every since i put the efan on my '97 4.0 my computer is giving a lean code....PO171 and PO172......i had this problem about a year ago and fixed it with the gaskets....could it be because there isn't a fan pulling the engine down a little bit? the truck isn't running any different that before?

 

The hotter an engine runs the leaner that it will run, its possible. There is fine line to walk there though. More power and performance when the engine runs hotter but too hot and boom. I never had any issues but my fan was not enough anymore after some recent events. I have since removed it and went back to stock.

 

i have the fan set to come on at right about were the stock gauge would be in normal conditions.......it prolly comes on a little lower than normal........

 

I think the lean codes are unrelated. Look for a manifold vacuum leak, an unmetered air leak or a contaminated MAF first, sluggish O2 sensors after that. A common leak to check for could be in the tubing to the PCV.

 

its very unlikely that the fan could cause this condition, i would do what rwenzing says

 

i put the gasket inbetween the upper intake and the lower intake last winter when i got the codes then, that solved the problem......my MAF is built into the air filter housing.......i will check the PCV tomorrow...