Fuel System:
* Fuel Pump: A weak or failing fuel pump might provide enough pressure for idling but not the increased flow needed during acceleration. Listen for the fuel pump whirring when you turn the key to the "on" position (before cranking). A weak whir or no sound suggests a problem. You can also test fuel pressure with a gauge.
* Fuel Filter: A clogged fuel filter will restrict fuel flow, especially under higher demand. This is a cheap and easy part to replace first.
* Fuel Injectors: Clogged or malfunctioning fuel injectors might deliver fuel at idle but fail to provide enough during acceleration. This often requires specialized testing equipment.
* EGR Valve: If the EGR valve is stuck open it can dramatically reduce engine power, especially at higher RPMs. It should close when acceleration is needed.
* Throttle Position Sensor (TPS): A faulty TPS prevents the computer from knowing how much throttle is applied, leading to poor fuel delivery. A bad TPS can cause erratic idle or lack of power.
Ignition System:
* Distributor Cap and Rotor: Cracks or corrosion in the distributor cap and rotor can interrupt the spark, leading to poor performance. Inspect for these visually.
* Spark Plugs and Wires: Worn or fouled spark plugs and damaged wires can prevent proper ignition. Check the plugs' condition and look for cracks or damage in the wires.
* Ignition Coil: A weak coil might provide enough spark for idle but not enough under load. Testing the coil's output requires specialized equipment.
* Mass Air Flow (MAF) Sensor: This sensor measures the amount of air entering the engine, critical for fuel/air mixture. A faulty MAF sensor can cause poor performance.
Other Possibilities:
* Catalytic Converter: A clogged catalytic converter will severely restrict exhaust flow, leading to a loss of power. This is less likely to only show up under acceleration if it's completely clogged.
* Vacuum Leaks: Leaks in the vacuum system can disrupt the engine's operation and affect power. Inspect all vacuum lines for cracks or disconnections.
Troubleshooting Steps:
1. Start with the easy checks: Replace the fuel filter. Inspect the distributor cap, rotor, spark plugs, and wires. Check for obvious vacuum leaks.
2. Listen for the fuel pump: Does it run when you turn the ignition on?
3. Check fuel pressure: If you can borrow or rent a fuel pressure gauge, this is a crucial test.
4. Get codes scanned: A code reader for OBD I systems (if your Bronco has one, most likely not fully OBD-I compliant) can give you clues about possible sensor problems.
5. Advanced diagnostics: If the easy checks don't solve the problem, you'll likely need to take it to a mechanic or use more advanced diagnostic tools to test the fuel injectors, ignition coil, TPS, and MAF sensor.
Don't jump to expensive repairs without systematically checking the more common and easily accessible components first. The fuel filter and a visual inspection of the ignition system are excellent starting points. Good luck!
