Fuel System:
* Fuel Pump: A weak or failing fuel pump might not deliver enough fuel at low speeds when engine demand is lower. The pump may struggle under load when you're accelerating, but when the engine speed drops (low speed driving), fuel delivery becomes insufficient causing a stall.
* Fuel Filter: A clogged fuel filter restricts fuel flow, mimicking the symptoms of a weak fuel pump.
* Carburetor Issues (if applicable): If it's a carbureted model, problems like a clogged idle circuit, a faulty accelerator pump, or low fuel level in the bowl can lead to stalling at low speeds. Dirty or malfunctioning jets are also common problems.
* Fuel Lines: Cracked, pinched, or deteriorated fuel lines can restrict fuel flow, especially at low pressures.
Ignition System:
* Distributor Cap and Rotor: Worn or corroded components in the distributor can cause intermittent sparking, leading to engine misfires and stalling.
* Ignition Coil: A weak or failing ignition coil might not provide enough voltage to spark the plugs reliably at low RPM.
* Spark Plugs and Wires: Worn, fouled, or damaged spark plugs and wires can cause misfires, especially noticeable at low speeds.
* Pick-up Coil (in the distributor): A failing pick-up coil won't send the proper signal to the ignition system, resulting in inconsistent sparking.
Electrical System:
* Alternator: A faulty alternator can cause the battery voltage to drop, potentially leading to power loss and engine stalling, particularly if the battery is already weak.
* Battery: A weak or failing battery can't supply enough power, exacerbating the issue if there's already a problem in the ignition or fuel system.
* Wiring: Loose, corroded, or damaged wiring connections can interrupt the power supply to essential components. Check all connections, especially around the ignition system and fuel pump.
* Neutral Safety Switch: This switch prevents the engine from starting unless the transmission is in Park or Neutral. A faulty switch might be preventing power to the system at low speeds. (Less likely but possible)
* Throttle Position Sensor (TPS): This sensor (if equipped, more common in fuel injected systems) tells the computer the throttle position. A faulty TPS can cause erratic fuel delivery or ignition timing.
Troubleshooting Steps:
1. Check the basics: Start with the simple things – fuel level, battery voltage, and visual inspection of wiring for obvious damage.
2. Listen for unusual noises: A whining sound from the fuel pump might indicate it's failing.
3. Check the spark: Test the spark plugs and wires for proper sparking. A weak or inconsistent spark points to problems in the ignition system.
4. Inspect the distributor: Look for wear and tear on the cap and rotor.
5. Check fuel pressure: If you have the tools, measuring fuel pressure will help determine if the fuel pump is delivering enough fuel.
6. Consult a repair manual: A manual specific to your 1985 Chevy C10 will provide diagrams and detailed troubleshooting steps.
It's crucial to systematically check these areas. Since the problem occurs at low speeds, it's likely a fuel delivery or low-RPM ignition issue. If you're not comfortable working on your vehicle yourself, take it to a qualified mechanic for diagnosis and repair.
