2004 Civic 1.7L Auto

Was overheating on me after a front end collision. After some time I figured out it was a cracked radiator. Fixed that and it works great.

After a few months my gas mileage started to suffer. I used to get 39 to 40 mpg on long trips now I get 33 pretty consistently on those same trips.

I've done a tuneup (spark plugs, oil change, air filter), timing belt, checked tire pressure, and it's still low gas mileage.

No power loss, runs and starts fine, no codes but the cat is right behind the radiator. What are the chances the cat is partially clogged? Maybe low flow at high speeds...

 

Doubt it is a clogged cat, if so I expect you would notice a loss of power. Did you check the timing with a timing light after changing the timing belt?

 

I haven't but all the maintenance stuff was done after loss of mpg.

 

What is the mileage on this car? If it has over 125k you might want to consider changing the upstream Oxygen sensor.

A malfunctioning sensor doesn't always trigger a check-engine light (at least it didn't for me) and could me causing the engine to run rich (i.e. too much fuel in the air/fuel mixture). This would explain why you might have a fine level of performance but bad gas mileage.

 

Ding ding ding. Next cheapest fix after the 02 sensor is grabbing a test pipe.