Fuel System:
* Fuel Pump: A weak fuel pump might provide enough fuel at idle and low speeds but struggles to keep up with the increased demand at higher speeds. The pump could be failing, the fuel filter could be clogged, or there might be a leak in the fuel line reducing pressure.
* Fuel Filter: A clogged fuel filter restricts fuel flow, leading to sputtering and stalling, especially under load. This is a cheap and easy thing to check first.
* Carburetor Issues: If your CJ5 has a carburetor (very likely), several problems could be at play:
* Dirty carburetor: Dirt, debris, or varnish buildup can restrict fuel flow or affect the air/fuel mixture. A rebuild or thorough cleaning is often necessary.
* Clogged jets: Small jets in the carburetor can become clogged, starving the engine of fuel at higher speeds.
* Float level: An incorrect float level can lead to inconsistent fuel delivery.
* Vacuum leaks: Leaks in the carburetor's vacuum system can disrupt the air/fuel mixture.
* Fuel lines: Cracks or kinks in the fuel lines, especially those that become stressed when the engine is under load, can restrict fuel flow.
Ignition System:
* Distributor Cap and Rotor: These components can wear out, causing poor ignition at higher RPMs. Cracks or corrosion can lead to misfires and stalling.
* Ignition Coil: A weak or failing ignition coil may not provide sufficient spark at higher engine speeds.
* Worn Spark Plugs or Wires: Worn spark plugs or faulty wires can cause misfires, especially under load. Check for cracks, corrosion, or excessive wear.
* Ignition Module (if equipped): Some CJ5s had an electronic ignition module; a faulty module can cause intermittent or complete ignition failure.
Other Possibilities:
* Crankcase Ventilation: A clogged PCV (Positive Crankcase Ventilation) system can build up pressure in the crankcase, affecting engine performance.
* Temperature Sensor: A faulty temperature sensor can cause the engine to run lean at higher temperatures, leading to sputtering and stalling.
* Throttle Position Sensor (TPS) (if equipped): If your Jeep has electronic fuel injection (less likely for an '81), a faulty TPS can cause incorrect fuel delivery.
Troubleshooting Steps:
1. Start with the easy checks: Inspect the fuel filter for clogging, check the fuel lines for cracks or kinks, and visually inspect the spark plugs and wires.
2. Check the fuel pressure: If you have the tools, measure the fuel pressure at idle and at higher RPMs to see if the pump is delivering adequate fuel under load.
3. Listen for unusual noises: Pay attention to any unusual noises from the fuel pump or other components.
4. Test the ignition system: Check the distributor cap, rotor, spark plugs, and wires for wear or damage. Consider testing the coil.
5. Clean the carburetor: If you're comfortable doing so, carefully clean your carburetor (or have a professional do it).
Since the problem is load-related, focus on the fuel delivery and ignition components that are more heavily stressed at higher RPMs. A mechanic familiar with Jeep CJ5s can help diagnose the problem more efficiently. Given the age of the vehicle, a comprehensive inspection of the entire fuel and ignition systems is highly recommended.
