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Q: Rough idle when it is hot. When I start it when it is cold, I have no issues.

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I have a Ford Explorer 2006 that has a rough idle when it is hot. When I start it when it is cold, I have no issues but If the car is hot, the rpm will increase and decrease and car may stall. If I leave it like 1 hour, it starts normally. No other issues at all. No check engine light. I cleaned maf sensor and T-body and replaced fuel filter and spark plugs but did not help. Please advise. Thanks

My car has 170000 miles.
My car has an automatic transmission.

Hi Ghary, What you are describing sounds like an ignition coil that is beginning 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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