Repair Process 

1. A road test confirmed the fault exists. ECU scan retrieved codes P0304 (Cylinder 4 Misfire) and P0300 (Random/Multiple Cylinder Misfire). Primary and secondary windings of ignition coils measured normal. Spark plugs and fuel injectors were swapped between cylinders, yet the fault remained after road test.

2.The owner stated the fault occurs more frequently in the morning. Live data was monitored under cold start conditions; coolant temperature, intake manifold pressure, intake air temperature, boost pressure and oxygen sensor readings all appeared normal. Suspecting poor fuel quality, the fuel was replaced and fuel tank cleaned, but the fault persisted on test drive.

3. Suspected CKP monitoring malfunction. Inspection showed normal CKP and CMP signal waveforms (Figure 1).

1. 
   

   

   
   

   4. Suspected carbon buildup caused intermittent misfire. Spark plugs and cylinder carbon deposits were inspected. Endoscope inspection revealed water-like stains on the top of the 4th cylinder piston, which appeared to be coolant residue. After cleaning the stains inside the cylinder, a pressure test was performed on the cooling system. Further endoscope check found accumulated antifreeze on the intake side of the 4th cylinder piston crown.

   
   

   

   
   

   5.Potential coolant leakage points were identified as intake manifold, cylinder head gasket, sand holes on cylinder head and cylinder block. After removing the intake manifold, a cracked sealing ring was found at the intake port of cylinder 4. Water droplets were visible at the cylinder head intake port and intake valve. It was concluded that the cracked intake port sealing ring of cylinder 4 triggered the engine warning lamp (Figure 3 & 4).

   
   

   

   
   

   6.Replace the intake port sealing ring and clean all water stains. Perform a new cooling system pressure test; no water droplets were found on the piston crown. The fault was eliminated after road test.

   
   

   

   
   

   Solution

   Replace the intake port sealing ring

   
   

   Case Summary

   1.Do not replace parts solely based on fault codes by experience. Troubleshoot step by step combined with the characteristics of fault symptoms.

   2. An endoscope can quickly locate the fault point during maintenance work.

   3.When troubleshooting such faults, routine inspections must not be overlooked, including coolant, engine oil, battery, cylinder compression pressure and other items.

   4.The 18K4G engine cylinder block has water passages on the intake port side of cylinder 1 and cylinder 4. Careful observation of the structure during repair will facilitate future troubleshooting work.