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2018 XT5 Rough Idle and CEL - Misfire Codes?

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Mike T. · rep 0 · May 24, 1:46 PM · 1 replies · solved · Question

Vehicle: 2018 Cadillac XT5 Luxury

Hey everyone,

My 2018 Cadillac XT5 Luxury, with about 75,000 miles, has started acting up lately. It developed a really rough idle, especially when I'm stopped at a light. The engine feels like it's shaking more than usual.

Yesterday, the Check Engine Light came on. I haven't had a chance to get the codes read yet, but based on how it's running, I'm suspecting a misfire. The car also feels a bit sluggish under acceleration.

I've kept up with oil changes, but I'm wondering if this could be spark plugs or something similar. Any thoughts on what might be causing this and what I should check first? Is this a common issue for the 3.6L V6 in these XT5s around this mileage?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 24, 1:46 PM

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Hello Chris,

It sounds like you're correctly diagnosing a potential misfire issue with your 2018 Cadillac XT5 Luxury. A rough idle and a Check Engine Light (CEL) are classic symptoms. Given your mileage, several maintenance-related items could be the culprit for the 3.6L V6 engine.

First Steps & Diagnostics:

  1. Get the OBD-II Codes Read: This is the most crucial first step. An auto parts store can often do this for free, or you can use your own scanner. Look for codes in the P030x series (e.g., P0301, P0302, etc.), where 'x' indicates the cylinder experiencing the misfire. This will help pinpoint the problem cylinder.

Common Causes for Rough Idle/Misfire at Your Mileage:

  • Worn Spark Plugs: Cadillac recommends spark plug replacement typically between 60,000 and 100,000 miles, depending on the specific engine and driving conditions. Worn plugs are a very common cause of misfires and rough idle.
  • Failing Ignition Coils: Each spark plug has an individual ignition coil. These can fail over time, leading to a loss of spark in a cylinder. If you get a P030x code, you can often swap the coil with an adjacent cylinder to see if the misfire code follows the coil, indicating a faulty unit.
  • Clogged or Failing Fuel Injectors: While less common than plugs or coils, a fuel injector that isn't delivering the correct amount of fuel can cause a misfire. A fuel system cleaner might help if it's a minor clog, but a failing injector will require replacement.
  • Vacuum Leaks: Unmetered air entering the engine through a cracked hose, gasket, or a failing PCV valve can disrupt the air-fuel mixture, leading to a rough idle and potential misfires.
  • Mass Air Flow (MAF) Sensor: A dirty or failing MAF sensor can send incorrect air intake data to the engine control module, causing incorrect fuel delivery and a rough idle.

DIY Checks You Can Perform:

  • Visual Inspection: After getting the codes, check the wiring harnesses to the ignition coils and spark plugs for any visible damage or loose connections. Also, inspect vacuum lines for cracks or disconnections.
  • Spark Plug Replacement: If your XT5 is due for plugs, replacing them is a good maintenance item. Use high-quality iridium or platinum plugs recommended by Cadillac.

When to See a Shop:

  • Flashing Check Engine Light: If the CEL is flashing, it indicates a severe misfire that could damage your catalytic converter. You should minimize driving and get it inspected immediately.
  • Persistent Issues: If you've replaced spark plugs and coils on the affected cylinder(s) and the problem persists, or if the codes point to fuel system issues, catalytic converter problems (e.g., P0420/P0430), or complex sensor failures, it's best to consult a qualified mechanic. They have advanced diagnostic tools to pinpoint more elusive problems.
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