High RPM with regular gas and other problems

Asked by Oct 05, 2016 at 08:44 AM about the 2004 Mercury Sable LS

Question type: Maintenance & Repair

I have a 2004 Mercury DiSable LS with 106,000 miles.  
Lots of work has been done within the last 15,000 miles
such as a new transmission, spark plugs, coils, struts, ball
joints, bearings and as if this morning even the fuel
pressure sensor.  It doesn't run bad but it's not great
either.  My fuel mileage is 20-21 mph, at best with
premium gas.  If I run it on regular gas, the RPMs shoot
up to almost 4000 regardless of speed.  This started a few
months after the transmission replaced and possibly after
the coils (can't quite remember).  The RPMs are fine on
premium.  If it matters, I ALWAYS get gas from the same
place when possible.  
Before all the problems started, I was getting about 28
MPG and now, like I said, I'm lucky to get 21 MPG on a
good day.  My mechanic brother-in-law said this is to be
expected given the age and miles but he also said it's not
a DOHC despite it being displayed as such so I don't
really trust him completely (or at all).  I'm hoping the $100
fuel pressure sensor that took me all of 3 minutes to
replace will help out somewhat but that remains to be
seen.  Is there anything else I should do and/or check?   

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