My 98 Legacy L wagon (auto, 110k) continues to get poor highway mileage. My commute: 13 miles, 95% highway, typically 60-70 mph w/ cruise control. Some slower stretches, but it's certainly not gridlock. Mileage: 20 mpg. Car is rated at 30 mpg! I am not leadfooted by any means. Air filter is a K&N, fuel filter is 6 months old.

 

I just had the spark plugs & wires changed when a plug went bad (they had a warranty). No change in mileage. The car has synthetic fluids (amsoil 10w30, royal purple 75w90 in diffs) and had a complete tranny rebuild in January. Tires are ex. condition Nokian WR's.

 

What could be the cause of such terrible fuel economy? I've seen several past posts where people complain of the same issue - has anyone ever pinpointed a cause?

sounds like maybe an o2 sensor.

sounds like maybe an o2 sensor.

 

I changed the front O2 sensor a few months ago...

Also inspect your pcv valve. I just replaced mine(00 OBW) and have consistantly gained about 30-40 miles per tank of gas since.

Also inspect your pcv valve. I just replaced mine(00 OBW) and have consistantly gained about 30-40 miles per tank of gas since.

 

Good idea! I just checked it, and while it's pretty dirty it does give that little click when you stick your finger on it. Since they only cost like $3 I will replace it today. I think I will also run some Seafoam through the throttle body to try to clean that out. Can't hurt, right?

Seafoam is an excellent idea, but do it through a vacuum line not the TB. Also might try cleaning your MAF sensor (gently, gently!), IACV and maybe even the TPS. A new Engine Temperature Sensor--the one for the ECU not the guage--made a big difference in running for me. Good luck.

... Since they only cost like $3 I will replace it today...

 

I've seen a lot of people recommend using only the Subaru part for this, and while it's still not a lot of money, it's definitely more than $3. Seems like it was more like $12-$15 or so? Can't remember exactly, but I replaced mine a while back.

 

Steve

And don't forget to check your tire pressure.

 

Olnick, any tips for cleaning the MAF?

Tire pressure is ok, and I have cleaned the MAF using spray electronics cleaner.

 

I will this week replace the PCV valve, clean the throttle body (using the vacuum port), and will look into the engine temperature sensor. I am leary of changing that since I had a heck of a time preventing air bubbles in the coolant system when I swapped out the radiator thermostat a few months ago.

JohnnyB--just what bfg9k said. I disassembled the intake ducting, removed the MAF box and sprayed the wires (used brake cleaner, hope that was okay!). I'm just very leery of touching those fine wires with anything, even a Q-tip.

 

bfg9k--I don't recall losing any coolant when I replaced the temperature sensor, but I might have. If you do lose some, use Setright's "front-end-uphill" technique and carefully burp the upper hose as you top up.

 

Brings up a question I've always had--why didn't Subaru provide a bleed valve at the hgih point of the cooling system to allow the release of trapped air? Might have even saved a few head gaskets along the way!

bfg9k--I don't recall losing any coolant when I replaced the temperature sensor, but I might have. If you do lose some, use Setright's "front-end-uphill" technique and carefully burp the upper hose as you top up.

 

I replaced the PCV valve today without any trouble. The way the temp.sensor replacement is described in the Hanes repair book it sounds like a lot of coolant leaks out. I'm glad to hear this is not necessarily the case . I will try that next if cleaning the TB this weekend doesn't help.