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I have a 2009 2500 Suburban. The owners' manual says it has a 39 gallon gas tank. On several occasions I have driven it 50 or more miles after the low fuel light came on to reach a gas station. At that point, filling it up usually only took about 34 or 35 gallons which seemed strange. It seemed like being very low on a 39 gallon tank should take 37 or maybe 38 gallons.
One day I actually ran out of fuel on a trip. I put approximately 1.5 gallons of gas in the tank and then drove about 10 miles pulling a 6,500 pound boat to a gas station. It only took a bit over 35 gallons.
So, I'm thinking the tank is not really 39 gallons. Does anyone else have a similar year / model Suburban? Can you ever get anywhere near 39 gallons in when empty? Or even 37 or 38?
It's disappointing that I'm missing around 3 or 4 gallons of advertised capacity. Nothing much to do about it I guess.

Thanks,
 

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How many gas tank does it have? I remember some of suburbans came with 2 fuel tanks. These trucks use the lift pump from a 6.5 to transfer fuel from the rear tank to the front tank. When the pumps fail you will see some strange behavior like running out of fuel when it says you have 1/4 tank. The rear tank will be full and the front tank empty. These systems average the fuel between tanks for the gauge. Not saying this is your problem, I just had to fix so many of these.
 

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I don't have a definitive answer for you, but I can tell you that the previous generation Yukon XL 2500 came with (2) tanks. The main is approximately 30 gallons and the reserve is about 7 gallons. Total capacity is rated at 37.5 gallons IIRC.

Similar comment, your fuel transfer pump that moves the secondary tank contents to the main tank could be failing. I am not aware of more than one sensor, which is in the main. I don't believe there is any way to determine the actual level of the secondary tank. When the primary gets low the secondary is continuously transferred to the primary.

The transfer pump only transfers from the secondary to the primary, never the other way around. The secondary will fill when you add at the pump and I believe it will fill last. i.e. when the level in the primary gets near full it will spill over into the secondary. I'm not positive about this but is consistent with my experience having owned this truck for nearly 15 years.

Another issue may be a partially clogged tank vent. When you stop pumping do you wait a little and try to pump again or do you assume the tank is actually full? As you pump the backpressure in the tank increases as the vent path is ultimately back out the filler neck (constricted). Backpressure is how the automatic fill stop works; when it hits a limit the handle clicks off. If the vent is partially constricted it won't be able to vent off quickly enough and you may think the tank is full when it's really almost full. Where it clicks off would be a function of the pump rate, the vent rate, and the tank level. If you like second-order differential equations we can have a more detailed discussion but I assume you don't...

I know we're told not to "top of your tank" but you may want to see just how much you can squeeze in there by allowing a few minutes to vent before you try to put more fuel in your tank. You may have to repeat this process more than once. What sometimes happens is the fuel vent filter medium will come loose and lodge in the vent line hose from the tank back to the filler neck. The filter material is a medium of small carbon balls. Let us know if you suspect this is the case as I can provide additional information if you care to know. Ironically, topping off your tank can be a contributing factor to filter media failure.

The low-level sensor works independently of the level sensor and the fuel gauge and is indicative of a true low fuel condition regardless of what may be indicated at the dash. If your low level-light comes on you should have about 5 gallons total remaining in the primary. Time to fill up as you likely have less than 50 miles until empty. If your level sensor or gauge ever fails, you can essentially drive off the low-level light.

Don't drive the truck to empty as the fuel pump is cooled by the fuel in which it sits. This is a good way to overheat a pump and reduce its useable life or cause outright failure.
 
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I believe its 36 gals total with dual tanks. If the transfer pump has burned up in it, there is a replacement pump balance module that is reprogramed to leave 3 gals in rear tank. They would empty the tank and keep running causing them to burn up. Go put 12 volts to the lift pump and see if it works.
 

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1/2 ton Yukon XL (1500) is 31 gal; 3/4 ton Yukon XL (2500) is 37.5 gal. OP say he has a 2009 model year so could be incorrect for that generation vehicle.

This is for 2001 - 2006 model years. This comes from a 2004 GMC Yukon/Yukon XL Owner Manual (attached).
 

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Yep. All the older members seem to have mostly disappeared.
There are still quite a few around. The real world has a way of being a priority sometimes..
(y)
 
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