Most Likely Culprits:
* Cylinder Misfire (on one bank of cylinders): A misfire causes incomplete combustion. Unburnt fuel passes through the exhaust, resulting in smoke (often bluish-gray or black) and rough running. The location of the smoke indicates which side (passenger or driver) of the engine the problem cylinders are on. Check for:
* Worn spark plugs: Inspect, clean, and gap them if necessary. Replace if they're damaged or fouled.
* Worn or damaged spark plug wires: Look for cracks, fraying, or loose connections.
* Bad ignition coil(s): A weak coil can cause misfires. Testing is needed.
* Distributor cap and rotor issues: Cracks or corrosion can interrupt the spark.
* Faulty ignition module: This is a more advanced component that needs testing.
* Low compression in one or more cylinders: A compression test is crucial here. Low compression suggests worn rings, valve problems, or a blown head gasket.
* Valve Problem (on one bank of cylinders): A sticking or burnt valve on one side of the engine could cause incomplete combustion, leading to smoke from that side. This might also result in oil leaking into the combustion chamber. This usually results in bluish smoke.
* Burnt valves: Often caused by overheating or lean air/fuel mixture.
* Sticking valves: Can be caused by carbon buildup or wear.
* Head Gasket Leak (on one side): A leak on one side of the head gasket could allow coolant to enter a cylinder or combustion gases to leak into the cooling system. This usually manifests as white smoke (steam) or milky oil.
* Worn Piston Rings (on one bank of cylinders): Worn piston rings allow oil to seep into the combustion chamber, resulting in bluish-gray or blue smoke, especially upon acceleration.
Less Likely (but possible):
* Exhaust Leak: While less likely to cause rough running and significant smoke, a leak *could* contribute.
Troubleshooting Steps:
1. Visual Inspection: Carefully inspect the engine for any obvious signs of damage, leaks (oil, coolant), loose wires, or worn components. Pay close attention to the side producing the smoke.
2. Compression Test: This is THE most important test. It will quickly tell you if you have a major issue in one or more cylinders on one bank.
3. Check the Spark: Use an inline spark tester to check if you are getting a spark at each spark plug.
4. Check for Vacuum Leaks: A vacuum leak can cause a rough idle.
5. Examine the Exhaust Manifold: Look for cracks or leaks.
6. Air/Fuel Mixture: A lean condition can lead to burnt valves. A rich condition can lead to black smoke.
7. Oil Level and Condition: Check for excessive oil consumption or milky oil (indicates coolant in the oil).
Important Note: Diagnosing engine problems accurately requires some mechanical aptitude and tools. If you're not comfortable performing these checks, it's best to take your Cheyenne to a qualified mechanic specializing in classic cars or big-block engines. They have the tools and experience to properly diagnose and fix the problem. Ignoring the issue could lead to more extensive and costly repairs.
