Ignition System:
* Worn spark plugs: This is a very common cause. Old plugs misfire, leading to incomplete combustion and unburned fuel. Check for fouling (black soot) or excessive wear. Replace them with the correct type and gap.
* Worn or faulty ignition wires: Cracked or damaged wires can cause misfires, especially when the engine is under load or at high RPM. Inspect for cracks, breaks, or arcing.
* Faulty ignition coil: If the coil is failing, it won't provide sufficient spark to ignite the fuel-air mixture consistently, leading to misfires and rough running. Testing a coil requires specialized equipment.
* Distributor cap and rotor: (If your 351 has a distributor) These components can wear and degrade, leading to poor spark distribution. Check for cracks, corrosion, or excessive wear.
Fuel System:
* Clogged fuel injectors: Clogged injectors don't spray fuel properly, resulting in a weak or uneven fuel delivery to the cylinders. This can lead to misfires and unburned fuel. Cleaning or replacing injectors may be necessary.
* Low fuel pressure: A faulty fuel pump or clogged fuel filter can reduce fuel pressure, leading to insufficient fuel delivery and incomplete combustion. Check fuel pressure with a gauge.
* Faulty mass airflow sensor (MAF): The MAF sensor measures the amount of air entering the engine. A faulty sensor can lead to an incorrect air/fuel mixture, causing rough running and unburned fuel.
* Vacuum leaks: Leaks in the vacuum lines can disrupt the air/fuel mixture, causing rough running and potentially unburned fuel. Inspect all vacuum lines for cracks or disconnections.
Other Possibilities:
* Faulty PCV valve: A clogged PCV valve can cause excessive crankcase pressure, affecting the air/fuel mixture and leading to rough running.
* EGR valve issues: A malfunctioning EGR valve (Exhaust Gas Recirculation) can lead to poor combustion.
* Camshaft position sensor (CMP sensor): This sensor tells the computer the engine's position, and a failing one can severely disrupt timing and cause misfires.
* Crankshaft position sensor (CKP sensor): Similar to the CMP sensor, a faulty CKP sensor can prevent proper ignition timing.
Troubleshooting Steps:
1. Check the easy stuff first: Start with the spark plugs, wires, distributor cap and rotor (if applicable). These are relatively inexpensive and easy to replace.
2. Listen to the engine: A distinct clicking or ticking sound could indicate a misfire.
3. Check for codes: Use an OBD-II scanner (if your vehicle has it) to retrieve diagnostic trouble codes (DTCs). This can narrow down the potential problems.
4. Inspect visually: Look for obvious signs of damage or leaks in hoses, wires, and components.
5. Fuel pressure test: Check fuel pressure to rule out fuel delivery issues.
If you're not comfortable working on your engine yourself, it's best to take it to a qualified mechanic for diagnosis and repair. Ignoring the problem could lead to more serious damage. The strong smell of unburned fuel is a fire hazard.
