How come when the truck is in park or neutral and I tap the gas it will rev up to 3000 rpm and then back down slow, but it doesn’t do it in reverse or drive
When the car is in gear, the load on the engine is sufficient to prevent the malfunction. If the engine idle speed on your vehicle is controlled by an idle control valve (ICV), that ICV valve can fail or a faulty or blown fuse in other electronically controlled systems can cause the IAC motor to malfunction, leading to a higher than normal engine idle speed. Failures in the ICV valve itself are often due to excessive carbon buildup in the intake manifold. Other possible causes of the excursion in RPM you are describing include vacuum leaks, a malfunctioning throttle and, rarely, a PCM/ECM malfunction. With regard to vacuum leaks, on a fuel injected engine a vacuum leak will cause high engine idle as the engine’s oxygen sensor detect the extra oxygen entering the engine through the vacuum leak. The newly lean (too much air relative to fuel) status will result in the engine’s computer pumping in additional fuel, resulting in an abnormally high idle. A malfunctioning throttle system can cause both a high idle as well as an engine stall. A dirty air intake or cracked air intake tube is often the culprit. If you desire that this high idle problem be diagnosed by a certified Mechanic, dispatched by YourMechanic right to your location, please request an engine idle speed diagnostic - high idle and the responding certified mechanic will get this taken care of for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.
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