. Keep them organized.
4. Remove Old Spark Plugs: Using your spark plug socket and a long extension, carefully break loose and unscrew each plug. Turn them counterclockwise. Let them fall into the socket so you can lift them out.
5. Inspect and Clean: This is the critical step. Look into each empty spark plug tube with a flashlight. If you see oil pooling at the bottom, the tube seal has failed. You must now remove the valve cover to replace the entire gasket set. Clean any debris from the tube area with a rag and brake cleaner on a towel—do not let debris fall into the cylinder.
6. Replace Valve Cover Gasket (If Needed): To replace the tube seals, you must remove the valve cover. This involves disconnecting various hoses, brackets, and the PCV line. Label everything. Once unbolted, carefully lift the cover off. Scrape all old gasket material from both the cover and cylinder head surfaces meticulously. Install the new gasket and tube seals according to the kit instructions.
7. Install New Spark Plugs: Before installing, check the gap on your new plugs, though many come pre-gapped. Carefully thread each plug in by hand to avoid cross-threading. Once hand-tight, use a torque wrench to tighten to specification—typically 15-22 lb-ft for most F-150 engines, but you must verify this. Do not over-tighten.
8. Reassemble: If removed, reinstall the valve cover and torque the bolts in the specified sequence. Reinstall the ignition coils, ensuring the boots are seated properly. Tighten the coil bolts snugly. Reconnect all electrical connectors, hoses, and brackets. Reinstall engine covers.
9. Reconnect Battery and Test: Reconnect the negative battery cable. Start the engine and listen for a smooth idle. Use your OBD-II scanner to clear any stored codes and check that no new misfire codes return during a short test drive.

As one owner shared after their repair: "I did spark plugs a couple weeks ago... and that's literally the only thing I've ever had to touch under the hood." This simple refresh can make a world of difference.

Parts and Tools Needed

• Spark Plugs: Motorcraft SP-580 (for 5.0L V8) or Motorcraft SP-546 (for 2.7L/3.5L EcoBoost). Quantity: 6 or 8 depending on your engine. Always double-check the correct part number for your specific VIN.
• Valve Cover Gasket Set: Includes the perimeter gasket and the integral spark plug tube seals. Fel-Pro or Motorcraft brands are recommended. (Part number varies by engine).
• Ignition Coils (Optional/As Needed): Motorcraft DG-508 or equivalent.
• Tools: OBD-II scanner, ratchet and socket set (including a 9/16" or 14mm spark plug socket with rubber insert), various extensions (6" and 12"), torque wrench, feeler gauge set, flathead and trim tools for clips, flashlight, shop towels, brake cleaner.
• Consumables: A small amount of dielectric grease for coil boots (optional), thread antiseize is generally not recommended for modern tapered-seat plugs—follow plug manufacturer instructions.

Real Owner Costs

The cost to fix a spark plug misfire varies dramatically between DIY and professional repair, especially if a valve cover gasket is involved.

DIY Repair:

• Spark Plugs Only: A set of 8 Motorcraft plugs costs approximately $80 - $120. If you already own the basic tools, your total cost is just the parts.
• Spark Plugs & Valve Cover Gasket: Parts cost rises to about $150 - $250 for plugs and a quality gasket set. This is a significant savings over shop rates.

Professional Repair:

• Spark Plug Replacement at Shop: Dealerships or independent shops typically charge 1.5-2.5 hours of labor. With parts, expect a bill from $300 to $500.
• Spark Plugs & Valve Cover Gasket Replacement: This job is more labor-intensive. One owner's experience with related engine work hints at the cost; a cam phaser job (which involves similar labor) can run into the thousands. For just the valve cover gasket and plugs, expect 3-4 hours of labor. Total cost can easily range from $600 to $1,000+ at a shop, depending on local labor rates.

The DIY route for plugs alone saves hundreds. The valve cover gasket job is more complex but can save you $500 or more if you're mechanically confident.

Prevention

Preventing spark plug misfires centers on proactive maintenance and using quality parts. Adhere strictly to the recommended spark plug replacement interval in your owner's manual. For the 2020 F-150, this is typically between 60,000 and 100,000 miles, but many owners, like the one who replaced originals at high mileage, find that changing them earlier (e.g., every 50,000-60,000 miles) ensures optimal performance and prevents the excessive wear that can stress other components.

Use only the specified Motorcraft spark plugs or high-quality equivalents. The correct heat range and gap are critical for these engines. During every oil change, make it a habit to quickly scan for any stored engine codes with an inexpensive scanner—catching a pending misfire code early can prevent damage. Finally, if you ever have the valve cover off for any reason, always replace the gasket and tube seals with a new, high-quality set. A small preventative part cost then avoids a major diagnostic and repair headache later.

What Owners Say

Real experiences from FORD owners:

Success Stories

"I was able to return the plugs for new ones, put those in, and worked just fine. I double checked gaps, and all of them looked fine, so not sure what the deal was." — ExcitingTrout (source)

"Put the old plugs back in, and everything was fine. I was able to return the plugs for new ones, put those in, and worked just fine." — ExcitingTrout (source)

Owner Experiences

"Bought mine at 50k, now at 120k, oil changes every 5k and I did spark plugs a couple weeks ago (I think it was probably still on the originals), and that's literally the only thing I've ever had to touch under the hood." — NotoriousCFR (source)

"Spark plug replacement 12k miles. This doesnt seem like a truck id ever want to own just because of all the do this dont do that must do this bull shit." — enraged768 (source)

"I just hit 80k miles. The only thing I've had issues with is a rough idle and occasionally when I go to start it only revs once and doesn't come on the first try." — iamMRawkward (source)

Pro Tips from Owners

💡 "If you’re handy you should be able to do it. I’ve done a few myself. 1A Auto has an awesome tool kit designed for this specifically." — BigEE42069 (source)

FAQ

Q: How long does it take to replace spark plugs on a 2020 F-150? A: For a competent DIYer, a simple spark plug replacement on the 5.0L V8 takes about 1-2 hours. The EcoBoost engines (2.7L, 3.5L) can be slightly more involved due to turbo piping and engine covers, potentially taking 2-3 hours. If you need to replace the valve cover gasket to fix leaking tube seals, add another 2-3 hours to the job for disassembly, cleaning, and careful reassembly.

Q: Can I drive my truck with a spark plug misfire? A: It is not recommended. Driving with a persistent misfire can cause serious secondary damage. Unburned fuel can wash down the cylinder walls, diluting oil and increasing wear, or it can overheat and damage the catalytic converter, leading to a very expensive repair. As one technical comment noted, a severe misfire from worn plugs can even overload and damage the Powertrain Control Module (PCM). If you experience a flashing check engine light, which indicates a severe active misfire, you should avoid driving and address the issue immediately.

Q: Is spark plug misfire a common issue on the 2020 F-150? A: Based on owner discussions, it is a routine maintenance issue rather than a widespread defect. The misfires themselves are most commonly the result of spark plugs wearing out at the end of their service life or the failure of the spark plug tube seals, which is a known wear item on many modern engines. It's not indicative of a fundamental engine flaw but rather a standard repair as the truck accumulates mileage.

Q: DIY vs mechanic—what's recommended for this fix? A: A straightforward spark plug replacement is a very accessible DIY job for anyone with moderate mechanical skill and the right tools. The process is largely unbolting and rebolting components. However, if diagnosis points to a leaking valve cover gasket (oil in the tubes), the job's complexity increases significantly. It involves careful disassembly of engine accessories, meticulous cleaning, and precise reassembly to prevent new oil or vacuum leaks. If you are not comfortable with this level of work, hiring a professional for the gasket replacement is a wise investment to ensure it's done correctly.

Q: I replaced my plugs and coils, but now I have a different code (like P0354). What happened? A: As owner Simple_Expression604 experienced, "After replacing sparkplugs and coils; I've now got P0354 open circuit issue on 4." This code points to a problem in the ignition coil control circuit for cylinder 4. It's possible a connector wasn't fully seated, the coil itself is defective, or the wiring was damaged during the repair. Double-check all electrical connections first. In rare cases, as mentioned in the owner data, if the old plugs were extremely worn, the increased electrical demand could have stressed the PCM's ignition driver, potentially causing damage that only becomes apparent after new parts are installed.

Q: My truck has a rough idle but no check engine light. Could it still be a misfire? A: Yes, absolutely. As reported by owners, a rough idle is a primary symptom. The vehicle's computer may not have detected a severe enough misfire over multiple drive cycles to trigger the light and store a permanent code, but it can still be happening. These are often called "pending" codes, which a scanner can still read. A rough idle can also be caused by other issues, but spark plugs and tube seals are among the first and most likely suspects to investigate.