So recently my 2004 f150 FX4 has been running rough. Checked vacuum lines no leaks , no codes except maf but it's new. I just saw where my inlet hose connection for the heater core has been leaking into #3 cylinder, could this be the cause of the rough idle ?

 

It sure can. Fix it, replace the plug (replace them all if they have more than 60k on them) and coil. What is the exact numeric code?

 

Gave a p0171 lean code. Replaced all the plugs and coils about 10,000 miles ago . Will replace the ones that got wet

 

P0171 is *not* a MAF code. It's usually a vacuum leak but it may go away when you stop the misfiring.