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1999 Ford Van 4.6L Engine Misses on Cylinders 1 and 7 After 10 Seconds of Running

Model: ford 4.6 Fault Code: P0300 Posted: 2008-11-30 06:56

I have a 1999 Ford van with a 4.6L engine that runs smoothly for the first 10 seconds after starting, but then begins to miss on cylinders 1 and 7. I've already replaced all spark plugs and installed new coil-over-plug assemblies, including swapping them with known-good units from non-misfiring cylinders. The issue is confirmed to be in those two cylinders. When the engine starts, injectors are working properly and show consistent 12 volts at the plug end. However, after about 10 seconds of operation, the injector signal drops—specifically losing the negative ground signal—while the spark remains present but weakens over time. I've tested all wiring harnesses to the PCM and found them intact. The engine runs fine at startup, but misfires occur shortly after ignition. This behavior suggests a potential issue with the Powertrain Control Module (PCM) coil driver circuits or a fault in the injector ground circuit. Since the spark is still present initially, it's unlikely that the problem lies solely with the injectors or fuel delivery. I suspect either a faulty PCM signal to cylinders 1 and 7 or a damaged harness near the EGR tube, which runs close to the coil and injector wiring.

Related fault codes
P0300P0301
Comments (8)
Anonymous 2008-11-30 09:22

Check the wiring harness carefully, especially along the EGR tube area. The harness often rubs against this component and can cause intermittent wire damage or shorting. Make sure you've verified all spark plug connections as well.

Anonymous 2008-11-30 11:39

I’ve replaced all plugs and coils, and tested by swapping them with working cylinders—no change in behavior. The engine runs fine at startup but drops cylinder 1 and 7 after about 10 seconds of operation. Injector voltage remains stable (12V), but the negative signal disappears after that period. All wires to the PCM test good, so it's likely a fault within the PCM’s coil driver circuits.

Anonymous 2008-11-30 12:54

Don’t jump to conclusions about injector drivers yet—first verify the coil driver circuits and their signals in the PCM. The PCM intentionally shuts off injectors on misfiring cylinders to prevent catalytic converter damage, so this behavior is expected under certain conditions.

Anonymous 2008-11-30 17:53

Spark plugs fire initially, but after a few seconds of running, the engine loses spark in cylinders 1 and 7. The coils have been replaced, and I’ve swapped them with working cylinders—still no improvement.

Anonymous 2008-12-01 08:25

The spark appears strong at startup but weakens within a few seconds of operation. This weakening causes the injectors on cylinders 1 and 7 to shut down automatically, which matches the observed misfire pattern. The issue occurs only after engine warm-up.

Anonymous 2008-12-01 17:38

When the PCM detects a misfire, it typically shuts off injector signals for those cylinders to protect the catalytic converter. Have you inspected the harness near the EGR tube? That area is prone to wear and can cause intermittent faults. All coil and injector wiring run through this harness. If that’s intact, consider faulty PCM driver circuits for cylinders 1 and 7. Also verify whether you’re losing power or ground signals—especially on the negative side of the injectors.

Anonymous 2008-12-02 06:36

All visible components appear fine, but I’m specifically losing the ground signal from cylinder 1 and 7 after startup.

Anonymous 2008-12-03 17:52

If all wiring and connections are confirmed good and the issue persists, it strongly points to a faulty PCM driver circuit for cylinders 1 and 7. This is consistent with the symptoms described—initial spark presence, loss of ground after 10 seconds, and no improvement despite new coils and plugs.