Thread: MPG issues
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#10: 01-26-2013, 09:50 AM
mnstrmech's Avatar
Title: Contributor
Location: South Bend
Car: 1993 legacy wagon
Posts: 406
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GET RID OF THOSE HORRIBLE BOSCH PLUGS!!!! they are a joke. Just get the NGK copper plugs that should be in there. Copper burns hotter, so you will get a more complete burn. Trust me on this one, Im an ASE certified technician.
Easy way to know if your cat is working is to look for moisture coming from the tailpipe, if you have moisture, your cat is working. Water is the by-product of a working cat.
18-19mpg in cold weather on the highway is fairly normal for your legacy. That was the mpg i was getting before i did a great mod that i will explain. Which brings me to your rpm's.
Next trip keep an eye on your rpm's. I learned about this myself. 2800 rpm's is a bit high at 70 mph. My 93 runs around 2400 fully warmed up. What i mean about this is the Subaru's ecu keeps the transmission out of torque converter lock-up (tcc lock-up) to warm the transmission fluid up faster, and takes a while before kicking it in. I noticed my tcc wont lock in for about 10 or so miles, KILLING my mpg's.
I posted in this post
on how to re-wire the "manual" button on the shifter to command the TCC on.

it was quite easy.... the "manual" button conversion i did a while ago is easy too. On the TCU module (under the dash, just next to the steering column), the one 20pin connector thats closest to the front, pin 6 is a blue w/ yellow stripe, that is the "manual" control. I snipped that wire, left at least 1 inch on the pin, and the other side of the wire i connected to a relay to turn the ground signal (which the other end of the blue\yellow wire runs to the switch, which goes to ground) into positive voltage. take the switched side of the relay that is positive and tap into the wire for the torque converter wire... which i believe is the red/yellow??? Dont quote me on that...
I should do a schematic for this.............................................. ...

"power" mode is easier.... i took the pin from pin 6 (the short 1" blue/yellow wire) and put that two spots over to pin 4... ran that to a switch that the other side is ground... so flip the switch... that pin see ground.... and power mode is active...... VOILA'!....... lol....

BTW.....IF you do the TCC switch mod..... DO NOT TOUCH THE "MANUAL" BUTTON ON YOUR SHIFTER TILL YOUR AT LEAST 40-45MPH!!!!! once you push the button... the TCC will kick in... and the amber "manual" light on your dash will light up....
I did this mod cause i drive on the highway about 20 miles one way to work, and i noticed the rpm's run about 2700 or so at 65... then when i was about to get off the highway... it was 2200... so i researched and found the tcc doesnt get locked up till it hits a certain temp,,, KILLING mpg.. so i went from approx 17/18 mpg to 25mpg by kicking on the tcc as soon as getting up to speed on the highway...... just got to remember to turn off the switch before slowing down on the ramp.......
if ya want pics... gonna have to wait...