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2005 Subaru OBW 3.0R LL Bean
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Discussion Starter #1
I just purchased a 2005 Subaru Outback 3.0 LL Bean wagon and a week after the purchase I discovered that the fuel gauge will not go past just above the half way mark even though the car is filled up all the way.


The reading on the gauge does go down what seems like normally as you drive the car. In other words, I will not have to drive the car for half a fuel tank’s worth of gas in order for the gauge to begin giving readings. So I guess this means that the half way point, or where it currently stops now, is the new full mark. Unfortunately, once the gauge starts reading “E” empty there is no way of telling how much gas is still in the tank but it is probably just a bit past half full. Does anyone know what I can do to start diagnosing the problem? I do have a warranty on the drive train and engine but I doubt that will cover the fuel gauge and the dealership probably won’t do much to help. Any input will be appreciated.
 

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03 H6 OBW & 06 WRX Sportwagon
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if you have no other symptoms and the check engine light isn't on, I'd assume the problem is confined to the 2 senders in the tank and possibly the gauge. If one of the senders were just gummed-up with schmutz (sorry for the technical terminology) it might behave like this.


So, perhaps a treatment or 2 of SeaFoam or Techron in the tank would clear things up.



Another possible issue, however, involves the fuel tank's vent/vapor system causing the station's fill pump to shut off too soon. Do you feel confident the tank is actually full when it's been filled at the station?
 

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Discussion Starter #3
Another possible issue, however, involves the fuel tank's vent/vapor system causing the station's fill pump to shut off too soon. Do you feel confident the tank is actually full when it's been filled at the station?
I had 3 different people fill the car at 3 different locations and it filled to that point every time so I don't think it is a matter of the pump shutting off prematurely, you can actually hear the gas start to rush up the fill pipe right before the pump shuts off.

I'm a bit sceptical about putting something like SeaFoam in the gas tank I heard it can lead to more trouble sometimes have you tried this and what was the result? I'm wondering if it is fairly easy to get to the sensors in the tank and just clean them by hand or just replace them if it is not too expensive...?
 

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Unfortunately, once the gauge starts reading “E” empty there is no way of telling how much gas is still in the tank but it is probably just a bit past half full.
Not exactly. The tank has a capacity that is indicated in the Owners Manual. When the gauge is at E, fill the tank and see what the gas pump says went in. The difference between what went in and the capacity is what was left in the tank.

Example: I believe the 05 Outback tank holds 16.9 U.S. gallons. When the gauge reaches the E, there's usually a gallon or two left. So when filling a tank that is at E, you should be able to put in something like 14 - 15 gallons. The difference between 16.9 and what you put in is what was left in the tank.

The tank is a saddle bag shape, straddling the drive shaft. So it's like two tanks connected across the top. There's two sensors, one on each side. The fuel pump is on the right side, and a jet pump (siphon) is used to transfer fuel from the left side to the right so that both sides should be fully used (or close to it) when the gauge is at E.

The two sensors are variable resistors wired in series. Their resistance is maximum when the tanks are empty, and very low when the tank is full. If one sensor is stuck at the empty position (high resistance) while the other is working properly, (or there's a bad connection anywhere in the circuit), then when the tank is full, the gauge won't go to the F. As fuel is used, the working sensor begins to lower in the tank, and the gauge will follow. If the transfer pump is working, the tank will reach close to the empty point when the gauge is at E. Thus the value of knowing how much is actually left when the gauge is at E. You might have to try this a few times to get a reliable result, but it could be helpful to narrow down the area of the problem, and if it's a sensor, the most likely one.

The sensors are accessed through hatches in the floor below the rear seat. The left side sub-sensor is part of a fuel pickup. The sensor on the right side is part of the fuel pump assembly.
 

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2016 Outback Ltd 2.5 eyesight Nav push button Hole in roof, Lapis Blue
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If it is really a sending unit sticking then Techron is the way to go, get 2 20 oz bottles (the black bottles not the blue ones) add a full bottle to the tank fill it up. The next 2 tanks of gas add 1/2 bottle each time, that should get the float freed up and if not at least your fuel injectors will be cleaned up. This stuff really does work and is likely the best fuel additive. Seafoam is great to help clean carbon out of the engine SLOWLY introduce 1/2 can via a vacuum line with the engine running, then shut off the engine for 10 - 15 minutes and restart boy will it smoke for a while!
 

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Discussion Starter #6
Not exactly. The tank has a capacity that is indicated in the Owners Manual. When the gauge is at E, fill the tank and see what the gas pump says went in. The difference between what went in and the capacity is what was left in the tank.

Example: I believe the 05 Outback tank holds 16.9 U.S. gallons. When the gauge reaches the E, there's usually a gallon or two left. So when filling a tank that is at E, you should be able to put in something like 14 - 15 gallons. The difference between 16.9 and what you put in is what was left in the tank.


The sensors are accessed through hatches in the floor below the rear seat. The left side sub-sensor is part of a fuel pickup. The sensor on the right side is part of the fuel pump assembly.
That makes sense, when I go to fill the car up it only accepts about 8 gallons of fuel and I was expecting it to be closer to 14 or 15...so there is roughly 9 gallons actually in the tank when the gauge says E "Empty" assuming its capacity is 16.9 gallons.

In trying to understand the system better, I looked at the fuel pump part online and saw that it had a float. You mentioned that there are two separate tanks with two separate sensors that communicate between each other. Does this mean there are two separate floats and one might be stuck in the empty position and does not rise when the fuel tank is filled up? I’m not sure how this variable resistor sensor works, is the resistance based on the position of the float? If so, does it make sense for me to try and see if both floats are free to move and do their job before pouring anything into my gas tank?

I suppose it would be less work to try and use Techron first but I heard of stories where it knocked a bunch of residue loose at once and that jammed up the fuel filter and that sounds like a big problem, car won’t be getting much gas that way…
 

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That makes sense, when I go to fill the car up it only accepts about 8 gallons of fuel and I was expecting it to be closer to 14 or 15...so there is roughly 9 gallons actually in the tank when the gauge says E "Empty" assuming its capacity is 16.9 gallons.

In trying to understand the system better, I looked at the fuel pump part online and saw that it had a float. You mentioned that there are two separate tanks with two separate sensors that communicate between each other. Does this mean there are two separate floats and one might be stuck in the empty position and does not rise when the fuel tank is filled up? I’m not sure how this variable resistor sensor works, is the resistance based on the position of the float? If so, does it make sense for me to try and see if both floats are free to move and do their job before pouring anything into my gas tank?

I suppose it would be less work to try and use Techron first but I heard of stories where it knocked a bunch of residue loose at once and that jammed up the fuel filter and that sounds like a big problem, car won’t be getting much gas that way…

items 33 and 52 work together;



http://opposedforces.com/parts/lega...system_turbocharger/fuel_tank/illustration_2/
 

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Discussion Starter #8
Thanks Texan! This diagram helps but again brings up more questions...I see the float is on the passenger side fuel pump assembly, I do not see the float on the driver side assembly, so how does it use more than one sensor to determine how much fuel there is in the tank? Does it not need two floats to do that? This is where I am lost, if I were to take the fuel pump assembly apart and determine that the float on the fuel pump works fine and has full range of motion what will be my next step in looking for the problem cause?
 

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Thanks Texan! This diagram helps but again brings up more questions...I see the float is on the passenger side fuel pump assembly, I do not see the float on the driver side assembly, so how does it use more than one sensor to determine how much fuel there is in the tank? Does it not need two floats to do that? This is where I am lost, if I were to take the fuel pump assembly apart and determine that the float on the fuel pump works fine and has full range of motion what will be my next step in looking for the problem cause?
there are 2 floats there. #37 pass (R) side w/pump, 52 is driver's (L) side

maybe do a google image search for subaru gas tank or w'ever would find some photos?
 

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Each float has a variable resistor attached which will be at it's maximum (more-or-less) when the float is down (tank empty), and at it's minimum when the float is lifted high (full tank).

The main float/fuel level sensor, which is at the fuel pump, has a maximum resistance of approximately 32 Ohms; the sub-sensor, on the left side, has a maximum resistance of 62 Ohms. Both variable resistors can go as low as a few Ohms.

Because the tank straddles the drive shaft, the center part is less deep than the two sides. When the tank is filled, fuel goes into the tank mostly on the right side (some might go to the left because of baffles inside the tank, and when the level in that side gets high enough, the fuel flows over to the left side. When the left side level reaches that of the right side the whole tank continues to fill at the same height.

I've attached a few more diagrams that show the pump assembly with the main sensor float, the sub-fuel pick-up and sensor float, and the fuel tank in profile so that the need for two sensors is more apparent from the shape of the tank. Also attached is the wiring diagram for the fuel gauge system.

When the car is running, the fuel pump, which is on the right side, pumps fuel from that part, but as long as the fuel is above the "bridge" between the two sides, the level goes down equally. Once the fuel level drops to the bridge, the pump is drawing fuel from the right side only. However, it delivers more fuel to the engine than is needed, and the extra fuel is sent back to the tank. This returning fuel passes through a jet pump (at the fuel pump assembly). The pump is connected to a fuel pickup on the left side (where the sub-sensor is located) and draws fuel from the left side to the right. This should ensure that all the available fuel in the left side is transferred to the right so that it can be pumped to the engine.

The fuel level sensors are wired in series. When the tank is empty, the total resistance is around 94 Ohms and this causes the fuel gauge to read at E. When the tank is full, the total resistance of the two sensors is very low, less than 10 Ohms, and this causes the gauge to read at F.

If, when the tank is filled, the gauge does not go to F, it means that the total resistance of the circuit isn't as low as it should be. This could be because one of the floats isn't moving up as far as it should. If it's stuck in one position, such as mid-way, the gauge will not go to E. If the float cannot go above the sticking point but can go down below, then the fuel gauge readings at the lower end will be correct, and it's not likely only 8 gallons could be added when the gauge reads E.

The other cause of the gauge not reaching F when the tank is full is because there's a bad connection in the fuel gauge circuit.

With a bad connection (or other improper source of resistance in the fuel gauge circuit), the total resistance in the circuit could be, for example, 35 Ohms, even when the tank is full and the sensor floats are high (individually they are at near zero resistance). That would prevent the gauge from going to F. As the fuel is drawn down, the floats would drop with the dropping fuel level in the tank and the individual sensor resistances would go up. But this means that the total circuit resistance of about 94 Ohms, when the gauge would read E, would be reached when the sensor floats themselves have not yet reached their lowest levels. Filling the tank at that point would result in less than the expected amount (gallons) being put in because the gauge is prematurely at the E.

Taking the fuel level sensors out to inspect them means removing the fuel pump assembly on the right side, and the fuel pick up on the left. This can lead to seal issues as well as being a somewhat dangerous task as the fuel in the tank is then exposed.

However, if you're so inclined, perhaps start by checking the resistance of the sensors when the tank is full. They are accessible through covers below the rear seat (seat bottom has to come out.) As noted earlier, when the tank is full, both sensors should be at their lowest resistance. If you want to go this route, let us know. There are connectors on each side that should allow you to measure the resistance without removing the sensors from the tank.

Another way to see what is happening is to connect a scanner to the OBD port. On the 2005, one of the engine parameters that can be displayed is the "fuel level sensor resistance", which varies between around 4 Ohms and 96 Ohms. The scanner will indicate the total resistance. If it's not near 4 Ohms when the tank is full, that would explain why the gauge isn't going to F. The next step would be to isolate the cause of the high resistance, starting by checking the individual sensor resistances as suggested in the previous paragraph.

If when the tank is full, the total resistance is incorrect, and both sensors resistances are quite low, then there's a bad connection or damaged wire in the wiring back to the Body Integrated Unit. (But this is a rare situation; unlikely unless the car had been damaged such that the wiring and connections were disturbed.)
 

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Discussion Starter #11
Thank you Plain OM that was a good bit of insight into how I can go about diagnosing the problem. I talked to the sales man at the dealership where we origionally got the car and explained my frustrating situation. He asked me to bring the car by so they can try to solve the issue, I'm hoping it will end at that and they will solve whatever might be causing this. If not, then I will be checking the resistance on the sensors...I'll keep ya posted and thanks again!
 

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apart and determine that the float on the fuel pump works fine and has full range of motion what will be my next step in looking for the problem cause?
With the senders out of the car take a multi meter to them (read the Ohms) and make sure they read the same from one extreme to travel to the other.
 

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With the senders out of the car take a multi meter to them (read the Ohms) and make sure they read the same from one extreme to travel to the other.
We'll have to see what rUsMishka reports back from the dealer, but in regard to your note, the two sensors have different resistances -- around 32 Ohms in one case and 62 Ohms in the other -- and therefore won't read the same when not at the "Full" position. They should vary from pretty close to zero Ohms (at "Full") to their maximum resistances (float all the way down). (See my post above.)

(I believe Br67ett was a spambot(?) and the posts have been removed.)
 

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If you haven't already done so, check your gas cap. Take it off and tighten it back on, making sure it clicks when you tighten it. Then put in about $20 of gas and try the same thing in a couple of days.

I had this same issue with my Honda CR-V where the fuel gauge was stuck on full fow a long time and then at most, moved maybe an 1/8. Did he above a couple of times and then it was fine. Might not solve the problem but give it shot!
 

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We'll have to see what rUsMishka reports back from the dealer, but in regard to your note, the two sensors have different resistances -- around 32 Ohms in one case and 62 Ohms in the other -- and therefore won't read the same when not at the "Full" position. They should vary from pretty close to zero Ohms (at "Full") to their maximum resistances (float all the way down). (See my post above.)

(I believe Br67ett was a spambot(?) and the posts have been removed.)

Oops thats what i meant ... more coffee before posting :)
 

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Discussion Starter #16
Thanks guys, dropped it off yesterday morning and they are still trying to figure out the issue...maybe I should have directed them to this forum first lol...
 

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Looks like the soobie is all fixed up and ready for pick up! The dealership took care of the problem and had their "go to" shop replace both of the sending units so everything is back to normal...and they did not mention anything regarding price so I'm guessing they took care of everything on their own, as I figured they should, crossing fingers for no surprises X X Thanks everyone for the helpful input, so I was able to ask all the right questions and knew what to expect.

Next up, a complete HID retrofit 4300K bi-xenon projectors and 3000K fog lights if anyone is interested I will start a new thread on this shortly.
 
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