MrRenoman

Hello,
I'm trying to find information on how the dual tank fuel tanks equipped with a fuel pump in each tank, connected together with a large fuel hose, and various evap hoses operates. I have both tanks out and can see how the hoses connect to the two tanks. It's very interesting. I'm experiencing the slow fill issue that I've read and viewed posts and videos about. I found out that one of the emission hoses that connects the rear tank rollover or vent valves to the carbon canister had standing fuel in it which I was able to blow out. There is a second emission hose from the rear tank that goes to the front tank rollover or vent valve and this line has a check valve in it. I could go into a lot of detail that can make one's head spin so I won't do that . I am interested in someone who can explain in detail how the system works. I know how the purge solenoid at the engine and the vent valve under the car operates in conjunction with the vacuum sensor on the rear tank sending unit. My issue is why the emission tube had fuel in it and if that is the same tube that vents the pressure causing the fuel nozzle when filling to click off. Being filled with fuel may explain why it tank won't fill since it can't vent. What also interesting is the front tank has the large vent hose to the filler neck. So pressure building in the rear tank when filling needs to be vented but it seems that the tank pressure must go through a line from the rear tank to the front tank then though the hose that goes up to the filler neck. Or does venting occur directly through the vent valve under the vehicle? I believe this valve is connected to the front tank and is always open until the ECM commands it to close to test the fuel tanks integrity. Any help is appreciated.
Thanks,
Art

MrRenoman

Attached is a sketch of the dual tank fuel filler, rear to front tank fuel hose, and the hose that goes from the front tank to the filler neck (vent hose?). I'm putting all back together today and leaving the evap hoses off the tanks, carbon canister, purge solenoid, and vent valve. I can hook them up one at a time til either nothing happens and the fuel can be filled or it reverts back to the slow fuel fill problem.

tech2

didn't know they made a dual tank system. I do know evap system operation. The diagram is very helpful.

the vent valve is normally open. it only is commanded closed during evap system integrity/leak testing. It should be open during refueling, purging and normal engine running. If it blocked and air can't flow...the tank could collapse if the fuel cap did not have a vacuum relief.

the vent valve should also have a fresh air filter (missing from your diagram). air must pass in and out of the vent valve.
remove the gas cap, remove the hose at the vent valve....blow thru the hose ( use your lungs, compressed air over 6 psi can damage components...air should flow out the filler neck. If it doesn't, there is a blockage. any restriction from the fresh air filter to the vent valve, canisiter or hoses will cause a slow fill.

things to check...fresh air filter must be clear. clogged cannister?

any dtc set...they might give an indication of the cause?

MrRenoman

I did have a code that triggered me to replace the purge valve solenoid on the engine and the vent valve under the truck. I had a spare vacuum switch that is attached to the rear tank fuel pump/sending unit. I installed it but still received a code once in awhile. As mentioned in a post awaiting review and posting by a moderator...I replaced the entire rear pump since I had the tank out. I'll see what happens once I put it all back together. I'm leaving all of the evap hoses disconnected from all units and then plug them in one at a time while testing my fuel filling effort. Access through the rear access panel will make it easy to disconnect the rollover vent valves since they are the only evap connections to this tank. If you notice, there are two fuel pumps, one for the rear tank and one for the front tank. A fuel line similar to the primary fuel line connects the rear to the front tank. At some point the rear pump feeds fuel to the front tank. The rear tank fuel pump also has a fuel level sensor but I don't think it's for the gas gauge. I think it's to let the ECM know that the fuel is below the orifice of the large fuel hose connecting the rear and front tank together. The rear hose is attached about halfway up the rear tank. I can only assume that this is to keep any sediment from flowing from one tank to another. I guess if the large orifice was on the bottom of the rear tank then a fuel pump wouldn't be required. The rear tank is located higher that the front tank. More to follow.

MrRenoman

Well, I connected all of the emission lines back up. It's been a couple of weeks and have no problem filling the tank or seeing any check engine light. I cut an access hole in the rear of the suburban to have access to the rear sending unit, fuel and emission lines. I may post my main and rear tank access holes in the near future. I'm not sure if anyone has posted this mod in detail. If so, send me the links, I'd like to see it.

Thanks for looking,
Art

CK2000YUK

Emissions Vapor? lines above spare tire cross member. I found them disconnected!
Thank you for posting this. I just bought a 2000 YukonXL 2500 and I'm chasing a gas fume smell. The prior owner had the gas tank and fuel lines replaced 3 years ago.
Searching for the source of the gas smell I found two hoses above the spare disconnected. I'm assuming they have been disconnected for the past 3 years. Your sketch explains alot, however why this is possible without a gas (liquid) leak. I'm a little uncertain however why there is not a low pressure or emissions code being stored. I'm assuming that since the main tank is connected to the canister, all the sensors are being satisfied. We have emissions testing every 2 years and this truck has passed as recently as December 2018.

The previous owner is a baffled as I am and he states that it fills up just fine and that the truck, when empty, always takes more than 30 gallons - so the rear tank is being emptied - I guess that means that the pump works.

I was hesitant to reconnect the lines in case the gas in the small tank was bad (as if it hadn't been refreshed in 3 years). I'll likely drop the rear tank and give it a good inspection and smell test - unless i can get some assurance that the gas in the rear tank is being refreshed with each fill up.

Please let me know if you have any insights or if you posted any pictures.

02Sub

I replaced all the fuel and vapor lines on my 2002 K2500 Sub a couple of years ago.

I am trying to remember the details the best I can.

BOTH fuel tanks fill at the same time when you add gas to the car. I think the rear tank actually fills full before the main tank does.

When the computer detects that the main tank fuel level has dropped below a certain point (not sure home much), it commands the fuel pump in the secondary tank to pump gas from the secondary tank to the main tank. This continues until the fuel level in the secondary tank drops to a low level.

So the black hose you see between the two unconnected lines may be the output of the secondary tank fuel pump going to the main tank. Can you follow that line and see if it goes to the main tank fuel pump assembly? If so, then the secondary tank is probably working fine, just the vent/EVAP lines are disconnected which would account for the fuel smell.

If one of those unconnected lines is the secondary tank fuel pump output, then you would have a gas gusher coming out of it as the computer tries to pump the gas into the main tank. Since this is not happening, either those are vent lines only, or someone has unplugged the secondary tank fuel pump electrical connection.

The reason you are not getting codes is that the 3/4 ton Suburban has a GVWR of 8600lbs, which places it into a different emissions category which is much less strict than the 1/2 ton Suburban. Notice that the 3/4 ton Suburban don't have an official fuel mileage rating? This is another side effect of its heavier GVWR.

Steve

MrRenoman

I’m adding comments to this thread which may duplicate some info already detailed. It’s been awhile since I posted the original post. Your comments added to my interest. It’s interesting that the rear tank has a fuel pump in it. The rear tank large fuel hose connects the rear tank to the front tank. The outlet of the rear tank is about midway up the forward side of the tank. A lot of the fuel seems to gravity feed from the rear tank to the front tank but some of it doesn’t. The rear tank has a fuel pump, a fuel level sending unit and also has a fuel vacuum or pressure switch that is opened or closed by the ecm to test the integrity or sealing of both tanks. I replaced the entire sending unit which came with a new pump, level and switch. This solved my check engine light that was coming on. So it seems that the ecm, I believe, senses that the rear tank is low and powers the rear tank pump to transfer fuel to the main tank. The rear tank has a couple of hard plastic lines one of which connects the rear fuel pump sending unit connector to the front tank fuel sending unit. This may be one of the lines the previous poster saw not connected. The other disconnected line is an emissions line that, on my 2500 goes to the emissions components in my diagram ultimately connecting to the main tank. As mentioned, fuel would flow from the rear tank gushing out unless the power was removed from the rear tank fuel pump by whatever means it was done such as disconnecting the rear tank wiring harness or cutting the power lead, who knows, the pump could be bad too. The disconnected lines may explain the gas fumes since the rear fuel pump outlet and the emissions hoses are disconnected. Fuel will still transfer to the main tank but the rear tank will probably never transfer the remaining fuel to the front tank. Every time the truck is fueled the gas entering the rear tank will mix with the fuel in the tank and this mixed fuel will flow to the main tank via gravity. I doubt very much if any old fuel would remain. I’d connect the two lines back onto the rear tank at the very minimum and investigate if the electrical connection is open or modified. Can’t think of why these lines are disconnected. I’ve made some assumptions here in this post but are based on the comments posted. Maybe someone will chime in and add to this.
I just realized that the reason the lines are disconnected may be for the same reason I originally created this post...slow or difficult fueling. Without those lines connected there is no back pressure on the gas stations filler that clicks off with back pressure. Mine resulted from topping off the tanks during a long road trip which, I believe, filled up the evap hose with fuel which is a path for gas tank venting. Reconnecting the lines may cause slow or no fuel filling.

MrRenoman

After reading the posts again and looking at the pic of the disconnected lines I can add that I believe the two disconnected lines are the emissions lines I detailed in the sketch. The fuel pump line is connected directly to the top of the sending units of both tanks. You can’t see the connections from under the truck. Seeing the two lines that are disconnected in the pic leads me to believe they are emission lines, may be the cause of gas smell and may be disconnected due to the common slow fuel fill I’ve read about. Hope this helps and I apologize for long posts and any redundant info.

CK2000YUK

Thank you again for the detailed response. (I actually sent you a TY last week, but when I pushed send I received a message stating that the "Administrator" would review the message prior to it being released - i guess that never happened).

So I've connected the lines and the gas smell is gone. I've passed inspection and I've got the truck on the road. I've not had to put gas in it yet - but will likely do that this weekend - so I'll find out then if it is filling up at a normal rate, and just how much fuel it takes.

In the meantime, i have other repairs to make, and questions to ask. I'll search the forum first, but hope you won't mind if i ping you if i run into a dead end.