Likely Culprits (in order of likelihood, generally):
1. Intermittent Sensor Issues: The most common reason for a check engine light with no stored codes is a sensor that's failing intermittently. The PCM (Powertrain Control Module) might detect a problem briefly, trigger the light, and then the problem resolves itself before a code is permanently set. Suspects include:
* Mass Air Flow (MAF) Sensor: Measures the amount of air entering the engine. A dirty or failing MAF sensor can cause erratic fuel delivery, leading to rough running. Clean it first (carefully, following instructions specific to your truck). If the problem persists, replacement might be needed.
* Throttle Position Sensor (TPS): Tells the PCM the throttle position. A faulty TPS causes inconsistent fuel and timing.
* Crankshaft Position Sensor (CKP): Crucial for ignition timing. A bad CKP will cause rough running, misfires, and potentially no start.
* Cam Position Sensor (CMP): Similar to CKP, but for camshaft timing. Problems will be very similar to a bad CKP.
* Fuel Pressure Sensor: Inaccurate readings will result in improper fuel delivery.
* Glow Plug Relay (if applicable): Particularly if the problem is worse when cold.
* Injectors (less likely to cause a light without a code, but possible): An intermittently failing injector might cause rough running but not always trigger a code. This is harder to diagnose and usually involves testing injector function.
2. Wiring Issues: Loose connections, corroded terminals, or damaged wiring harnesses can cause intermittent sensor problems and might not always set a code. Inspect all wiring to and from the sensors listed above. Look for broken insulation, corrosion, or loose connections.
3. PCM Issues (Least Likely, but Possible): Although rare, a failing PCM can cause intermittent problems without storing codes. This is usually a last resort diagnosis and is expensive to fix.
Troubleshooting Steps:
1. Check the obvious: Make sure your fuel is clean and the fuel filter is not clogged. Low fuel level can sometimes cause erratic operation.
2. Clean MAF Sensor: This is the easiest and cheapest first step. Use MAF sensor cleaner specifically designed for this purpose. Do not touch the sensor element.
3. Inspect Wiring Harnesses: Carefully examine the wiring harnesses for any signs of damage, corrosion, or loose connections. Pay close attention to the wiring for the sensors mentioned above.
4. Use an OBD-II Scanner with Live Data: While your scanner didn't find codes, a more advanced scanner might allow you to monitor sensor data in real-time. Observe the values of the MAF, TPS, and other relevant sensors while the engine is running and compare them to the specs in your truck's repair manual. Look for erratic or out-of-range readings. A good scan tool can often even show pending codes, that haven't tripped the light yet.
5. Professional Diagnosis: If you can't find the problem yourself, take the truck to a mechanic specializing in diesel vehicles, especially those familiar with older Powerstrokes. They have access to more advanced diagnostic tools and expertise.
Important Note: Ignoring a check engine light, even without codes, is not recommended. An underlying problem could be causing damage to your engine, leading to costly repairs later.
