vehicle is a 2000 5.4 motor with about 170,000 miles. Runs decent at idol speeds, but still misses out slightly. When I approach 40 MPH major loss of all power. The harder I push the gas, the worse it is, almost sounds like the motor is not even running. Codes I have received are P0171/P0174. So far I have tried cleaning the MAF sensor, check for vacuum leaks and new fuel filter, what do I try next? any advice is greatly appreciated.

 

Clogged exhaust???

 

Plugged cats. Take it to a muffler shop for a test. Check the rubber PCV elbow behind the intake manifold/throttle body for holes and rot.

 

I did check that boot that you mentioned.looks like I will be pulling the exhaust apart. will a fuel pump go completley out or can it slow down and not work properly?

 

Sparkplugs and COP's ?

 

Check your fuel Pressure.

If that's OK, Have your Cylinder Head temperature switch
checked to see if it's in Specs.

This was the cause of my No power problem and
both PO171- PO174 Codes.

 

Xcfingers - did you find out anything? I have exactly the same symptoms with a 2000 5.4 hi-mileage engine. This engine did show codes P0171 and P0181 recently, but they have not come back since I cleared them... I'm not sure they are related to the loss of power.

 

could also be the mass air flow sensor, but more likely the cats are plugged.

 

Is it me or are there a ton of p0171/p0174 codes popping up lately. Cold weather maybe?