my check engine light came on and the code was P0171, which means its running to lean, I have been getting bad gas mileage around 12 mpg and I have also noticed the engine will run rough from time to time. I think it might be a fuel injector issue but not sure. I have no idea how to go about fixing this problem and any help is much appreciated.

thanks

 

What motor,year, and milage?

Have you changed the fuel filter?
Did you check the PCV system for leaks?
Any mods?
Exhaust leaks/changes?

Help us out here so we can help you.

 

It's 1998 4.2l v6 with 155000 miles

 

Have you fixed the potential of hydrolock on your motor?

You be need to swap you intake gaskets for the redesigned version.

 

^That and check your PCV hoses for leaks. IIRC the 98 has the aluminum plenum, but if it's the composite then it may need isolator bolts.

The P0171 can be caused by all sorts of things, even an exhaust leak on that bank. When you have unmetered air entering the motor the computer and O2's will pick it up as lean and then richen up the fuel to compensate, so it may be actually running rich and that is where the milage goes.

First off like I said check the PCV system, then clean the MAF and maybe change the fuel filter-clear the codes and see when and how soon they come back.

At 155,000 it's probably ready for some maintainence.

 

Isolator bolts didn't come until 2000. I think that's when composite plenum debuted too.

As jgger said, just try knocking out all possible causes linked to that code. Most of them are easy inexpensive ones, some a little more time consuming. The intake gasket can be done for $50 and about half a day of work since no isolator bolts are involved.

 

The check engine light comes back on after about 10 miles after I reset it. I now have two different codes the P0171 and P0174 which is the same thing but on the other side of the engine. I also pulled out a spark plug and it had some black residue on it, which i think may mean the engine is actually running rich. im thinking that there might be a leak somewhere in the intake system which is bringing in air that is unaccounted for and thats where the O2 sensors are picking up the extra oxygen in the exhaust and so its compensating for that by injecting more fuel into the engine causing it to run richer and that explains why I'm getting horrible gas mileage. I think the fix might be to get a new intake system. any other ideas?
thanks

 

If it is the aluminum plenum then it's probably the intake gasket.

If it's the composite plenum then plenum gaskets and NEW isolator bolts.

Have you totally checked your PCV system (including a new motorcraft PCV valve)? Check every hose and fitting.

 

That's what happened to my 5-month gas fiasco. I used fuel injector cleaner and a day later I dropped to 9-10 mpgs. It clogged my rear cats and I had new ones welded in and viola!

 

I finally fixed it, I found two pcv elbows with big holes in them so I replaced them and everything is running good now

 

Good to hear-I love cheap fixes!