* Ignition System Issues: A faulty ignition switch is a prime suspect. If the switch isn't fully breaking the circuit when you turn the key off, power continues to feed the ignition system, keeping the engine running. The brake pedal might be interrupting a ground wire somewhere or introducing a resistance that breaks the stray current. This is the most likely cause given your symptoms. Check the ignition switch itself and its wiring carefully.
* Fuel System Issues: A sticking fuel pump relay or a faulty fuel pressure regulator could be keeping fuel pressure up, even after the engine is switched off. The brake pedal interaction is less likely related to a fuel issue, but a very unusual fuel leak might be affecting a brake-related sensor creating the stopping effect.
* Vacuum Leak: A vacuum leak after the throttle plates could allow the engine to continue drawing fuel and running even with the ignition switched off. The brake pedal's interaction is less likely linked here, unless there's a very unusual vacuum line routing problem near brake components.
* Faulty Neutral Safety Switch: This switch prevents the engine from starting unless the transmission is in Park or Neutral. A faulty switch might not be fully cutting power to the starter and could contribute to the continued running, though less directly than the ignition switch problem. The brake pedal involvement in this case is less clear.
* Ground Issues: A poor ground connection somewhere in the electrical system could cause all sorts of erratic behavior. The brake pedal might be inadvertently improving a ground connection.
Troubleshooting Steps:
1. Start with the Ignition Switch: This is the most likely culprit given your description. Inspect the ignition switch carefully. Does it feel solid when turning it on and off? Is there any play or looseness? Wiggle the wiring harness connected to the switch while someone tries to turn the key.
2. Check the Wiring: Trace the ignition wiring harness thoroughly, checking for any loose connections, frayed wires, or corrosion. Pay close attention to the ground connections.
3. Fuel System Pressure Test: Use a fuel pressure gauge to check fuel pressure both when the engine is running and after it's turned off. A persistently high pressure suggests a faulty fuel pressure regulator or pump relay.
4. Vacuum Test: If you have a vacuum gauge, check for leaks in the vacuum lines.
5. Neutral Safety Switch: Test this switch for continuity according to its specifications.
6. Grounds: Clean all ground connections. Apply a dielectric grease to prevent future corrosion.
Because of the brake pedal's involvement, focus your attention first on the ignition switch and its wiring. A faulty ignition switch is a relatively inexpensive and easy fix compared to extensive investigation of the fuel system or vacuum lines. If you are not comfortable working with automotive electrical systems, take it to a qualified mechanic.
