01 Chevy Prizm hesitates when engine cold

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Apr 26, 2007
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2001 Chevrolet Prizm sometimes hesitates on acceleration when engine is cold. Peculiarly, it only seems to do this during warm weather (50+).
We've taken it in twice to a local repair shop. The first time they checked the fuel, ignition and computer control systems and replaced the mass air flow sensor. The second time they replaced the fuel pump. Neither fixed it.

Recently, I found the following technical service bulletin on NHTSA:
Service Bulletin Number: 3053A
NHTSA Item Number: 10009598
Summary Description: Hesitation on acceleration while the engine is cold and possible P0171 diagnostic trouble code. *TT

Any ideas?