Buick Repair Questions? Ask a Mechanic for Answers ASAP
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There are 2 possible and very likely causes for this. First is the fuel pump. If the filter was plugged, it could have caused premature wear on the pump and the lower fuel pressure as a result.
The second possible and even more likely cause is a plugged converter. These engines are notorious for the catalytic converter plugging up. It may set the Mass Air Flow sensor code because the engine is not breathing correctly. I would start with a back pressure test. This is done be taking the front oxygen sensor out and installing a gauge in there to see what back pressure there is from the cat. There should not be more than 1psi at the most.