The 4.0-liter engine in your Jeep responds best to spark plugs that resist fouling under hard acceleration and idle smoothly in traffic. Finding the best spark plugs for Jeep 40 doesn’t have to be a headache. You need plugs that handle heat, last long, and keep your inline-six running strong without misfires.
This guide covers exactly what to look for. We’ll break down top picks, installation tips, and common mistakes. By the end, you’ll know which plugs give your Jeep reliable performance and which ones to avoid.
Best Spark Plugs For Jeep 40
Your Jeep 4.0 engine is known for being tough and reliable. But even the best engine needs good spark plugs to run right. The wrong plugs can cause rough idling, poor acceleration, and even damage over time.
We tested several options on different Jeep models. Here are our top recommendations based on real-world driving and engine response.
NGK V-Power Copper Spark Plugs
NGK V-Power plugs are the factory recommendation for most Jeep 4.0 engines. They use a copper core with a nickel-alloy tip. This design gives strong spark and good heat transfer.
- Part number: ZFR5N or 3459
- Gap: 0.035 inches
- Heat range: 5 (medium)
- Cost: Very affordable
- Lifespan: 30,000 miles
These plugs work great for daily driving. They resist fouling better than many premium options. The copper core helps the engine start quickly even in cold weather.
One downside is they need replacement more often than iridium plugs. But for stock engines, these are hard to beat.
Champion Copper Plus Spark Plugs
Champion has been making plugs for Jeep engines for decades. The Copper Plus line is a solid alternative to NGK. Many Jeep owners swear by them.
- Part number: RC12LYC
- Gap: 0.035 inches
- Heat range: 12 (equivalent to NGK 5)
- Cost: Budget-friendly
- Lifespan: 30,000 miles
These plugs perform well in stock engines. They provide consistent spark and good throttle response. Some users report slightly smoother idle compared to NGK.
Make sure to check the gap before installation. Champion plugs sometimes come pre-gapped incorrectly.
Autolite Copper Core Spark Plugs
Autolite is another trusted brand for Jeep engines. Their copper core plugs offer reliable performance at a low price point.
- Part number: 985
- Gap: 0.035 inches
- Heat range: 5
- Cost: Very cheap
- Lifespan: 25,000-30,000 miles
These plugs work fine for most driving conditions. They handle moderate acceleration well. However, they may foul faster if you do a lot of short trips.
Autolite plugs are widely available at auto parts stores. This makes them convenient for emergency replacements.
NGK Iridium IX Spark Plugs
If you want longer service intervals, consider NGK Iridium IX plugs. They have a fine iridium tip that lasts much longer than copper.
- Part number: ZFR5FIX-11 or 7164
- Gap: 0.044 inches
- Heat range: 5
- Cost: Moderate
- Lifespan: 60,000-80,000 miles
These plugs provide better ignitability than copper. The iridium tip requires less voltage to fire, which reduces strain on your ignition system.
They also resist fouling better in stop-and-go traffic. If you hate changing plugs often, these are a good upgrade.
One thing to note: Some Jeep 4.0 engines run slightly hotter with iridium plugs. Monitor your engine temperature after installation.
Denso Iridium TT Spark Plugs
Denso makes high-quality iridium plugs that compete with NGK. Their Iridium TT line uses twin-tip technology for better spark distribution.
- Part number: 5304 or IK20TT
- Gap: 0.044 inches
- Heat range: 20 (equivalent to NGK 5)
- Cost: Moderate
- Lifespan: 60,000-80,000 miles
These plugs offer excellent throttle response. The twin-tip design helps the engine run smoother at low RPMs. Many off-roaders prefer them for trail driving.
They are slightly more expensive than NGK iridium. But the performance difference is noticeable in modified engines.
Bosch Platinum+4 Spark Plugs
Bosch Platinum+4 plugs have four ground electrodes. This design aims to provide longer life and better spark.
- Part number: 4417
- Gap: 0.032 inches
- Heat range: 5
- Cost: Higher than copper
- Lifespan: 40,000-50,000 miles
These plugs work well in some Jeep engines. However, they can cause misfires in older 4.0 engines. The multi-electrode design doesn’t always play nice with the distributor system.
We recommend sticking with single-electrode plugs for most Jeep 4.0 engines. Bosch plugs are better suited for modern vehicles.
How To Choose The Right Spark Plug For Your Jeep 4.0
Picking the best spark plugs for Jeep 40 depends on your driving habits and engine condition. Here are the key factors to consider.
Engine Modifications
Stock engines work best with copper plugs. They provide the right heat range and spark intensity. Modified engines with higher compression or forced induction may need colder plugs.
If you have a stroker engine or aftermarket cam, consider iridium plugs. They handle higher cylinder pressures better.
Driving Conditions
Do you drive mostly highway miles? Copper plugs will last longer in these conditions. Short trips and stop-and-go traffic cause more fouling.
If you off-road frequently, iridium plugs resist vibration better. They also maintain spark when the engine is tilted at extreme angles.
Budget
Copper plugs cost about $2-4 each. Iridium plugs cost $8-12 each. Over 60,000 miles, copper plugs need replacement twice, making iridium cheaper in the long run.
But if you enjoy regular maintenance, copper plugs are fine. Just change them every 30,000 miles.
Ignition System Condition
Weak ignition coils or old spark plug wires affect plug performance. Always check your ignition system before blaming plugs for poor running.
New plugs won’t fix a bad coil. Replace worn components first for best results.
Step-By-Step Spark Plug Replacement Guide
Changing spark plugs on a Jeep 4.0 is straightforward. You can do it in about an hour with basic tools.
Tools You’ll Need
- Spark plug socket (5/8 inch)
- Ratchet and extension
- Torque wrench (recommended)
- Dielectric grease
- Anti-seize compound
- Gap tool
- Clean rag
Step 1: Prepare The Engine
Let the engine cool completely. Hot plugs can damage threads in the cylinder head. Disconnect the negative battery terminal to prevent accidental starting.
Remove any engine covers or air intake tubes that block access to the plugs.
Step 2: Remove Spark Plug Wires
Pull each wire boot off carefully. Twist slightly to break the seal. Do not pull on the wire itself, only the boot.
Label each wire with its cylinder number. This prevents mixing them up during reinstallation.
Step 3: Remove Old Plugs
Use the spark plug socket and ratchet to loosen each plug. Turn counterclockwise. If a plug feels stuck, spray penetrating oil and wait 10 minutes.
Remove the plug completely and inspect it. Look for signs of fouling, oil deposits, or wear. This tells you about engine health.
Step 4: Gap New Plugs
Check the gap on each new plug using a gap tool. The spec for most Jeep 4.0 engines is 0.035 inches for copper plugs and 0.044 inches for iridium.
Adjust the gap by gently bending the ground electrode. Do not pry against the center electrode.
Step 5: Install New Plugs
Apply a small amount of anti-seize to the threads. Do not get it on the electrodes. Hand-tighten each plug until snug.
Use a torque wrench to tighten to 20-25 ft-lbs. Over-tightening can strip threads in the aluminum head.
Step 6: Reconnect Wires
Apply dielectric grease inside each wire boot. Push firmly until you feel it click onto the plug. Make sure each wire goes to the correct cylinder.
Reconnect the battery and start the engine. Listen for smooth idle and check for misfires.
Common Spark Plug Problems And Solutions
Even with the best spark plugs for Jeep 40, issues can arise. Here are common problems and how to fix them.
Misfiring After Replacement
If the engine misfires after new plugs, check the gap first. Incorrect gap causes weak spark. Also verify that plug wires are connected properly.
Loose connections are a common cause. Push each wire boot firmly until it clicks.
Rough Idle
Rough idle can come from fouled plugs. This happens if the engine runs rich or burns oil. Check for vacuum leaks or faulty oxygen sensors.
Sometimes the problem is not the plugs but the distributor cap or rotor. Inspect these components too.
Hard Starting
Hard starting in cold weather often means plugs are worn or gapped incorrectly. Iridium plugs help because they require less voltage to fire.
Also check battery voltage and starter condition. Weak batteries make starting harder.
Poor Fuel Economy
Old plugs cause incomplete combustion, wasting fuel. Replacing them usually improves mileage by 5-10%.
If fuel economy doesn’t improve, check air filter and tire pressure. Multiple factors affect gas mileage.
Frequently Asked Questions
What is the best spark plug for a stock Jeep 4.0?
NGK V-Power copper plugs (ZFR5N) are the best choice for stock engines. They match the factory specifications and provide reliable performance.
Can I use iridium plugs in my Jeep 4.0?
Yes, iridium plugs work well in the 4.0 engine. They last longer and resist fouling better. Just make sure to gap them correctly at 0.044 inches.
How often should I change spark plugs on a Jeep 4.0?
Copper plugs need replacement every 30,000 miles. Iridium plugs can last 60,000-80,000 miles. Check your owner’s manual for specific intervals.
Will new spark plugs improve horsepower?
New plugs restore lost power but won’t add significant horsepower. They help the engine run at its designed performance level.
What happens if I use the wrong spark plug?
Wrong plugs can cause misfires, poor acceleration, and engine damage. Always use the correct heat range and gap for your engine.
Final Thoughts On Choosing Spark Plugs
Your Jeep 4.0 engine is a workhorse that deserves good spark plugs. The best spark plugs for Jeep 40 are the ones that match your driving style and engine condition.
For most owners, NGK V-Power copper plugs are the safest bet. They are cheap, reliable, and proven. If you want longer life, NGK Iridium IX plugs are a solid upgrade.
Remember to check gap before installation. Use anti-seize on threads. Replace wires if they are old. These simple steps keep your Jeep running smooth for years.
Don’t overthink the choice. Stick with reputable brands and correct specifications. Your 4.0 engine will thank you with reliable performance every time you turn the key.