Mu engine will occasionally lope at startup. If I look at my scanner the fuel trims will suddenly go negative up to - 20 and then the engine smooths out. It seems to happen in open loop and when in closed loop the fuel trims balance out. I changed my maf sensor, but it did nothing. I don't smell any gas, no Check engine light, any ideas?
1998 Chevy Venture LS
240,000 miles
Engine replaced at 165,000
Platinum plugs replaced at 165,000
Plug wires replaced at 190,000