Q: Rpm shoots high >2000 when starting in the cold then drops low almost to 0 like its about to stall out, rides around 2000 rpm

asked by on January 16, 2018

When I start my car, mostly on cold mornings the RPMs either shoot up really high (between 2-3000) and then drop to almost 0 like it wants to stall out or will immediately be that low and rumble for a few moments. It rides smooth but at a high rpm around 2000.

I've recently replaced the mass air flow sensor, O2 sensors, fuel pressure sensor, fixed a cracked exhaust piece, and replaced spark plugs.

Could I have a problem with the iac or TPS? I don't hear any hissing coming from under the hood so I don't think it's a vacuum leak?

My car has 190000 miles.
My car has an automatic transmission.

In the majority of vehicles, engine idle speed is controlled by an idle control valve (ICV). The 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 a high idle include vacuum leaks, a malfunctioning throttle (including the TPS) and, rarely, a PCM/ECM malfunction. With regard to vacuum leaks, on a fuel injected engine a vacuum leak will cause a 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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