2004 Jeep Liberty 3.7L Engine Code P2311 - Cylinder 4 Misfire After Coil and Plug Replacement

Are you in North America? The 'EC' prefix suggests an OBD-II system. I assume you're seeing code P2311—do you have any other diagnostic codes? What brand of spark plugs did you use? Some models are known to cause issues. Using a noid light or test light, probe each side of the coil connector while cranking: does it flash, stay steady, or show no light at all?

Thanks for responding! I'm in South Carolina. No other codes are showing on my scanner. The old plugs were NGK; the new ones are AC Delco. I now realize that might have been a mistake—NGK is likely more compatible. I don’t currently have a noid light, but I plan to get one soon. I’ve measured 13V at the center plug with a multimeter and haven't tested the test light yet—would that be from the plug terminal to ground?

Yes—by 'test light from plug,' you mean connecting one end of the test light to the spark plug terminal and grounding it on a metal surface, correct?

Just use a test light to complete the circuit as shown in diagrams. Crank the engine—what happens? 'No other codes' is concerning—how are you checking for diagnostic trouble codes? Why did you replace plugs and coils? Was it because the vehicle started misfiring, or were you addressing an existing code? Also, where does the 'EC' prefix come from?

EC stands for Engine Control. I used a Bluetooth OBD-II scanner with the Torque Pro app. I replaced both plugs and coils after noticing the engine started misfiring—running fine when going to work, but then exhibiting rough idle and surging around 2000 RPMs.

I'll try using the test light shortly.

AC Delco plugs are not ideal for this engine—check the radiator support area for emission label. The correct plug is likely Mopar #SPZFR6F11G. Also, verify that your new ignition coils have proper primary resistance—should be between 0.6 and 0.9 ohms.

No light at all when testing with the test light during cranking. Both new coils measured 1.1 ohms of resistance—above the acceptable range. Neither wire shows a ground fault, and I didn’t see any visible damage behind the engine block. I was planning to check continuity on the PCU plug—do you know which specific connector that would be?

Ignition coils from aftermarket brands often don't meet specifications—I suspect these weren’t original Jeep parts, and they may not work properly. Don’t panic yet—they might still function in the short term. Did you use a proper OBD-II scanner? With the plug connected to the coil, perform a KOEO (Key On, Engine Off) test: measure voltage on terminal 2 and terminal 1 of the harness. Report your readings. Often, ignition connectors degrade over time—check for corrosion or loose connections. Using a proper scanner should reveal more detailed codes.

Here's a helpful manual: http://www.mediafire.com/file/xd09c9dvv...Manual.pdf

Thanks for the help! I reseated all PCM connections and the misfire has stopped. I’ll clean up the connectors with a better contact cleaner and monitor performance going forward.

'Deoxit' contact cleaner is my go-to—available at Home Depot, Lowe’s, or Walmart.

Thanks for the support!