Q: 98 buick runs perfect when first starting then begins to thunk when it gets warm.

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car runs great when first starts then when warm it thunks in passing gear. I have changed fluid and filter (STP) fluid and added Lucas . I have cleaned radiator very well (both parts).

My car has 100000 miles.
My car has an automatic transmission.

If you are experiencing slow acceleration when the engine is under load, there are a variety of causes that would be considered. Fuel injectors can become clogged over time as the tiny passages become clogged. When an injector is clogged it can degrade the performance of the vehicle and the car may accelerate slowly or feel that it does not have enough power. Other symptoms include a decrease in fuel efficiency. A defective fuel pump will not supply enough fuel to the injectors. This can cause the vehicle to accelerate slowly, or even sputter and stall, especially at high speeds. A clogged fuel filter can also restrict the amount of fuel that reaches the injectors. A dirty airflow sensor can cause the wrong data to be be sent to the computer. A slow accelerating car is the result of this incorrect data. It may also trigger the Check Engine light. Other causes of poor acceleration are vacuum leaks, a defective throttle position sensor and a defective idle air control valve. To get the underlying cause of the problem resolved, you can request an engine diagnostic for slow acceleration. 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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