Check engine light is on?

Assuming it is the cause of the issue, burn it out.

what is the code?? This guide will help get it

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

Please run down this guide and report back. Roy

Drive it out.

Once again, I need the code

Roy

The 2009 Ford Taurus is not flex fuel, So if you decide to burn it out, Here is what will happen, The vehicle powertrain management can not handle it, What makes a flex fuel so different is all flex fuel vehicles has a fuel composition sensor which you do not have, So if you decide to use it you will have no power, yes the check engine light will appear as lean banks, Here is what will happen on the engine management side, The fuel trim pulse width injector will increase, The knock sensor will retard the timing, So no power on the streets. You can use this fuel it is suppose to be the future fuel. Who can not use this fuel is carburetor cars, And vehicle that require 92 octane. And by the way where did you purchase this fuel? And I call this the big lie fuel supply.

What gaskets were changed?

You did not have any lean bank 1 or bank 2 codes?

The best way to look for a vacuum leak is have a smoke test performed did they do one of those on it?

I have to collect information I am not there helping people fix their car over the internet is very challenging. All I have to rely on is what information you give me. Most of the time I have to ask questions beyond what information is given. You would hook up the smoke machine hose to the hose that goes to the brake booster. Make sure you block off the throttle body before you start smoking it. I cannot tell you the problem with the information I have so far. Bad fuel can cause a random misfire have seen that before. Also I do not know what the last guy found what exactly he did to fix it etc. So again the need for all the questions. You could have a vacuum leak that could be a lot of different places so that is the need for a smoke test. Also you could have a egr valve that could be sticking partially open. I am here to help people with their car problems. Giving my free time to help people in need.

It is the big round black part on the drivers side mounted to the firewall with the one large vacuum line going to it.

I wish I had a picture of that, you want one that leads directly into the intake manifold.