Greetings Everyone! I am new to the group and looking for a little help with a couple of problems on my truck. I have a 99 GMC Sierra 1500 Z71 with 195K miles. Last year during the winter two different times the SES came on and when I checked the codes I got P0171 & P0174 which are "system too lean bank 1" and "system too lean bank 2". The two times is happened were when it was very cold and the vehicle had been sitting for an extended period and was very cold soaked. When the SES came on everything else worked fine and it reset a couple days later, so I didn't really worry about it. This year whenever it is very cold and the truck has been sitting for several hours and gets cold soaked, the SES comes on with the same codes. Some of the times when this happens the engines surges 200-400 rpm at idle. It runs fine when driving and idles fine once it is warmed up. After doing a little research and talking with some people, I decided to change the MAF sensor and that seemed to fix the problem for about two weeks. The other day we had a snowstorm and it got down around 15 F and the problem is recurring. The plugs, wires, fuel filter were all changed within the last 15,000 miles earlier this year. Any suggestions as what to look at next?

 

typicaly(sp?),its starving for fuel when BOTH banks go lean...it could be a vacuum leak,but I'D BET ON A WEAK PUMP...TRY GETTING THE PRESSURE CHECKED

 

second on the vac leak, but also check air intake tube downstream from the MAF sensor, before the throttle plate. Leaks there will cause troubles too