How to Diagnose and Troubleshoot Unstable Engine Idle?
2 Answers
Methods for diagnosing and troubleshooting unstable engine idle include: 1. Check for air leaks in the intake pipe, PVC valve hose, and oil dipstick area; 2. Inspect whether the air filter element is excessively dirty; 3. Verify the proper functioning of the cold start injector and temperature-time control switch; 4. Check if the fuel system pressure is too low; 5. Examine the injector spray pattern; 6. Inspect cylinder pressure and valve clearance; 7. Check the ECU. When the engine operates normally at medium to high speeds but exhibits unstable idle or even stalls at idle or near-idle speeds, this indicates an idle malfunction.
With over a decade of experience in car repair, I've encountered countless cases of unstable idle. The first step is to check for any trouble codes. If you don't have a diagnostic tool, listen to the engine sound. If there's obvious misfiring, it's likely an ignition issue—try unplugging each ignition coil one by one to identify which cylinder isn't working. The second step is to inspect the idle control valve or throttle body for carbon buildup. If the throttle gets stuck due to thick carbon deposits, it can cause shaking—just remove and clean it. The third step is to measure fuel pressure. A clogged fuel pump filter or a faulty fuel pressure regulator can affect fuel supply. The fourth step is to check for intake leaks. Spray carburetor cleaner around the pipe connections—if the engine RPM changes suddenly, there's a leak. Finally, inspect rubber components like vacuum hoses and the PCV valve for aging. If you can't handle it yourself, take it to a shop.
