I have asked about P0171 and P0174 lean codes before. Now it is turning cold again and my truck kicked the codes again. Is there anything that I can or need to do to correct the problem. My truck is a 99 5.3 with a custom cold air and 2.25 split at the y pipe with no muffler.

 

The intake gaskets are leaking and causing a vacuum leak. If you have a scan tool to monitor O2 sensor voltage you can confirm this buy spraying carb cleaner around intake gaskets and watch voltage. Try doing it on cold start once it just goes into closed loop. Before spraying to will see sensor voltage low and once you spray them voltage will go to 900mV +.

 

I have checked for a leak and never found anything. I have read that the ethonal blend fuels cause cold start issues for older vehicles, and thought that maybe with the cold air I built it was allowing to cold of a charge in. The only time it has kicked the codes is when it gets really cold. Once the truck is up to temp it runs fine. I was thinking that bigger injectors or a tune might cure the problem but wasn't for sure.