I have a 2005 with a 4.6 that has 106k miles on it. Last year it started missing real bad and threw a code for a misfire on one of the cylinders. I changed that and it fixed the missing problem. But shortly after, I noticed it being sluggish in the morning leaving for work. So I replaced all plugs and fuel filter. It did nothing. Now the truck almost completely falls on it's face until it is warmed up. When it's warmed up, the only problem I seem to have is the 13 mpg it gets. When I bought the truck, it got 20 mpg. I know you lose some with age but that seems like a lot. So my question is now what should I look for? Cats, maybe?

 

Your right that is a lot of drop in economy. get the codes scanned.

It maybe the o2 sensors....but I would clean the throttle body housing 1st and make sure the EGR ports are clear...then clean the MAF sensor & clean the IAC.

Check it over for vacuum leaks..and have the fuel pressure tested.

the cats would last on the list...they could be affected, but, you would have had to let missing/bad coil go for some time to do real harm.

 

It's not showing any codes but I will check the throttle body this week. How would I check for vacuum leaks?

 

for vacuum leaks the dealership or good service garage can perform a "smoke test".

you can DYI with starting fluid ...spray down the vacuum lines..listen to / for RPM changes...usually RPMs will pick up. Where the RPMs pick up is your suspect area. look and flex lines a bit looking for cracks and general deterioration. the elbow on rear of the throttle body housing is a known area for leaking.