1999 Yukon Denali PO300 Code: Diagnosis and Solutions After Multiple Repairs
2000 1999 Yukon Denali with 114,000 miles, 5.7L V8 4x4 engine. Recently encountered a persistent PO300 random misfire code after replacing the intake gasket. All components were replaced using AC Delco parts including spark plugs, distributor cap (flat style), rotor, ignition wires, crankshaft position sensor, coil pack, fuel filter, and upgraded to new-style fuel injectors (replacing older poppet-style units). After extensive troubleshooting with assistance from friends who own auto repair shops, we have replaced the following: spark plugs, cap, rotor, wires, crankshaft position sensor, fuel filter, fuel injectors, EGR valve, and mass airflow sensor. The truck still exhibits a PO300 code and stumbles under light throttle. The engine does not idle smoothly and shows poor acceleration during takeoff. A mechanic recently suggested checking valve spring tension as a potential cause. However, no clear root cause has been identified yet. We have verified that the distributor is properly installed, timing was checked and adjusted using a Snap-on scanner, and vacuum lines are intact. Fuel pressure remains stable (no drop after engine shutdown), and initial fuel trim readings were within normal ranges. After replacing injectors and rechecking O2 sensor data, the check engine light returned after 10 miles of driving. Current question: What other diagnostics or repairs should we consider for a persistent PO300 on this high-mileage 1999 Yukon Denali? Any insights from experienced mechanics or owners would be greatly appreciated.
The misfire began after the intake gasket replacement. A vacuum leak should be ruled out as a primary cause—check all hoses and connections for cracks or disconnections.