2005 F150 XLT 5.4L. I recently did a full plug and COP change on all 8 cylinders at 100k. Before the change, I was getting about 16.5 MPG with the truck misfiring on accel. After the change, the truck runs smooth with no misfire, but now am getting around 14 MPG on average, driving the same, no other changes to the truck. New fuel filter 3k before the change.

The codes for cylinder misfire are gone, but am now getting a catalytic converter code. I have tried pulling the battery to reset the system but no change. Could the cat be what is causing the MPG decrease?

 

Yes the cat could be contributing to the decrease. The codes should be the first priority before worrying about mileage, the mileage is probably just the symptom of the bigger problem. How long did you let the misfire go before changing everything? If let go for a while, excess fuel from a misfire can land on the cat and burn there, melting the combs in the cat. Also did you use Motorcraft plugs and COPs?

 

The misfire was going on for about 2-3k, but only under harder acceleration. If I stayed light on the gas, I wouldn't notice a misfire.

 

Sounds like a clogged CAT....answer what Otto asked you about the COP's and plugs you installed....

 

Sounds like new cats are in order, but get a flow test to confirm. A low grade misfire still allows some fuel past. The PCM is slow to throw a code, so just because only hard acceleration trips the CEL, the problem was still there at lower throttle positions.

 

Oops, missed that line in Otto's response.

I used Motocraft plugs.

I don't remember the brand of COPs used. They were purchased of Ebay bsed on the reccomendation of several people here.