Q: Engine jerks sometime. on 1998 Ford Expedition

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Sometime engine jerks, it gets worst when engine heats up and going up steap hills. I did a dignostic check on engine and everything was ok no problems found. I repplaced the TPS sensor and it still doing the samething. Check engine light is not on, no misfire! help
(1) Answer
I have to assume that this vehicle has high mileage, if so and you have not had the spark plugs changed yet, you may have a bad spark plug or spark plug boot that is cracked. These two items can cause you symptoms given the clean self test results. If the spark plugs are new or recently replaced, then you need to remove the coils and inspect the spark plug boots for cracks or arc through, (where secondary ignition spark leaks through the spark plug boot and goes to ground instead of the spark plug) signs of arc through are small pin holes with discoloration of the boot rubber around that pin hole. Sometimes, dirt makes a circle around those pin holes. The easiest way to find if you have an ignition misfire is to drive the vehicle under the conditions that cause the rough running and see if the check engine light flashes when it is acting up. The longer you can sustain the rough running the better chance of pinpointing it. Once the check engine light flashes, the engine control module should set a code and retain what cylinder is causing the misfire. If none of these are the concern then you could anything from a bad ground to a bad Mass Air Flow (MAF) sensor. Good luck.
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