Steves72 Posted February 25, 2009 Share Posted February 25, 2009 First, let me tell you that the car is a 2000 OBW Limited. This has been occuring since the weather turned pretty cold in mid December. I began having the low fuel light turn on at around 200 to 220 miles of travel into a tank of gas. Normally on summer fuel I would make to around 285 to 305 miles before the light would turn on. While my fuel mileage has dropped on the winter blend of gas this was quite a surprise to have it come on this early. I should also note that the gas gauge did not support the fact that my fuel was that low. I was around half way between empty and the 1/4 mark. Not wanting to take a chance I stopped and filled the car. It took less than 11 gallons the first time I reacted to the low fuel light indicator. This trend has repeated itself since that time. Another instance was when I first started my car one morning in my driveway. The driveway has an incline of maybe 10 degrees and the car was nose up. The gas gauge was midway between 1/4 and 1/2, and the low fuel light came on! It did go out about one mile down the road but something is not right about that light. Tonight I started the car at my work, on flat ground, and again the low fuel light appears. About one mile down the road the light disappears and remained out for the remainder of my commute - around 14 miles. There was just under 1/4 of a tank of gas in the car according to the fuel gauge. Now I had the car on a lift this past weekend doing some routine maintenance and I noticed that there is a double well to the gas tank - one on either side of the drive shaft. Is it possible that the low fuel indicator light works off of an indicator that sits in one of these wells and that for reasons unknown to me that fuel is only getting picked up from that well? perhaps because of a clogged pickup on that side of the tank? If the low fuel light worked on an indicator in one well and the fuel gauge off of a float in the other well, that might explain why the two indicators are out of sync with one another. This is all speculative on my part and someone who knows where things are located in the tank might be able to provide a better answer. Thanks, Steve Link to comment Share on other sites More sharing options...
Cougar Posted February 25, 2009 Share Posted February 25, 2009 There are two fuel level sensors in the tank I believe. They work together to provide a reading on the gauge. One of them may have some bad contacts that is causing the trouble. Some folks have cleared this problem by using some Techron in the tank to clean the contacts. Link to comment Share on other sites More sharing options...
Suzam Posted February 25, 2009 Share Posted February 25, 2009 I used the Chevron's Techron gas additive to help with a lazy gas needle so it may help with the warning light too, although I don't recall ever hearing of one coming on as early as yours. Pick a couple of bottles up at Walmart or Pep Boys. It would be worth a try for a few tanks. Beats starting to pull things apart. Link to comment Share on other sites More sharing options...
crash321 Posted February 25, 2009 Share Posted February 25, 2009 I agree with previous posts, sulpher is probably the culprit, use a good quaity cleaner to see if the contacts clean up. Link to comment Share on other sites More sharing options...
Steves72 Posted February 25, 2009 Author Share Posted February 25, 2009 Why not? It is certainly cheaper than ripping everything apart. I will give it a try. Steve Link to comment Share on other sites More sharing options...
nipper Posted February 25, 2009 Share Posted February 25, 2009 Why not? It is certainly cheaper than ripping everything apart. I will give it a try.Steve Actually it doesnt cost anything but time to get to the fuel sensors. They are extreemly easy to get to (look under the carpet behind the back seats). The technron does work, but sometimes you still need to dig into them and using a hard bencil eraser clean then manually, and even then the results are not stellar. Also keep in mind that i think these are temp sensors. When they are exposed to air they are warmer then sitting in gasoline and triggering the light (I think). The cold temps can be confusing it. Besides better too early then too late Can also be that it is so cold that the volume of the fuel is low enough to trigger the light, but the energy potnetial of the fuel has not changed, so you get the same range. Fluids do expand and contract in temp changes. nipper Link to comment Share on other sites More sharing options...
Suzam Posted February 25, 2009 Share Posted February 25, 2009 Also keep in mind that i think these are temp sensors. When they are exposed to air they are warmer then sitting in gasoline and triggering the light (I think). The cold temps can be confusing it. OK Nipper, I challenge that statement. Why would the fuel be warmer than the air in the tank? The ambient temperature should keep both relatively the same give or take. Besides underground tank fuel is around 54° for the most part, so your light would go off in warmer weather if you fill up with cooler fuel? I know you quantified your explanation with the "I think" sooooo, please enlighten us, oh great purveyor of knowledge. Link to comment Share on other sites More sharing options...
nipper Posted February 25, 2009 Share Posted February 25, 2009 OK Nipper, I challenge that statement. Why would the fuel be warmer than the air in the tank? The ambient temperature should keep both relatively the same give or take. Besides underground tank fuel is around 54° for the most part, so your light would go off in warmer weather if you fill up with cooler fuel? I know you quantified your explanation with the "I think" sooooo, please enlighten us, oh great purveyor of knowledge. HA! This is from cougar http://www.ultimatesubaru.org/forum/showthread.php?t=86194&highlight=fuel+light The low fuel sensor works a little different than the normal fuel level sensor. The low fuel sensor is a thermistor and has no moving parts or contacts that can cause trouble. So when it shows low I would tend to believe it and stop at the next station for a fill up. From wikipedia A thermistor is a type of resistor with resistance proportional to its temperature. The word is a portmanteau of thermal and resistor. Thermistors are widely used as inrush current limiters, temperature sensors, self-resetting overcurrent protectors, and self-regulating heating elements. nyahnyahnyahnyah *whew* cause my electronic skills suck Link to comment Share on other sites More sharing options...
yohy Posted February 25, 2009 Share Posted February 25, 2009 Why I will always read a post from nipper, thanks for your knowledge and your humor. Link to comment Share on other sites More sharing options...
nipper Posted February 25, 2009 Share Posted February 25, 2009 Why I will always read a post from nipper, thanks for your knowledge and your humor. hehehehe untill cougar posted it i could never figure out how the silly things worked. For all i knew it could have been a really timy floating cork on a switch Link to comment Share on other sites More sharing options...
abog Posted February 25, 2009 Share Posted February 25, 2009 It is the temperature differential that the thermister uses to tell if it is wet or dry. YAY nipper, good call! Butttt, doesn't the temperature difference become relevant to ambient temperature? Sort of just moves down the scale in the cold? I have run into problems with the sensor before, but it seems like they either work or they dont. Link to comment Share on other sites More sharing options...
nipper Posted February 26, 2009 Share Posted February 26, 2009 but it seems like they either work or they dont. That is how most things are, either it works or doesnt. EUREKA! http://forum.allaboutcircuits.com/showthread.php?t=9053 "For a minimally intrusive possibility, you could try a thermistor inserted in the tank at a level, like 1/8 up from the bottom. With enough current (in the milliamp range) thermistors become self-heating. When immersed in the gas, the liquid will carry off heat, so the resistance will stay fairly high. When in air, the resistance will drop. " Google is your friend. Link to comment Share on other sites More sharing options...
abog Posted February 26, 2009 Share Posted February 26, 2009 Aha! Link to comment Share on other sites More sharing options...
Cougar Posted February 26, 2009 Share Posted February 26, 2009 (edited) That is how most things are, either it works or doesnt. EUREKA! http://forum.allaboutcircuits.com/showthread.php?t=9053 "For a minimally intrusive possibility, you could try a thermistor inserted in the tank at a level, like 1/8 up from the bottom. With enough current (in the milliamp range) thermistors become self-heating. When immersed in the gas, the liquid will carry off heat, so the resistance will stay fairly high. When in air, the resistance will drop. " Google is your friend. This is how I believe the circuit works. I haven't tested this myself. The only way I could see the circuit working is using the fuel as a coolant. When the fuel level goes below the sensor, the current running through the sensor causes it to heat up a little, and that lowers the resistance of the thermistor even more. With the series resistance to the light lowered, enough current can now flow to turn on the light. Edited February 26, 2009 by Cougar Link to comment Share on other sites More sharing options...
Suzam Posted February 26, 2009 Share Posted February 26, 2009 OK it all makes sense now, I couldn't figure it out until the self heating part came into play. So even though it was Cougar's original Nipper gets a nod as well. Good Job everyone! Link to comment Share on other sites More sharing options...
forester2002s Posted February 27, 2009 Share Posted February 27, 2009 I'm relieved to find out how this circuit works. I was worried at the thought that there were bare electrical sparky-contacts in my fuel tank. I kept saying to myself: "Surely not!" Link to comment Share on other sites More sharing options...
nipper Posted February 27, 2009 Share Posted February 27, 2009 I'm relieved to find out how this circuit works. I was worried at the thought that there were bare electrical sparky-contacts in my fuel tank. I kept saying to myself: "Surely not!" er um Well................... yes there are. Its called the fuel level sensor for the gauge. But buillions of cars have been like that because its a variable resistor that is in constant contact with the wiper arm of the sendor for no sprak. Also in a sealed gas tank the tank isnt exploive unless you add oxygen. Worry more about getting hit by lightening. nipper Link to comment Share on other sites More sharing options...
Steves72 Posted February 28, 2009 Author Share Posted February 28, 2009 So .... Is the thermistor and the float device, or whatever operates the gauge, in the same well? or is one in one well and the other in the opposite well? Remember my original question that it appears to me that the OBW has two fuel wells in its gas tank, one on either side of the drive shaft. Now, if and only if, these two separate measuring devices were in different wells it would be possible for them to be out of sync. Let's say one pickup in one well failed to pick up any fuel, this would cause one well to be drawn down while the other well stayed full. If the float was in the full well and the thermistor was in the empty well, I would be experiencing many of the symptoms I have described. Steve Link to comment Share on other sites More sharing options...
nipper Posted February 28, 2009 Share Posted February 28, 2009 (edited) In an AWD sooby you have two fuel senders and one light sensor. A left float amd a right float. They dont get out of synch, as they are are independent vairable resistor devices. There is alos a "jet pump" (siphon) that shifts fuel from one side of the tank to the other. Either or both sensors can be fouled by sulphur. They are wired in series. nipper Edited February 28, 2009 by nipper Link to comment Share on other sites More sharing options...
Steves72 Posted February 28, 2009 Author Share Posted February 28, 2009 Thanks for the information about how the tank operates. Now I know the thermistor must be the issue. I think I will try the Techron. You also mentioned accessing the fuel sending units under the carpet behind the rear seat. I will take a look there. Am I looking for an access panel down to the top of the fuel tank? Thanks Steve Link to comment Share on other sites More sharing options...
nipper Posted February 28, 2009 Share Posted February 28, 2009 There is a very obvious access plate on either side of the car. Then under thoe plates are the fuel sensors. Be careful as they are small nuts and easily lost. Be patient pulling out the sender as it will take some manuvering. nipper Link to comment Share on other sites More sharing options...
johnceggleston Posted February 28, 2009 Share Posted February 28, 2009 (edited) nipper, what is the down side to eliminating the sender that's not working well, you'll have less fuel than indicated?? the light will still work, your trip odo will still work, it shouldn't be hard to learn that 1/2 means empty. if one side is bad, just use the other, not both. if you did this, would adding a resistor for the missing sender make the gauge more accurate? no the resistance varies. more accurate when ful;l but less when empty, or vice versa. Edited February 28, 2009 by johnceggleston Link to comment Share on other sites More sharing options...
nipper Posted February 28, 2009 Share Posted February 28, 2009 Gauges are for refernce only. Also it may not be that one is bad, but that one has an innacurate spot in it, or they both do. I would never recomend just removing one. Its worse then having an innacurate sauge, because the gauge is predictable. nipper Link to comment Share on other sites More sharing options...
bulwnkl Posted February 28, 2009 Share Posted February 28, 2009 Techron is believed to clean sulfur deposits off the contacts, huh? I've had 2 relatively late-model Subarus whose fuel gauges read lower over time (3 if you count the XT6), and only 1 which didn't (got rid of it at under 12,000 miles). One of them wouldn't read much above 3/4 even when stuffed full to the point of spilling. I use other products that clean injectors and such as well as or better than Techron in every tank. The issue happens anyway. Subarus are the only vehicles I can call to mind that I've had this issue with in my lifetime, excepting a '76 Dodge pickup which was that way from new. Why are Subarus (at least some of them) so much more prone than other makes to having their fuel level sensors/senders accuracy degrade over time? Link to comment Share on other sites More sharing options...
nipper Posted February 28, 2009 Share Posted February 28, 2009 Techron is believed to clean sulfur deposits off the contacts, huh? I've had 2 relatively late-model Subarus whose fuel gauges read lower over time (3 if you count the XT6), and only 1 which didn't (got rid of it at under 12,000 miles). One of them wouldn't read much above 3/4 even when stuffed full to the point of spilling. I use other products that clean injectors and such as well as or better than Techron in every tank. The issue happens anyway. Subarus are the only vehicles I can call to mind that I've had this issue with in my lifetime, excepting a '76 Dodge pickup which was that way from new. Why are Subarus (at least some of them) so much more prone than other makes to having their fuel level sensors/senders accuracy degrade over time? Its not just subarus, other cars do it also, but realize that we also keep our cars much longer and much more aware of what they do then other drivers are. It also happend regionally. Now with cleaner fuel its less of a problem, but there have been a few incidents of fuel with too much sulphur in them (happened in FL and TX) that ruined alot of fuel senders and the gas supllier had to cough uup money for repairs. Soobys with over 150,000 miles on them is common place, as oppsed to almost any other brand (excluding volvos). I am sure if you had a bunch of nissans or chevys rolling around with that many miles on them in large numbers, they would show the same issues. Usually by that time any other car has so many things falling off them, no one notices the gas gauge as much, if it even works. nipper Link to comment Share on other sites More sharing options...
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