Why does the engine idle normally but misfire at high speed?
2 Answers
The reasons for normal engine idle but misfire at high speed are as follows: 1. Excessive spark plug electrode gap; 2. Improper adjustment of distributor contact gap; 3. Weak contact arm spring; 4. Faulty ignition coil or capacitor. The solutions for engine misfire at high speed are as follows: 1. Open the distributor cover and check the distributor gap, which should be 0.35-0.45 mm. Otherwise, adjust the contact gap. 2. Check the spring tension of the contacts. Use a spring scale to lift the contacts to the normal gap, and the reading on the spring scale indicates the spring tension. The normal value should be 500-700 grams. If a spring scale is not available, replace a set of contacts. 3. Remove the spark plug and observe the spark plug gap. The normal value should be 0.6-0.8 mm. If a specialized measuring feeler gauge is not available, a hacksaw blade with a thickness of 0.7 mm can be used for testing. This type of fault can be easily detected using an oscilloscope in a professional repair shop.
From my experience, if the engine idles smoothly but stalls at high speeds, it's most likely due to insufficient fuel supply. At low speeds, the fuel pump can barely cope, but once the RPM increases and the throttle is pressed harder, the demand for fuel surges. An aging fuel pump or a clogged filter can lead to inadequate fuel delivery, causing the engine to cut out. Another possible factor is excessive carbon buildup in the fuel injectors, which may still work fine at idle with low flow but get stuck during high-speed, high-volume injection. I've been through this—my car suddenly started shaking on the highway and nearly broke down. Luckily, we diagnosed it and fixed it by replacing the fuel pump. Regularly checking the fuel tank cleanliness and timely replacing the filter can prevent such dangers. Don’t underestimate it—delaying repairs can easily damage internal engine components, and the loss outweighs the gain.
