About a month ago, my Navi started misfiring. Being that it has over 90k miles, I figured now it's the time to replace the coils and plugs.


The insights from threads on changing plugs/broken plugs/plug removal issues, were a great asset. I ended up with 3 broken plugs (Lisle tool was a lifesaver).


Anyway, my gas mileage now is terrible. Before, I was doing 18-19 mpg on the highway. Now I'm barely getting 14 mpg on the highway, and forget city driving, I'm hitting 9 mpg. I'm thinking I need to replace the fuel injectors, and take a look at the air filter. Is there anything else I should be looking for to get my mileage back to what it was?


I know this ain't no Prius but still, I was actually happy with my sub-20 mpg for this tank of a vehicle!

 

What brand plug did you use?

 

Motorcraft SP-515

 

Was there actually anything wrong with your coils? I've often found that coils don't need to be replaced unless they crack or are otherwise defective. Replacing them when they have no problems is, in my honest opinion, a waste of money.

As for your gas mileage issues, does it feel like it is running rougher, or giving you a check engine light? It could simply be the new coils need to break it, or you may have a faulty connection at one of them. I highly doubt an injector issue. If you kept the old coils, I'd suggest putting them back on to see if your issue goes away. Never a bad idea to backtrack and see if you perhaps did something wrong.