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Q: Vehicle's engine stops turning over when at speed.

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At first, the truck was losing power and the engine stopped turning over only when idling. The truck would start back up after sitting for a second and then the engine would stop turning over anytime it idled. I did a full tune -up and for about a week the truck had no issues. Today, while at speed on the highway the engine stopped turning over twice and started back up after a few minutes. The third time, It sounded like it was trying to turn over but didn't and had to be towed. Please assist.

My car has 420000 miles.
My car has an automatic transmission.

Hi There, It sounds like you may have one of two possible issues. The issue of idling and stalling may be related to a dirty or faulty idle air control valve. The idle air control valve monitors the air intake as it is mixed with fuel prior to being injected into the engine at low speeds and at idle. This valve is controlled by the vehicle’s computer and will adjust idle speed based upon other measurements such as engine temperature, intake air temperature and electrical system load or voltage. This is also an important function when starting the motor as it allows the motor to run and idle on it’s own once the motor fires. When you accelerate, the engine RPM increases, and as you let off the gas, the RPM slowly returns to the normal idling speed with the help of the idle air control valve making the transition from a higher RPM back down to idle speed while adjusting the air/fuel ratio constantly to allow this to happen smoothly. When the engine RPM drops below the normal range of about ~800 RPM, this often times will cause the engine to stall indicating a dirty or faulty idle air control valve.

When the truck stalls at highway speed and does not immediately start right away after, this suggests you may have an ignition coil that may be starting to fail. Ignition coils are coated with a varnish-like insulation that becomes brittle over time after being stretched then contracted repeatedly The insulation develops small fractures that open when heated and close when cooled. When open, they allow shorting of coil windings and decrease or eliminate the coil’s ability to function. This type of temperature cycling happens because every time you shut the engine off, the fan stops running, the coolant stops flowing and the temperature under the hood rises. Every time you start the engine again the coolant flows, the fans turn on, and the temperature drops. When this heat dissipates and is absorbed under the hood, it is trapped causing this cycling effect. Components that contain heat sensitive materials include parts like coils, injectors, igniters, and some sensors. Any of these may be potentially subject to this same kind of heat problem which can be very difficult to trace. I would recommend having an expert from YourMechanic come to your location to take a look at your vehicle.

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