When I bought my Prius used a couple years ago it had a P0171 "system too lean" error. Over the past couple years I tried different fixes that had no effect - replacing MAF sensor, changing spark plugs and fuel injectors. They also didn't seem to affect the fuel economy, which hovered around 46mpg as measured by the amount driven (odometer) / amount of fuel refilled. A couple months ago I tried exchanging the fuel pump with a used one from eBay - immediately my LTFT dropped from ~+30% to around 0, and since then the check engine light hasn't come back on, which all seems to indicate the original P0171 error is fixed. However, exactly after the fuel pump replacement, my fuel economy has sunk to around 42mpg. From other threads where people fixed their P0171 by replacing the fuel pump, they reported significant increases in fuel economy, so I was expecting the same. Any ideas what the issue might be? Maybe I should've just kept the error. I don't have smog checks in my state anyway. It's like I paid all this money to troubleshoot and make the error go away, only to effectively decrease my fuel economy and nothing more...
You probably have a small evap leak that will show up after a couple more drive cycles. If it does you'll have to put some silicone grease on the seal
This may have happened to me too. I replaced the fuel pump in my 2012 with a cheap one when I was trying to track down issues with misfires. It turned out to be counterfeit plugs, and putting the old ones in fixed that. However, fuel economy is down from about 52mpg to 47mpg. Did you ever resolve this?
I got new plugs from Ebay, but on close inspection they appear to be counterfeit. I figured out something was up after uncontrollable misfires in cylinder #4. I was just getting back from a 500mi trip when things started going bad, and I managed to get the last few miles home (mostly downhill. The engine was a JDM low mileage swap that I had just completed. On the long drive I was getting a bit over 50mpg and going about 70mph much of the way. With the newer fuel pump in and the old plugs I can't seem to get above 46mpg under the same conditions. I do not have a baseline for the older plugs, but getting new ones did not seem like a bad idea. That is, until the misfires. I have some Denso twin tips on order from rock auto now, and hopefully that will help out.
