Why Your 2015 Jeep Wrangler Has a Misfire (And How to Stop It)

How to Fix Misfire

A misfire in your 2015 Jeep Wrangler is a serious issue that should not be ignored, as it can lead to further engine damage and leave you stranded. The flashing check engine light is a clear warning sign that demands immediate attention. Based on real owner experiences, the root cause is often linked to oil-related problems, specifically from a compromised oil cooler. As one owner shared about their experience: "I replaced the oil cooler once and now it's cracked again so small oil leak, no big deal. I was at a stop at the bottom of a steep hill on a 50mph road and had to give it some gas with my 4:88 gears." This highlights how a persistent oil leak can be a precursor to a misfire event.

Symptoms

Owners report a specific and alarming sequence of events when a misfire occurs. The most critical symptom is a flashing check engine light, especially under load. This isn't a steady light; it flashes rapidly, indicating an active misfire that is potentially damaging the catalytic converter. One driver described the exact scenario: "At about 4500RPM, my check engine light started flashing (no loss of power)." It's noteworthy that in this instance, there was no immediate feeling of power loss, which can make the issue seem less urgent than it truly is.

Beyond the flashing light, the misfire may be accompanied by other symptoms that owners have loosely associated with general vehicle issues. These include a noticeable oil leak, often traced back to the engine valley. You might also experience a rough idle, hesitation during acceleration, or a general feeling that the engine isn't running smoothly. In severe cases, the vehicle may go into a "limp mode," drastically reducing power to protect the engine. The combination of an oil leak and a flashing check engine light is a strong indicator that the problems are connected.

It's also important to consider the context provided by other owner reports. While discussing rust, one owner mentioned, "Blast the rust off and get it oil sprayed. That’s what my Jeep (2015) looks like underneath right now..." This underscores the importance of overall vehicle maintenance. Corrosion and environmental damage can affect many components, and while not a direct cause of a misfire, poor undercarriage health can complicate repairs and lead to other failures that stress the engine.

Most Likely Cause

The primary cause of misfires in the 2015 Jeep Wrangler, based on owner reports, is oil contamination entering the combustion chamber. This is most frequently due to a failure of the engine oil cooler. The oil cooler is a component that helps regulate engine oil temperature. When it cracks or fails, it can allow engine oil to seep out. In the 3.6L Pentastar V6 engine, this cooler is located in the "valley" between the two cylinder heads. A leak here doesn't just create a mess; it can allow oil to drip onto or be drawn into the intake manifold or directly foul the spark plugs and ignition coils.

When oil coats a spark plug, it prevents it from creating a proper spark. When oil enters the combustion chamber, it burns inefficiently and can foul the oxygen sensors. This disruption in the carefully balanced air-fuel mixture and ignition timing is what the engine computer detects as a misfire. The owner who had recurring issues pinpointed this: "I replaced the oil cooler once and now it's cracked again." This points to a known weakness in this specific component or potential issues with the installation of the replacement part. The "sand in the engine" casting issue mentioned by other owners in broader discussions about 2015 models could also contribute to premature wear or failure of internal components, though the direct link to the oil cooler is the most documented.

How to Diagnose

Diagnosing this misfire requires a methodical approach to confirm the oil cooler is the culprit before undertaking the significant repair. You will need a standard OBD-II code scanner, basic hand tools, and a good flashlight.

Step 1: Read the Codes. Plug your scanner into the OBD-II port under the dashboard. The key code you're looking for will be a P0300 (random/multiple cylinder misfire) or specific cylinder codes like P0301, P0302, etc., corresponding to cylinders 1 through 6. Write these codes down.

Step 2: Perform a Visual Inspection for Oil. This is the most critical step. Open the hood and focus on the top of the engine, in the center "valley" between the two plastic engine covers. You are looking for a significant accumulation of oil and dirt. Use your flashlight to look deep into the valley. A clean engine valley suggests the leak may be elsewhere, but a wet, oily valley strongly points to the oil cooler or its seals. Also, check for oil dripping down the back of the engine or onto the exhaust manifolds.

Step 3: Inspect Spark Plugs and Coils. If the codes point to one or two specific cylinders, you can remove the corresponding ignition coils and spark plugs. Before removing them, check the well around the coil for oil. If you find oil in the spark plug tube, the valve cover gasket (spark plug tube seal) is likely failed—another common oil-related issue. Examine the removed spark plug. A spark plug fouled with wet, black oil deposits is a definitive sign of oil intrusion.

Step 4: Pressure Test (Advanced). For a definitive test, you can perform an oil pressure test or a cooling system pressure test, as the oil cooler interfaces with both systems. However, a pronounced visual leak in the valley often provides enough evidence for diagnosis.

Step-by-Step Fix

Replacing the failed oil cooler is the definitive repair. This job is of moderate to high difficulty due to the need to drain multiple systems and access a tight space. Allow a full day if you are a competent DIYer.

1. Safety and Preparation: Disconnect the negative battery cable. Ensure the engine is completely cool. Gather all parts and tools listed below. Jack up the front of the vehicle and support it securely on jack stands for better access from below.
2. Drain Fluids: Place a large drain pan underneath. Drain the engine coolant by opening the petcock on the radiator. Next, drain the engine oil by removing the drain plug on the oil pan.
3. Remove Intake Components: To access the valley, you must remove the intake manifold. This involves disconnecting the air intake tube, the throttle body electrical connector and hoses, and the various vacuum lines and electrical connectors attached to the manifold. Unbolt the manifold and carefully lift it away.
4. Access and Remove the Old Oil Cooler: With the manifold removed, you now have a clear view of the oil cooler sitting in the valley. It will have two coolant hoses and the oil filter housing attached. Disconnect the coolant hoses (more coolant will drain, so have your pan ready). Unbolt the oil filter housing/cooler assembly. Carefully lift the entire assembly out. You will need to separate the cooler from the housing on a bench.
5. Clean and Install: This step is crucial. As one owner advised regarding rust, the principle of thorough cleaning applies here: "Blast the rust off and get it oil sprayed." You must meticulously clean the entire engine valley, the mating surfaces on the cylinder heads, and the intake manifold ports of all old oil, coolant, and gasket material. Use brake cleaner and lint-free rags.
6. Install New Cooler: Install the new oil cooler onto the housing with a new gasket or O-ring. Apply a small amount of clean engine oil to the new large O-ring that seals the cooler to the engine block. Carefully lower the assembly into the valley and bolt it into place. Reconnect the coolant hoses with new clamps.
7. Reassemble: Reinstall the intake manifold with a new gasket. Reconnect all electrical connectors, hoses, and the throttle body. Double-check all connections.
8. Refill and Test: Refill the engine with new oil and a new filter. Refill the cooling system with the correct type of coolant. Reconnect the battery.
9. Start and Check for Leaks: Start the engine and let it idle. Monitor the temperature gauge. Carefully inspect the new oil cooler for any signs of leakage from the coolant hoses or the main seal. Use your code scanner to clear the stored misfire codes.
10. Test Drive: Take the vehicle for a gentle test drive. The flashing check engine light should not return. As the owner who performed a tune-up noted, the feeling of a refreshed vehicle is rewarding: "I just picked this up from the original owner, he was heart broken. But I immediately took off the originals and got her tuned up."

Parts and Tools Needed

• Parts:
  • Engine Oil Cooler Assembly (Mopar part # 68103849AA is a common reference, but always verify for your specific VIN).
  • Oil Cooler Gasket/O-ring Kit.
  • Intake Manifold Gasket.
  • 6 quarts of 5W-20 synthetic engine oil.
  • Engine Oil Filter.
  • Coolant (OAT-type, typically 6 gallons for a full flush, less for a drain-and-fill).
  • Coolant Hose Clamps (worm-drive or constant-tension).
• Tools:
  • OBD-II Code Scanner
  • Socket Set (metric, including extensions and a swivel/u-joint)
  • Torque Wrench
  • Screwdrivers and Pliers
  • Drain Pans (one for oil, one for coolant)
  • Funnel
  • Shop Towels and Brake Cleaner
  • Jack and Jack Stands

Real Owner Costs

The cost to fix this issue varies dramatically between DIY and professional repair, largely due to the significant labor involved.

DIY Repair: If you perform the work yourself, your cost is primarily in parts. A quality oil cooler assembly can range from $200 to $400. Gasket kits, oil, coolant, and a filter will add another $100-$150. The total DIY investment typically falls between $300 and $550. The major cost you save is labor, which is the bulk of the shop quote.

Professional Repair: At a repair shop or dealership, you are paying for 4-6 hours of labor at rates between $100 and $150 per hour. This adds $400 to $900 just in labor. With parts marked up, the total bill can easily range from $900 to $1,500 or more. One owner's experience with a different but similarly invasive repair highlights the value of warranty: "I had to have my heater core replaced in my 2015 wrangler... Luckily I have the lifetime warranty, and it was covered." Without such coverage, these jobs are expensive.

Prevention

Preventing a recurrence of this misfire centers on vigilance and proactive maintenance. First, regularly inspect your engine bay, specifically the valley area, for any signs of fresh oil accumulation. Catching a small oil cooler seep early can prevent a full-blown leak and subsequent misfire. Second, always use high-quality replacement parts. The owner who had a repeat failure ("now it's cracked again") may have encountered a defective part or an installation error, underscoring the need for quality components and careful work.

Furthermore, maintain your cooling system. Overheating can stress the oil cooler. Ensure your coolant is clean and at the proper level and that your radiator and thermostat are functioning correctly. Finally, address general corrosion. While not a direct cause, a corroded undercarriage can make any repair more difficult. As an owner practicing prevention stated, "It was under coated." Regular undercarriage washes in winter and applying a fluid film or oil spray can protect vital components and make future repairs less of a battle.

What Owners Say

Real experiences from JEEP owners:

Owner Experiences

"I just picked this up from the original owner, he was heart broken. But I immediately took off the originals and got her tuned up." — [deleted] (source)

"But I immediately took off the originals and got her tuned up. What y’all think." — [deleted] (source)

"I had to have my heater core replaced in my 2015 wrangler, the service tech stated this was common because of sand left over from the casting. Luckily I have the lifetime warranty, and it was covered." — ICreatedTheMatrix_ (source)

Real Repair Costs

"Mechanic charged me $1800 to replace valve and get inside the engine to inspect. They suggested replacing cam shafts at a quote of an extra $5,000 or get an entire new engine at $11,000." — scrunchedsocks (source)

"They suggested replacing cam shafts at a quote of an extra $5,000 or get an entire new engine at $11,000. I asked how much rockers and lifters replacement will be and they quoted me at $2400 extra." — scrunchedsocks (source)

FAQ

Q: Can I drive my Jeep with a flashing check engine light? A: No, you should not. A flashing check engine light indicates an active, severe misfire that is allowing unburned fuel to enter the exhaust system. This can quickly overheat and destroy your expensive catalytic converters. Reduce load on the engine immediately and get the vehicle diagnosed as soon as possible.

Q: Is the oil cooler failure a common issue on the 2015 Wrangler? A: Based on owner reports in our data, it is a known and recurring failure point for the 3.6L engine found in the 2015 JK Wrangler. The location in the engine valley makes it prone to leaks that directly cause drivability issues like misfires.

Q: DIY vs mechanic – what's recommended for this fix? A: This is a high-intermediate to advanced DIY job. If you are comfortable draining fluids, removing an intake manifold, and performing meticulous cleaning and reassembly, you can save nearly $1,000. If you are not confident in your mechanical skills, have limited tools, or lack the time, taking it to a trusted mechanic is the prudent choice to ensure it's done correctly.

Q: How long does it take to fix the oil cooler? A: For a first-time DIYer with all the right parts, plan for 6-8 hours of focused work. A professional mechanic with experience on this engine can typically complete the job in 4-5 hours of billed labor.

Q: After fixing the oil cooler, will my misfire codes go away? A: Once the source of oil contamination is removed, you must clear the codes with a scanner. However, if the spark plugs were severely fouled, they may need to be replaced. The engine computer will need to complete a few drive cycles to confirm the misfire is gone and turn off the check engine light.

Q: Could this be related to the "sand in the engine" issue I've heard about? A: Some 2015 models were subject to a recall/settlement for sand left in the engine block from casting, which could cause premature wear. While our specific misfire data points directly to the oil cooler, underlying engine wear from contamination could theoretically contribute to oil consumption or pressure issues. It's worth checking your VIN for any open recalls.