Sorry - too many years as an instrument & process control engineer in petrochem. We often had to deal with multiple level transmitters on tall tanks or vessels so this is bread and butter to me. It does get confusing especially when dealing in percents.

From what I have read, the fuel tank levels are added together and the total is displayed. You don't seem to be able to drive as far as 18 gallons will take you, but only about half of that. I don't see how you could be using the fuel from the right tank, but not the left because the electric fuel pump is in the left tank. So - you are only using the left tank's fuel or the car is unable to fully empty fuel tanks for some other reason. Since your problem started after the crossover system was apart it is suspect as the problem's cause.

Maybe this will help: say the left tank gauge is 0 - 9 gal and the right tank gauge is 0 - 9 gal and the dash indication is the total fuel on board 0 - 18 gal. If both tanks are full, you have 18 gal (100% full). If one tank is full (9 gal) and the other is half full (4.5 gal) your total fuel is 13.5 gal (or 75% of 18gal). The fuel gauge really doesn't know or care where the fuel is, just the total.

Normally, if the transfer pump is working, as the car uses gas from the left tank the transfer pump moves fuel from the right tank to keep the left tank full - any excess will overflow through the corrugated pipe back into the right tank. So - the left tank stays at 9 gal as the right tank level drops to 0 gal. Then the left tank fuel is used and the volume drops since there is nothing in the right tank to transfer.

But - if the transfer pump is not working the right tank stays at 9 gal and the left tank fuel is used until you are out of fuel. Total fuel in the car is 9 gal in the right tank or 50%. The engine quits because you can't get fuel from the right tank to the electric low pressure fuel pump that feeds the engine.

I don't have a clue about HP tuners, but you can look through the data register descriptions.

Hope this helps...

Ron

 

Hey Ron, I get how the system works so thanks. Isn't the transfer pump just the fuel pump that sends fuel to the engine? There is just a Y that sends the excess to the RT and that same pump actually siphoned from RT to LT as it runs, only one pump in the system. So my right tank is full and the excess goes back to the LT this means the Venturi pump is not working. I can only burn 10 gallons and I can only put 10 gallons in.

 

I'm going to act like an engineer again and use the same terms as the document attached to post #6. The electric pump in the left tank is referred to as D turbine fuel pump. This feeds the engine via M Fuel feed pipe to engine and a slip stream via N Fuel feed pipe to RH tank to G Secondary fuel pressure regulator and H Siphon jet pump. Fuel from the fuel feed line goes through the siphon jet pump venturi and creates a low pressure at the smallest internal cross section of the venturi. Right tank fuel is sucked into the low pressure region of the venturi and this fuel along with what came across in the fuel feed line go to the left tank via P Fuel return pipe to LH tank.

To answer your question: yes and no - the D turbine fuel pump is the only "real" electric pump and the only device that adds pressure (energy) into the fuel tank system. Transfer in the document refers to moving fuel from the right tank to the left tank.

H Siphon jet pump is really a fluidic device that GM chose to call a transfer pump. Motive force for this transfer pump is an incoming flow into the venturi which sucks additional fuel into the device. Motive force fluid plus sucked in right tank fluid is returned back to the left tank.

The motive force fuel pressure (flow) is controlled by G Secondary fuel pressure regulator just upstream of H Siphon jet pump. Excess fuel not required by the engine or H Siphon jet pump is returned to the left tank by pressure relief through V Primary fuel pressure regulator.

So - why does fuel not transfer from right to left?
Jet Pump Clog (C5, XLR) -- If the jet pump in the RH tank becomes clogged, fuel will not transfer to the LH tank. When
this occurs, the vehicle eventually runs out of fuel, even though there is actually some in the RH tank. When the DTC
(1431, 2066 or 2636) sets, the fuel gauge drops to empty and the customer perceives an “erratic gauge.” One cause of
this condition was a piece of plastic left in the jet pump during the manufacturing process. This has been remedied.

Not mentioned is insufficient feed flow into the H siphon jet pump due to leaky connections, plugged feed line or plugged / bad G Secondary fuel pressure regulator.

And by the way, there is yet another E Venturi pump that scavenges fuel from the left tank into the internal sump containing the main fuel pump D turbine fuel pump to get the last drop of fuel.

I can only guess that the fuel system grew this complicated to smooth out fuel flow to the engine under various racing high G force conditions and to make sure all of the fuel is used. I doubt it was initially designed this way.

As I said, I'm an engineer - I enjoy trying to understand this stuff. The only venturi I've actually played with was used to drain my water bed. You connected one end to a faucet, water bed to the low pressure (side) connection, and other end to a drain. I also just threw a hose out the second story bedroom window to siphon water from the waterbed.

Ron

 

Yeah, I'll get my Mass Transfer book out I guess, Ha Ha. I doubt there is any clogging in the transfer line so it must be the sealing system. The place where you push the lines in from the corrugated pipe has a decent amount of play so it may not be seating past the 2 little O-rings. I"ll give that a look.

 

Ron, Would it be beneficial to blow some air through these lines? I know there are check valves but if I blow it direction of travel it should tell me something, correct?

 

I would suggest blowing through a clean piece of tubing held on the tube you are checking. I don't see how it could hurt if you use lung pressure. Since the right tank is full, it will take about 1/2 psi to overcome the fuel head pressure. (27" of water head level = 1 psi, gasoline is about 73% as dense as water, so about 38" of gasoline head = 1 psig). I'll guess the tank is less than 2 feet tall. I would certainly not use air at more than a couple pounds for testing.

You will at least prove there is no blockage. I would expect the secondary pressure regulator on the feed line in the right tank to pass flow in the normal direction. Not sure you will get air back through the fuel return line since the suction port of the siphon jet pump is probably the path of least resistance. Air blowing back through the fuel return line should also exit the suction port of the siphon jet pump. If you connect at the crossover I don't think there are check valves in your way. Check valves can be spring loaded and take some pressure to open in the normal direction. Double check the tank schematic to be sure.

Be careful - enough air flow through the feed line might bring some fuel back via the fuel return line.

Good luck

Ron

 

it happened to me! C6 zr1....

 

That is what I get from his description. However, when I look in the factory service manuals I can't find anything that describes where the fuel shut off valve is located. That valve shuts when the passenger side tank is full and causes a pressure build up that shuts off the gas pump. I suspect it is somewhere in the crossover tube area. If that valve is damaged or stuck in the closed position that would explain the gas pump shutting off when the drivers side tank is filled and fuel tries to move to the passenger side tank. If the valve is all the way to the left side of the car it could have been damaged when the driver's side tank was removed/replaced or maybe if something was stuck into the fuel inlet port.

Bill

 

I can blow air from the crossover pipe for the jet pump (N Fuel feed pipe to RH tank), air just keeps going in, no bubbling sound. I can blow air in the lower tube. (P Fuel return pipe to LH tank) and I hear the fuel bubbling. I am blowing air into the RT. There is first kind of a big pop like check valve opened and then it just flows. Hope this will help in diagnosing this problem.

 

Bill,

See schematic on page 5 of the GM doc in post #6. From the GM explanation, the float valve that closes the vent and causes the gas station pump to shut off is internal to the right tank level sender module. In fact, all of the little stuff appears to be part of the module:

On the right side, the sender module contains a secondary fuel pressure regulator (G) and a siphon jet pump (H). There is
also a fill limiting vent valve (FLVV) (J)

Operation
When the fuel tanks are filled, fuel first fills the left tank. As the fuel rises to the level of the crossover (K), fuel flows into
the right tank. As fuel occupies the interconnected tanks, air is forced to vent from the tanks, through the FLVV in the right
tank. When both tanks are full, the FLVV float in the right tank closes, preventing fuel from entering the vent system. This
also causes fuel to back up in the fill hose, causing the gas pump nozzle to shut off.

K1600 - curious. If you can hear bubbling when blowing into the return hose - the air is discharging into the fuel below the right tank liquid level. The pop when blowing into the return line almost sounds like debris in the venturi siphon pump as mentioned in the GM troubleshooting or the secondary pressure regulator preventing back flow. The venturi side path into the tank then becomes the path of least resistance and you get the bubble.

No noise when blowing into the feed line - if you are sure there is flow and not just putting pressure against a blockage of some sort - air is not discharging below the liquid level - so where else can air escape above the fuel level? Anywhere between where you are introducing your air and internal tubing at the right tank liquid level - crossover connection on the driver side or passenger side or the feed line going into the right tank (look at pg 5 of the GM description). But no pop and no bubbling noise when blowing into the feed line doesn't seem like you are moving debris back and forth somewhere. You apparently can't hear air escaping from a connection or damaged feed line.

I don't expect that any modifications to the left pump / sending module are problems since you are connecting directly to the fuel feed and return lines.

Since you don't mention that the right tank or crossover was disturbed, I'm still thinking the problem is related to the left crossover tube connection or damage somehow to the feed line. But I'm really just guessing based on the GM doc description and your observations.


Wish I had a solution for you.

Ron

 

So the dealer read my voltage from the two level sensors and both read 0.71 V. He also wrote down that this equates to 99.6% full which is accurate because I filled it up right down the street. My question is why does the the GM document posted in post 6 on page 3 say that 2.5 Volts is full and 0.7 is empty. Is there a way the wiring could have gotten messed up? I do have the Crawford Racing 3 pump system for port injection and this is a custom made wiring harness. Or is the GM doc a typo?

 

i think it was 2008 where gm changed the sensors to read the opposite. It was changed to more resistance - system errs on the side of having less gas. So when resistance increases you dont think you have more gas on board than in reality. .4 ohm = full. 240 ohm = empty

 

Remember that the GM document that I attached was for 2003 - 2004 XLR and Corvette. I'm not surprised that something has changed since then. I would also prefer a false low fuel indication - less likely to run out of gas.

Thanks for the update Don.

Ron

 

Let me ask this about connecting the crossover tube. Do you put the o-rings and gaskets on the male end of the crossover tube and then push it together, or do you insert them in the female end and then slide the male end in a very straight manner?

 

I don't know. Are there grooves of any kind on the male end to locate the O-rings? Or are there grooves inside the female end? That would be how I would decide lacking clear instructions.

ANOTHER THOUGHT: The retainer should only grab either the male or female side of the connection. This may be the best indication.

Some hints from a thread I linked to in Post 15 apparently written by a Chevy tech. He also mentions using dielectric grease.

Here is how the fuel tank I have looks. Looking into the fuel tank opening it goes: green o-ring goes in first, then the clear ring, then the yellow seal with the groove facing toward the inside of the tank, then the hard black plastic ring with inside chamfer facing the crossover tube. Make sure all 4 pieces are in the smaller tubes. Yellow o-ring on top (smaller), green o-ring on the bottom (larger) Each of these two have a black hard plastic retainer installed after the o-rings are in place.

Make sure you put lots of lube on all the o-rings and onto all the nipples in the crossover tube and the o.d. of the crossover. I use dielectric grease. Also make sure when you install the crossover, you install it straight into the fuel tanks. If it goes in a little bit off or the two nipples in the crossover don't go into the small fittings in the fuel tank opening, there is a chance of breaking the nipples off.

GL

Ron

 

Found this thread and picture. Not sure if it is from a C7 or not. (Crossover pipe is disconnected from the left tank):

https://www.corvetteforum.com/forums...l#&gid=1&pid=1

 

Bringing this back up bc still have problems. it has become very difficult to fill the LH tank (RH is full). Sometimes the fuel just goes on the ground and not into the empty LH tank. Blowing air through the bottom line to the RH tank it blows below fuel level in RH tank and sends a fine mist of fuel back through the corrugated crossover tube. Blowing air into the top line to the RH tank sends the air back through the corrugated crossover tube but with no misted fuel. Does this help with the diagnosis? Dealer wants $4k to drop everything and fix it. Covid has made this a little hard to swallow right now so I am looking to fix myself. Can I hook up a pump on the return line so I can just flip a toggle switch to transfer fuel from RH to LH tank?

Also, when filling a normally operating system, when LH tank is full and reaches the corrugated crossover, the fuel just flows through the corrugated tube to fill the RH tank, and not through either of the smaller lines inside the corrugated tube? This is just while adding fuel to the car at the fuel pump.

Thanks
Thanks

 

Drop passenger tank. Take apart siphon jet pump assembly. Attempt to blow through orfice. I bet u cant. Buy a brand new siphon jet pump. Theres some great after market options too. I jimmied up a siphon jet using a carburetor jet installed into a T joint. Problem solved.

 

I just don’t believe my passenger tank is plugged. No problems ever until asshats dropped drivers tank to install 3 pump system.

 

So what will happen if I energize the fuel system with the crossover disconnected from the driver’s side tank? That should tell me something. Either fuel all over or no fuel all over.