naimouasta Posted September 4, 2010 Share Posted September 4, 2010 so i decided to top off my oil and rotate my wheels before my 200mi road trip today . i have about 2800 miles on my oil. i added about a little over 1/4 quart. anyway totally forgot to screw my oil cap back on. i usually just leave it on top of the battery while i'm servicing. i drove like that for 60mi until i stopped at toll booth and smelled oil fumes, then i thought to myself "AHHH shit did i screw the oil cap back on?!?" . anyway pulled over and checked. whew, oil cap still on top of the battery. and there was absolutely no oil splashing anywhere in the engine bay, i was amazed...and thankful . and oil level was still good. one thing i did notice according to my MPG display; i was getting 31.9 mpg, for those 60mi, then it went back down to 29.1mpg for the rest of the trip with the cap on. driving conditions were the same through out and all highway. too much crank case pressure?!? do i need a new PCV? i'm at 62k and i need a tuneup asap; i'm just surprised at the difference in mpg with oil cap on and off. i have a 08 2.5i. where is the PCV on these newer cars? on all the older non-turbo's i know it was directedly screwed into the intake manifold on the passenger side. Link to comment Share on other sites More sharing options...
DRoc7822 Posted September 4, 2010 Share Posted September 4, 2010 I personally believe that you can breathe the wrong way inside the car and the MPG would change lol. I could go on a wild tangent about the fuel system and that trip computer but I wont. All Im gonna say is I dont trust it as far as I can throw it. Link to comment Share on other sites More sharing options...
Brady Posted September 4, 2010 Share Posted September 4, 2010 Don't read into the mileage. The way the Legacy calculates mileage, it's very common for the mileage to "deteriorate" over the course of a tank. Link to comment Share on other sites More sharing options...
naimouasta Posted September 4, 2010 Author Share Posted September 4, 2010 theres a difference between being precise and accurate. i think its precise, not accurate. i think its a useful tool to gauge changes in the condition of the motor. from comparing my calculations vs the MPG gauge, its more accurate when you take long drives, but when you start doing a lot of mix driving thats when the gauge starts to be less accurate, maybe 1.5mpg off at the most. its a "moving average". Link to comment Share on other sites More sharing options...
Brady Posted September 4, 2010 Share Posted September 4, 2010 I wouldn't even call it precise. Every Legacy I've been around, the mileage per the trip computer trends down over a tank of gas. It's a useful tool if you drive similar conditions and are comparing from tank to tank. But I wouldn't ever compare the first half of a tank and the second half of a tank, as my highest mileage has always been on the first 1/4 - 1/2 of a tank. Always. Link to comment Share on other sites More sharing options...
naimouasta Posted September 4, 2010 Author Share Posted September 4, 2010 *shrugs* i dunno. my went up after i got home @ 30.3. i'll go fill it back up tomorrow to see what the "real" avg mpg is. is it just coincidence that it drops within 2-3 miles after i put the cap back on? Link to comment Share on other sites More sharing options...
fx4me Posted September 4, 2010 Share Posted September 4, 2010 When my tank is all city driving, my mpg reading is 1 to 1.5 mpg optimistic, when it's all freeway driving on a tank my reading is usually only about .5 mpg high. Link to comment Share on other sites More sharing options...
BAC5.2 Posted September 4, 2010 Share Posted September 4, 2010 I wouldn't even call it precise. Every Legacy I've been around, the mileage per the trip computer trends down over a tank of gas. It's a useful tool if you drive similar conditions and are comparing from tank to tank. But I wouldn't ever compare the first half of a tank and the second half of a tank, as my highest mileage has always been on the first 1/4 - 1/2 of a tank. Always. Of course your highest mileage is at the top of the tank! There are more miles in the upper portion of the tank! Do you actually know how it measures fuel economy? The ECU knows injector duty cycle, it knows how big the injectors are, and it knows how far you've driven. The ECU monitors distance traveled and divides it by the volume of fuel spit into the engine. You get better economy at the top of a tank because the top of the tank has "more miles" in it, than the bottom. Ever notice, you'll get 100 miles on the first quarter tank, maybe 80 on the second quarter, and then 120 on the remaining half? While you DO need data points to drive the MPG calculation to an appropriate average, what you can't rely on is the amount of fuel actually in the tank. [URL="http://legacygt.com/forums/showthread.php/proper-flip-key-interesti-159894.html"]Flip Key Development Thread[/URL] "Genius may have its limitations, but stupidity is not thus handicapped." - E. Hubbard Link to comment Share on other sites More sharing options...
apexi Posted September 4, 2010 Share Posted September 4, 2010 PCV valve is a pain to see on our cars http://home.comcast.net/~dsmphotos/subie/2.5ipcv.jpg Link to comment Share on other sites More sharing options...
Brady Posted September 5, 2010 Share Posted September 5, 2010 Of course your highest mileage is at the top of the tank! There are more miles in the upper portion of the tank! Do you actually know how it measures fuel economy? The ECU knows injector duty cycle, it knows how big the injectors are, and it knows how far you've driven. The ECU monitors distance traveled and divides it by the volume of fuel spit into the engine. You get better economy at the top of a tank because the top of the tank has "more miles" in it, than the bottom. Ever notice, you'll get 100 miles on the first quarter tank, maybe 80 on the second quarter, and then 120 on the remaining half? While you DO need data points to drive the MPG calculation to an appropriate average, what you can't rely on is the amount of fuel actually in the tank. I'm not talking about the fuel guage though. I know the mileage computer measures off of injector cycle duty. I'm literally comparing to the same trip twice on the same tank. It was about 167 miles to the family cabin. I always got better mileage (per the trip computer that is) on a fresh tank of gas than a tank say "1/2" full. It doesn't make sense to me, and honestly, I don't think the mileage was actually any different if I took the time to calculate it out for miles driven versus fuel used to fill the tank. I'm just saying that if he's using the computer as his measure of whether his mileage actually changed before and after the oil cap was in, I would disregard it. Link to comment Share on other sites More sharing options...
Connect4 Posted September 8, 2010 Share Posted September 8, 2010 Drove an outback last month with 1/8 of a tank left. Thought I had another 30 miles left. It dropped below "E" in about eight miles. Not cool SOA! Link to comment Share on other sites More sharing options...
Mega Users seabass07 Posted September 8, 2010 Mega Users Share Posted September 8, 2010 It gives a total average of every reading. It gives a number around once a second. With a full tank and a clear trip, every input of the gas has a greater effect on the average because it won't get washed out by a lot of numbers. Think of it as exaggerating the average for the first half of the tank you use. If you want to get an accurate reading over a whole tank, the only truly accurate way is to divide the miles on the previous tank by the amount of gas you use to completely fill up. Link to comment Share on other sites More sharing options...
Ozzy Posted September 8, 2010 Share Posted September 8, 2010 Or if you have an Android phone, download "acar." It calculates the mileage for you and you can input service records as well. Link to comment Share on other sites More sharing options...
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