Subarunation 713 Posted January 9, 2007 Share Posted January 9, 2007 Here is an interesting problem. I have a 96 OBW 138,xxx miles, 2.2 5 speed. I towed a 6 x 10 tandem axel U-haul trailer (about 900#) with a foosball table in it (about 250#). 125 miles there empty and with the table 125 miles back. I got 19 mpg. Understandable, the trailer was heavy and offered a lot of wind resistance. Now for the last 2 weeks all we are getting is 21-22 mpg combined driving where before we were getting 25-27 mpg. On Sunday morning I had to go about 220 miles, 110 each way, on I-94. Very little traffic and it is really pretty flat across southern Michigan. Cloudy with temps steady at about 33-34 degrees. I filled it full (topped off full) drove to Marshall and back and filled it again (same way) and on the open road going 70-72 mph with cruise I only got 24 mpg. 3 weeks ago that would have been 30-31 mpg. Same drivers, same basic weather, same basic gas from the same basic stations, same basic destinations the only thing different is the bad MPG. Your thoughts of what might have happened and how it might be fixed? Thanks, Greg btw-to the search nazis ;-) I did do a search about MPG and read tons about gas and 02 sensors and filters etc, but I didn't see anything about an abrupt change from weight/towing and then not going back to previous. If you know or see something please pass on the links. Link to comment Share on other sites More sharing options...
idosubaru Posted January 9, 2007 Share Posted January 9, 2007 you'll want to keep an eye on it. one or even a couple tanks worth may not show accurately what you're getting. if it's only been a week, then it's likely only been a tank or two right? that's not enough to verify what mileage you're really getting. your next tank could easily be fine...or suck even worse! towing shouldn't affect anything. the only thing i can think of is if you used your brakes alot maybe your calipers are dragging some. i've had vehicles get substantially better mileage after just installing new pads. my guess is greasing the slides helps the caliper keep the pads off the rotors better. at 138,000 miles greasing the slides on the calipers may not be a bad idea. could have just been coincidental or the extra heat from towing could have caused it to come sooner than had you not been towing. that's a very light load, i think this is mostly coincidental. i would bet your low mileage would have come had you never towed....might have been delayed a little longer, but probably would have come anyway. Link to comment Share on other sites More sharing options...
Subarunation 713 Posted January 9, 2007 Author Share Posted January 9, 2007 you'll want to keep an eye on it. one or even a couple tanks worth may not show accurately what you're getting. if it's only been a week, (snip) the only thing i can think of is if you used your brakes alot maybe your calipers are dragging some. (snip)Thanks grossgary, It has been over 5 tanks in the span of 2+ weeks. That was one of the dead giveaways. We were spending our spare time at gas stations :-) There is no smell of brakes but I will check the feel of the wheels after driving and see if one of them is a little warmer then the rest. My 99 legacy 2.2 is getting the same mpg now as before Christmas. That is why I feel it is not gas or weather. Link to comment Share on other sites More sharing options...
idosubaru Posted January 9, 2007 Share Posted January 9, 2007 i doubt you'll find anything by your senses. i think a lightly dragging caliper can reduce mileage without any other side affects. on the ones i've encountered the pads didn't even show any sign of abnormal wear nor did the rotors need turning. but the new pads and grease helped. it was always an after-thought, the next couple tanks would be better gas mileage. Link to comment Share on other sites More sharing options...
robm Posted January 9, 2007 Share Posted January 9, 2007 Maybe resetting the ECU would help? Link to comment Share on other sites More sharing options...
OB99W Posted January 10, 2007 Share Posted January 10, 2007 Maybe resetting the ECU would help? Assuming that the clutch didn't get "unhappy" due to the extra half ton and is now slipping, the concept that the ECU learned some bad behavior isn't unreasonable. If that's the case, it should correct itself after a bit more "normal" driving, or resetting it might indeed be worth trying. Link to comment Share on other sites More sharing options...
1 Lucky Texan Posted January 10, 2007 Share Posted January 10, 2007 Maybe resetting the ECU would help? I'd try this (the reset) and feeling of all 4 rims after some extended driving to see if one is hot from a dragging brake pad. Link to comment Share on other sites More sharing options...
Subarunation 713 Posted January 10, 2007 Author Share Posted January 10, 2007 Assuming that the clutch didn't get "unhappy" due to the extra half ton and is now slipping, the concept that the ECU learned some bad behavior isn't unreasonable. If that's the case, it should correct itself after a bit more "normal" driving, or resetting it might indeed be worth trying.The clutch "seems" normal but then people tell me I appear normal too :-) I will pay a little better attention to it and see if I get a hint of clutch smell. I will reset the ECU. Thanks robm, OB99W and 1 Lucky Texan for the advice on that. It is a simple thing to check/rule out. BTW-where did this concept come from that the ECU "learns" driver styles? I have seen the same thing posted in a bunch of the other gas mileage threads when I was searching. I don't recall this beeing soemthing we talked about 3+ years ago? Is this true on the new cars? New like 04, 05, 06 and 07? My car is 11 years old. If this were true of the cars my age I am sure we would have talked about the ECU learning stuff back a few years ago but I don't recall it in posts from when I first stumbled across USMB. Could this be internet legend and not "actual factual"? It has had me curious for a few days. Link to comment Share on other sites More sharing options...
OB99W Posted January 10, 2007 Share Posted January 10, 2007 [...]BTW-where did this concept come from that the ECU "learns" driver styles? [...]Could this be internet legend and not "actual factual"? It has had me curious for a few days. It's not internet lore. The ECU stores certain parameter data based on what it "learns" as the car is driven; examples that may be germane are fuel trims. If it didn't store those averages, the engine management system would have to respond instantaneously to all the sensors, requiring a much more powerful computer and probably still leading to uneven response characteristics. Link to comment Share on other sites More sharing options...
Subarunation 713 Posted January 10, 2007 Author Share Posted January 10, 2007 It's not internet lore. The ECU stores certain parameter data based on what it "learns" as the car is driven...HOLY COW! I did some searching and talk about a range of answers. From what it appears my 96 will track knock sensor, MAP and O2 sensor input (but this is from the Web). Some say this happens quickly, some say 15 minutes some say over the period of days. I wish I could find something from Subaru. I even read where the ECU can only be reset so many times? Huh? God bless the internet! I will reset the ECU and see what happens. I hope it isn't the last reset allowed before the ECU self destructs Mission Impossible style :-) Link to comment Share on other sites More sharing options...
OB99W Posted January 10, 2007 Share Posted January 10, 2007 [...]I wish I could find something from Subaru. I even read where the ECU can only be reset so many times? Huh? God bless the internet![...] You may be getting conflicting information because '96 was a time of transition for the auto industry. In order to meet more stringent emission standards, better engine management systems were developed. The ECUs of "newer" cars include OBD-II (on-board diagnostics, second version). First came non-OBD, then OBD-I; the '96 Legacy incorporates OBD-II. Here are a couple of links to articles that might help clarify things a bit: http://endwrench.com/images/pdfs/EWPreOBDAug05.pdf http://endwrench.com/images/pdfs/OBDEWWin05.pdf Link to comment Share on other sites More sharing options...
newsance Posted January 10, 2007 Share Posted January 10, 2007 Almost all fuel injection systems rely on some version of a "long term fuel trim". Essentially, this is designed to account for variances in operation between different vehicles to allow for smoother running and more tolerance for wear. Essentially, the long term trims apply an adjuster to the base reference tables that determine fuel delivery given airflow, throttle position, rpm, and all other factors. As the computer sees consistent rich or lean situations over a period of time it will update the long term trims to either add or subtract more fuel from that entry in the fuel map. Over time, it will continue to change until it reaches a relatively stable map that gives the desired A/F ratio with minor adjustments. This allows it to account for an injector that flows more or less fuel than it should, or vacuum leaks, reduced compression, and many other factors. Some will even account for engine knock in determining whether to add more fuel. So, it is entirely possible that the trailering created different running conditions that caused the computer to update the long term fuel trims based on the increased load, and it has not relearned "normal" values yet. Link to comment Share on other sites More sharing options...
nipper Posted January 10, 2007 Share Posted January 10, 2007 Do a hard reset of the ecu. Leave the ecu or battery unplugged for an hour. Next do a clutch test. Try to start the car off from 3rd or 4th gear the car should stall. If it stalls then the clutch is fine. being one of the search nazis , i wouldnt tell you to do a search on it as this is a different question then what we have had in the past. nipper Link to comment Share on other sites More sharing options...
Subarunation 713 Posted January 10, 2007 Author Share Posted January 10, 2007 You may be getting conflicting information because '96 was a time of transition for the auto industry. In order to meet more stringent emission standards, better engine management systems were developed. The ECUs of "newer" cars include OBD-II (on-board diagnostics, second version). First came non-OBD, then OBD-I; the '96 Legacy incorporates OBD-II. Here are a couple of links to articles that might help clarify things a bit:http://endwrench.com/images/pdfs/EWPreOBDAug05.pdf http://endwrench.com/images/pdfs/OBDEWWin05.pdf Thanks a TON! OB99W My car was manufactured 10-95 and so it fell in one of the "troubled times" mentioned in one of the tech articles. Nipper, I am going to do the "hard reset" as you call it. I take the negative terminal off and leave it off overnight. Not because I think longer is better but because my memory fails and when I go to start the next morning somehow it won't start. I then put the terminal back on and the ECU has the "double secret probation" reset. The seach nazi was in good fun! Link to comment Share on other sites More sharing options...
nipper Posted January 10, 2007 Share Posted January 10, 2007 Thanks a TON! OB99W My car was manufactured 10-95 and so it fell in one of the "troubled times" mentioned in one of the tech articles. Nipper, I am going to do the "hard reset" as you call it. I take the negative terminal off and leave it off overnight. Not because I think longer is better but because my memory fails and when I go to start the next morning somehow it won't start. I then put the terminal back on and the ECU has the "double secret probation" reset. The seach nazi was in good fun! i know it was i think the reset time is ten minutes, but usually everyone deos it over night. This wil also reset the TCU which should help. It takes about 1000 miles (give or take) for it to fully learn you again. nipper Link to comment Share on other sites More sharing options...
Subarunation 713 Posted January 10, 2007 Author Share Posted January 10, 2007 It takes about 1000 miles (give or take) for it to fully learn you again. nipper Ahhhh, if only wives were that easy.... Link to comment Share on other sites More sharing options...
1 Lucky Texan Posted January 11, 2007 Share Posted January 11, 2007 Ahhhh, if only wives were that easy.... would'nt help - even the factory map is set to 'annoying'. Link to comment Share on other sites More sharing options...
nipper Posted January 11, 2007 Share Posted January 11, 2007 HAHAHAHA i'm glad i dont have that OS system to deal with. I just have elderly mother v7.8 which refuses to upgrade. nipper Link to comment Share on other sites More sharing options...
Setright Posted January 13, 2007 Share Posted January 13, 2007 Nice one, Texan Link to comment Share on other sites More sharing options...
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