1997 Ford F-150 4.6L Engine P0308 Missfire: Diagnosis & Fixes After Rebuild (Non-COP Ignition)

Since you've already replaced the spark plug, ignition wire, and coil pack for cylinder 8, and the misfire persists specifically on that cylinder, the issue is likely localized to cylinder 8. Check for vacuum leaks in the intake manifold near cylinder 8, inspect the fuel injector harness (look for loose connectors, chafed wires), or verify if any components are unique to cylinder 8. Please update with your findings.

When you rebuilt the engine, were the valves and heads fully overhauled? What was the original failure? Could there be an exhaust restriction on the bank containing cylinder 8?

The engine was rebuilt due to head gasket failure. Heads were machined, valve guides replaced, and valves and seats were ground. I noticed that the #8 injector connector was slightly loose—this will be my next step. If it doesn’t resolve the issue, I’ll investigate for intake vacuum leaks.

The engine has a new bottom end. The crankshaft and connecting rods were not replaced, but all components including rod bearings, main bearings, pistons, rings, and oil pump have been replaced. No cylinder leakage was detected, and the compression test on cylinder 8 remains stable at approximately 170 psi. I'm still unsure of the root cause.

I’ve confirmed that the engine has a new bottom end. The only components not replaced are the crank and rods—though these have new bearings, pistons, rings, and oil pump. No cylinder leakage was found, and compression on cylinder 8 is consistent at ~170 psi. I’m still stumped. If I could only drive downhill all the time, this issue wouldn’t matter. 😄 I’ll check for vacuum leaks today—weather is mild at about 3°C. Also planning to upgrade from Champion to NGK spark plugs and wires, as well as switch from Autolite to NGK plugs to see if that helps.

I don’t believe a vacuum leak is the primary cause—especially since it only occurs under load and intermittently. A true vacuum leak would typically cause a constant lean condition or misfire at idle, which isn't what’s happening here. Have you checked for weak injector pulse to cylinder 8? That could be a contributing factor.

No, I haven’t tested the injector pulse yet—though it's now on my list after multiple plug and wire replacements still result in P0308. I don’t think it’s a vacuum leak either. I’ll investigate further today. The weather is mild at around 3°C—perfect for outdoor work.

I have a 1999 Ford F-150 with the same 4.6L engine and experiences similar symptoms. After installing new plugs and wires, it improved slightly—but I haven’t fully resolved the issue yet.

Is there a vacuum hose located near the inlet of cylinder 8 on the intake manifold?

Yes, I suspect it might be related to the throttle position (TP) sensor. The truck runs fine at idle and light throttle, but when slightly accelerated, it begins to misfire—similar to a sudden air or fuel imbalance. I recently worked on my brake line and pulled it slightly; could this have affected vacuum routing? Could it be a vacuum-related issue or a TP sensor fault?

I recall that the air filter housing sensor was suspected—someone tried popping it out by the bottom, thinking it might be a clip. Possibly related to airflow sensing.

I replaced the sensor two days ago, but the issue persists—the misfire now appears on cylinder 6 instead of 8. This suggests either a wiring fault or a more complex system-level problem.

The symptom occurring at around 1/4 throttle strongly points to a throttle position (TP) sensor issue, though no matching diagnostic code is present. The behavior also resembles engine instability under load—possibly due to wiring shorts or other mechanical issues. Since the engine was completely disassembled during rebuild, it's worth checking the engine bay for overlooked connectors or hoses near cylinder 8.

My engine hasn’t been rebuilt like this—only had minor maintenance. I’m planning to clean my injectors next. What could cause a TP sensor failure? I don’t have much money to spend on expensive diagnostics right now.

Are your F-150 symptoms exactly the same or slightly different?

Yes, it's identical—runs fine at idle and light throttle, but skips when given a slight acceleration. This started recently after the rebuild.