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Q: Engine hesitating in the first 20 minutes of driving when cold

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Hi. I have problem with my car when the weather is cold. It would start stumbling and hesitating in the first 20 minutes of driving. The dash would show check engine light flashing as well. After 10-20 minutes of running with these issues, the engine would gather speed and run fine. Also, it would occasionally stall at traffic lights. I went to the dealership but they couldn't fix the problem. They used a code reader and no error codes came. They had the fuel filters and fuel pump checked and couldn't find anything wrong. The issue is still there and I would like to know what could be wrong. The car has gone around 70,000 miles. Do you have any ideas? Thank you.

A: This could be caused by a cold state vacuum...

This could be caused by a cold state vacuum leak. Basically, this is a condition where a vacuum leak is present until the engine heats up and the induction parts expand and seal the leak. When the vacuum leak is present, you may be getting too much air, causing the engine to stumble. Because this happens during the warm up phase, the engine is running on preprogrammed values, so no fault codes are stored.

A smoke test on the induction system while the engine is cold might make it possible to locate the source of this leak. You may want to enlist the help of a mechanic, such as one from YourMechanic, who has the tools and expertise to diagnose the hesitation issue and find the best path of repair.

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