P0308 and P0174 Misfire Diagnosis - 2005 Ford F250 with Used Engine Replacement
I've thoroughly inspected all common causes of misfires in my 2005 Ford F250, including spark plug degradation. The original issue was a spark plug that had worked its way out of the cylinder. I replaced both the spark plug and the coil pack (cop), as well as the fuel injector for cylinder #8. Using a noid light, I confirmed spark presence, with signal current readings between 0.05 and 0.09 amps—consistent with other cylinders. Compression testing shows approximately 150 psi, matching the other cylinders on that side. I've also removed the catalytic converters (cats), which were previously hollowed out. When starting the truck, it runs roughly and I can feel a misfire. Under load, it backfires through the exhaust, and when pushed harder, the check engine light flashes and the vehicle becomes unresponsive—requiring extremely gentle throttle inputs to avoid stalling. The engine was replaced in August 2015 with a used engine from Thrifty Auto Sales under warranty. The issue began shortly after the warranty expired in February. I’m now concerned about potential mechanical faults such as valve or camshaft issues in cylinder #8. Since I can't afford to pull the valve cover, what diagnostic tests can I perform without doing so? If I do need to remove the valve cover, what should I specifically look for? This is my only vehicle, and I cannot use my company truck for personal needs—any guidance would be greatly appreciated.
Are there any additional diagnostic codes present? If not, that raises concern. The system typically sets P0420 or B1S2 O2 sensor-related codes when a catalytic converter is missing or ineffective—this should be addressed first. However, note: removing emission devices illegally may violate regulations. With an OBD-II scanner, check key parameters including ECT (engine coolant temperature), MAF (mass air flow), MAP (manifold absolute pressure), fuel trim values, and oxygen sensor voltages. If no scanner is available, perform a fuel pressure test under KOEO (Key On, Engine Off) conditions—monitor for a drop in pressure over 5 minutes. A significant drop may indicate a faulty or stuck-open injector. Clean the MAF sensor thoroughly—ensure no oily air filters are present. Check for vacuum leaks using the carburetor spray method: apply starting fluid near suspected leak points and observe for engine response. Regarding your concern about legal recourse: did the seller provide a catalytic converter or install one at the time of sale? Note: Cylinder #8 is on the driver’s side—confirming this helps with accurate diagnostics. KOEO = Key On, Engine Off.