* Throttle Cable/ linkage: The most common cause. A sticking or improperly adjusted throttle cable can keep the throttle plate slightly open, causing a high idle and making the truck creep forward. Inspect the cable for binding or fraying. The linkage itself could also be bent or improperly adjusted.
* Throttle Position Sensor (TPS): This sensor tells the computer the throttle's position. A faulty TPS can send incorrect signals, leading to a high idle.
* Vacuum Leaks: A vacuum leak anywhere in the system can disrupt the idle. Common places to check include:
* Intake manifold gaskets: These are notorious for leaking on older vehicles.
* Vacuum hoses: Inspect all vacuum hoses for cracks, holes, or loose connections. Pay close attention to hoses connected to the brake booster (if the brake booster is causing a vacuum leak it will also make it hard to brake effectively).
* Vacuum advance on the distributor: A leak here can affect idle and timing.
* Idle Air Control (IAC) Valve: This valve regulates air flow at idle. A dirty or malfunctioning IAC valve can cause a high idle. It's usually a small motor near the throttle body.
* Transmission Issues: While less likely to cause a *high* idle, a problem with the transmission's internal components (like a sticking valve) could cause the truck to creep forward even when in park or neutral. This is particularly true if the issue only happens when the engine is running and not when it is off.
* Computer/ECM Issues: While less common, a faulty engine control module (ECM) could be sending incorrect signals, leading to the symptoms you describe.
Troubleshooting Steps:
1. Check the throttle cable and linkage: Manually try to move the throttle plate (the butterfly valve inside the throttle body) by hand. If it feels sticky or doesn't return smoothly to the closed position, the cable or linkage needs attention.
2. Inspect for vacuum leaks: Start the engine and listen carefully for hissing sounds. Use a propane torch (carefully and only near suspected leak areas, NOT near the engine!) to check for vacuum leaks. If there is a leak the engine's RPM will increase when the flame nears the leak. Spray carburetor cleaner or brake cleaner to see if a change in RPMs occurs.
3. Clean the IAC valve: If accessible, remove the IAC valve and clean it thoroughly with carburetor cleaner.
4. Check the TPS: A multimeter can be used to test the TPS voltage output. This requires checking the voltage at different throttle positions according to your vehicle's specifications.
If you're not comfortable performing these checks, it's best to take your truck to a qualified mechanic. They'll have the tools and expertise to diagnose and fix the problem efficiently and safely. The combination of high idle and forward creep strongly suggests a problem with the throttle system, but investigating the transmission isn't entirely out of the question if other checks don't reveal the root cause.
