1996 Plymouth Voyager P0300 and P0306 Misfire: Causes, Fixes, and Cost-Saving Solutions

What engine size does your vehicle have? I suspect fuel injectors aren't the root cause. Have you performed a compression test yet? This can help identify mechanical issues like worn valves or piston rings.

I'm currently dealing with P0306 in my 2000 Plymouth Grand Voyager. The only symptom is rough idle, especially at stoplights—otherwise the car runs smoothly. Over the past two months, I've spent over $2,000 trying to fix it without success. I replaced burned valves (which were supposedly the cause), two camshafts, and various engine electrical components including spark plugs, wires, and coil packs—all with no improvement. A local shop cleaned all six injectors for $270; that didn't help either. Now a Plymouth specialist claims it's likely due to a burned valve or broken piston ring—even though I've already had a valve job. I suspect a sensor issue like the MAP, oxygen, or idle air control sensor, but most mechanics don't agree. I'm now relying on forums and my own research instead of professional diagnosis.

Start with basic, low-cost fixes: replace spark plugs, clean fuel injectors, inspect spark plug wires. If they're old or worn, replace them with high-quality new ones. Document the results and update your findings here for others to reference.

I still suspect a valve-related issue unless the EGR sensor is faulty or the flexplate is cracked—both of which can occur after transmission rebuilds, especially in older vans with repeated repairs.

Kyle123456 wrote: The only symptom is rough idle at stoplights. I used to have a similar issue on my 96 Grand Voyager 3.0L (206k miles). A forum post suggested that the idle air control valve (IAC) may develop gum or dirt buildup, restricting airflow and causing rough idling. I started spraying intake cleaner into the throttle body every time I changed oil—about every six months—and it significantly improved idle smoothness. Recently, my vehicle has started showing P0306 again. I’ve already replaced spark plugs, wires, cap, and rotor with no lasting fix. A STP fuel treatment temporarily resolved the issue for two months. My freeze frame data shows: Load=3.9%, Coolant Temp=206.6°F, Short-term Fuel Trim=-0.78% (lean), Long-term Trim=+5.47% (rich), Intake MAP=5.7 psi, RPM=608, Speed=0 MPH. I’ve observed a periodic misfire on wire #6 when using a timing light—before and after plug/wire changes. Since the fuel treatment worked temporarily, I’m considering repeating it. Do you have any recommendations for better injector cleaners or alternatives?

The freeze frame data appears normal. The misfire is likely substantial—when the CEL flashes, that indicates a significant issue. Start with fundamentals: verify compression and cylinder leakage. Swap the fuel injector from cylinder #6 to another cylinder to test if the problem follows. Given the age of this 3.0L engine (over 25 years), a valve-related issue is likely the most probable cause.

Jeff Compton wrote: I'd be leaning towards a likely valve problem. I don’t have constant smoke like older 3.0L engines, but I’ve noticed heavy oil smoke when idling in Colorado (7,000 feet elevation) on a few occasions. Since then, it hasn’t returned. I also experienced a weak catalytic converter code at high altitudes—now resolved after returning to sea level. I use one quart of oil every 800–1000 miles, and I suspect some is leaking from the valve cover gaskets (only a few drips on the driveway). I can sometimes smell burning oil dripping onto the exhaust when stopped. My question: What happens to valves over time? Is it just normal wear? And at this point, should I invest in repairs—or is it better to accept that the engine will eventually fail? The car runs fine and passes emissions inspections if I clear codes about a week before testing (to allow self-tests). I’ve replaced catalytic converter, fuel pump/filter, injectors, hoses. Timing belts, water pump, transmission, and muffler have all been replaced. Only issue was a leaking fuel line clamp—cut an inch to fix it. I’m skeptical of fuel additives—but they did help once. Is there any better alternative for cleaning injectors or preventing oil burn?

The 3.0L engines in this model were prone to poor valve guides, leading to oil burning and leaks. If you already have leaking valve cover gaskets, I recommend removing the cylinder heads, reconditioning them, replacing water pump, timing belt, camshaft, and crankshaft seals. With proper maintenance, these engines can last well beyond 200,000 miles. The 3.0L is known for a strong bottom end. I strongly advise against clearing engine light codes to pass inspections—this vehicle emits pollutants and should be repaired or retired responsibly.