1. Ignition System: Even with new plugs, the problem could be elsewhere in the ignition system:
* Worn Ignition Coils: These are responsible for delivering high voltage to the spark plugs. A faulty coil will cause a misfire in one cylinder. This is a very common cause of a 3-cylinder misfire. Testing the coils is crucial.
* Worn Spark Plug Wires (if applicable): Older Cobalts might have distributor-less ignition systems, using individual wires. Cracked or damaged wires can prevent proper spark delivery.
* Ignition Control Module (ICM): This module controls the ignition coils. A faulty ICM can cause misfires in multiple cylinders.
2. Fuel System: While dirty injectors are a possibility, there are other fuel-related issues:
* Clogged Fuel Injector(s): Cleaning might not be enough; a severely clogged injector might need replacement. A fuel injector cleaner treatment in the fuel tank *might* help, but often professional cleaning or replacement is necessary.
* Fuel Pump Issues: A weak or failing fuel pump might not deliver enough fuel to one or more cylinders.
* Fuel Pressure Regulator: This regulates fuel pressure. A faulty regulator could cause inconsistent fuel delivery.
3. Engine Mechanical Issues:
* Compression Issue: A low compression reading in one cylinder could be due to a valve problem (burnt valve, sticking valve), a piston ring problem, or a head gasket leak. A compression test is essential to rule this out.
* Vacuum Leaks: A significant vacuum leak can disrupt engine operation and cause misfires.
4. PCM (Powertrain Control Module - the "computer"): While possible, a PCM fault leading to a single cylinder misfire is less likely than the other issues. A bad PCM would usually cause more widespread problems. However, it's not impossible and should be considered if all other causes are eliminated.
Troubleshooting Steps:
1. Get a Diagnostic Code Scan: A scan tool can retrieve Diagnostic Trouble Codes (DTCs) from the PCM. These codes will provide valuable clues about the problem. Auto parts stores often offer free code scans.
2. Visual Inspection: Carefully check the ignition system components for damage (cracked wires, loose connections).
3. Compression Test: This is crucial to check the mechanical health of the cylinders.
4. Fuel Pressure Test: Test the fuel pressure to ensure the fuel pump is working correctly.
5. Ignition Coil Testing: Most auto parts stores can test your ignition coils.
It's unlikely you can effectively troubleshoot this yourself without some basic tools and mechanical knowledge. The most cost-effective approach is to get a diagnostic code scan and then focus on addressing the specific problem(s) identified by the scan and tests mentioned above. A mechanic specializing in GM vehicles will be able to efficiently diagnose and repair the issue. Attempting extensive repairs without the proper diagnostic steps could lead to wasted time and money.
