Just replaced the T5 in my ‘05. The seller threw a rod and toasted the original at 169K. Replacement has 102K and with lots of new parts (timing-belt, tensioner, water-pump, etc) the car is a very good drive. I did also have the Haldex serviced as well as a new PCV oil-trap.
Disappointed in the mpg. Serious issue at 17-18 mpg. I have just put in a full can of SeaFoam to clean the injectors.
What is common for this vehicle? Current mpg seems quite low as I expect it to be at least as good as most Subarus of the same era. They typically get 22 but at moderate highway speeds I know they can get 25 (I know this from experience). And hat is with AWD Subies.
Any idea on what to check on this car?
Thanks in advance.

 

Are you new to this car? And have no previous MPG readings/estimates from this car?

If your car is rich or misfiring causing too much fuel to be used - the self diagnostics will pick that up and set a code turning on the check engine light. You could check fuel trim to see if OBD thinks it's running slightly rich, but not enough to set a code. (certainly you could spend $$$ on new O2 sensors and possibly get .00x more miles per gallon. But how many miles will it take to pay $$$ for those new o2 sensors in more MPGs/fewer gallons used?)

Does your user name indicate you are from a cold northern state? Winter blend gas also gives fewer MPGs. Wait a month or so until summer blend is on your region and check again.

For magic can injector cleaning - I've always used Techron. (but those additives are already in top tier gasolines)

"What is common for this vehicle?" See below courtesy the US dept of energy. Looks like 16 in the city is normal using premium. For Subarus that info is here
https://www.fueleconomy.gov/feg/bymake/Subaru2005.shtml

 

your gas mileage will also vary if your area has a "winter fuel" blend. On my 850T I would see a 2 MPG difference summer to winter when the northeast would get the winter fuel blend with a bit more ethanol. If you can find a station that sells ethanol free gas, try out a tankful to see what happens. Also what grade gas do you use? If you are using 87 regular, the car will retard the spark and enrich the mix to control pre-detonation which shows up as lower gas mileage. Try running premium 91 or 93 octane to see if that makes a difference.