Easy Checks:
* Choke: Is the choke sticking in the closed or partially closed position? This is especially likely if the problem started in cold weather or after the engine was cold. Manually try to open the choke fully (there's usually a lever or cable). If it's sticking, you'll need to investigate the choke mechanism itself.
* Vacuum Leaks: A vacuum leak can cause a high idle. Inspect all vacuum lines for cracks, holes, or loose connections. Pay close attention to lines connected to the brake booster, distributor advance, and the carburetor itself. Spray a little carburetor cleaner around suspected areas while the engine is running. A significant change in idle speed indicates a leak.
* Throttle Cable: Make sure the throttle cable isn't sticking or binding. Try gently pulling and releasing the cable at the carburetor end to see if there's any resistance.
* Idle Adjustment Screw: On the carburetor, there's usually a screw that adjusts the idle speed. (Consult your owner's manual or a repair manual for the exact location). Only adjust this screw slightly, and only after you've ruled out other potential causes. Incorrect adjustment can damage the engine.
More Involved Issues:
* Carburetor Problems: This is a common culprit. A dirty or malfunctioning carburetor can cause a variety of problems, including high idle. The carburetor might need cleaning, rebuilding, or even replacing. This often requires specialized tools and knowledge.
* Throttle Position Sensor (TPS): While less likely on a 1988 carbureted van, some models might have a rudimentary TPS. A faulty TPS can send incorrect signals, affecting the idle.
* Ignition System: A faulty ignition component, such as a bad ignition coil or distributor, could cause erratic engine behavior, including a high idle.
* Computer Issues (ECM/PCM): Even though it's a carbureted system, some 1988 models might have a basic computer controlling some aspects of the engine. A malfunctioning computer is less likely but possible.
Troubleshooting Steps:
1. Start with the easy checks: Address the choke, vacuum leaks, and throttle cable first.
2. Visual Inspection: Carefully inspect all components related to the idle system.
3. Consult a Repair Manual: A factory service manual or a Haynes/Chilton repair manual specific to your 1988 Chevy van will provide detailed diagrams and troubleshooting information.
4. Seek Professional Help: If you're not comfortable working on your carburetor or other engine components, take your van to a qualified mechanic. This is especially true if you've ruled out the simple fixes.
Remember safety first! Always disconnect the battery negative terminal before working on any electrical components. Also, be cautious when working around hot engine parts.
