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Hi. I have a 2005 Avalanche with a 5.3 Vortec engine, automatic transmission. I went on a road trip from about 6300 feet and went to northern Idaho, thru mountains, and ended at around 1400 ft. (Up and down trip). Then on the way home, some 1200 miles later, my engine kind of stumbled at a gas station, at idle right before accelerating. I didn’t notice any reason for it, and runs super while driving. Now it is real bad! Blows black smoke, and stumbles badly. Check engine light is on with engine code of random misfire (PO300). What do you suggest I do?

Get it looked at by a pro A.S.A.P. I suspect the MAF (Mass Air Flow Sensor) may have died. Just find a good shop with a Tech 2 Scanner (OE type scanner) and let them figure it out. We know it is running rich! The question is why at 45K? Seek help soon before you do serious damage to the catalytic converter. The good news is, if the cat converter is damaged, it is covered for 8 years or 80,000 miles by the manufacture.

 

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