Why does the engine idle normally but misfire at high speed?

[NolanRose]

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.

[DelTravis]

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.

[McAna]

From a car enthusiast's perspective, if the engine idles fine but misfires at high speeds, it's most likely the ignition system failing under high RPM. Worn-out spark plugs can barely ignite the mixture at low speeds with weak sparks, but can't keep up with the demand for stronger, more frequent sparks at high RPM. Similarly, aging ignition coils may fail to deliver sufficient voltage output at high speeds, causing intermittent misfires. Don't forget to check the high-tension leads - cracks there can also cause problems under heavy load. I've personally disassembled ignition components before - cleaning or replacing spark plugs can prevent this. This issue not only affects acceleration but could also clog the catalytic converter, leading to bigger troubles.

[DeRyder]

As a family car owner, I've noticed that stable idling but stalling at high speeds is often caused by a clogged intake system. Over time, the air filter accumulates dust and thickens, barely providing enough air at low speeds. However, when the engine demands a large amount of fresh air at high RPMs, the insufficient supply leads to an overly rich air-fuel mixture, resulting in stalling. My previous car exhibited exactly these symptoms. Upon inspection, the air filter was as dirty as a carpet, and replacing it made an immediate difference. Regularly cleaning or replacing the filter is crucial. Delaying maintenance can cause engine surging, a sharp increase in fuel consumption, and pose a threat to driving safety. Remember, don't let a stable idle lull you into complacency.