It is my understanding (from reading other threads) that the left tank stays full while the siphon pump (jet pump) empties the right side tank, then the left tank empties once the right tank is empty.

If the right tank emptied after the left tank would the fuel gauge still work properly? ie: Would the computer still properly average out the level of fuel between the two tanks (right side full + leftside empty = 1/2 tank on the gauge), or does the left tank HAVE to empty first to keep the gauge working properly?

 

Don't understand your question?

If the right tank emptied after the left tank, you would have a fuel pump problem ?

The computer looks at both fuel levels and determines how much fuel is left. If it saw the left tank empty and the right full, I would think it would set a code for a bad fuel pump.

Even if the pump was bad, there is a connecting tube which would at least level both tanks, so the right tank could never be higher than the left.

Why are you even asking this question? Do you have a problem?

 

I don't get the question either.

The vehicle "self diagnoses" problems between the fuel level sensors, and sets one (or more) of possible DTC should a situation like I think you're describing occur. These DTC codes are P1431, P1432, and P1433.

If one or more of these set, your fuel gauge will immediately drop to EMPTY, you will get a LOW FUEL warning on the DIC, and the CHECK GAUGES lamp will illuminate.

 

From what I've read, the "jet" pump in the pass side tank (which operates on fuel flow from the driver's side tank), is constantly pulling fuel from the pass side and topping up the driver's side. Once the pass side runs dry the driver's side tank is drained.

I'm just wondering if the computer cares which tank drains first? And would that have an effect on the way the dash displays the fuel level? I'm thinking that for better weight distribution I may look into having the pass side stay full until the driver's side tank is empty.

 

The PCM most certainly cares which tank drains first. The first DTC I mentioned above, P1431, is set when the PCM believes that the right tank fuel level sensor is "stuck". One of the ways it does this is if it sees the right tank fuel level HIGHER than the left tank level (there are some other parameters here ... I'm giving you the Cliff Notes version), then the PCM believes the right fuel level sensor is stuck, posts the DTC, and sets your fuel gauge to EMPTY, the DIC displays "LOW FUEL", and the CHECK GAUGES warning comes on.

I guess GM could have done like the old Jaguar XJ6 sedans with dual tanks had, a rocker switch on the dash below the fuel gauge marked LEFT and RIGHT. If you set the switch to LEFT you were running the left fuel pump and the gauge was showing the fuel level in the left tank ONLY. Switch to RIGHT and you were running off the right tank and the gauge showed the level in the right tank only. You could actually do some cool things with this set-up. For example, fuel one tank with premium, the other with regular. For around town driving you'd run off the premium tank, and for highway cruising you'd run off the regular tank. Also made filling the car faster, since each tank had its own filler, you could attach the car to two station pumps at the same time. Freaked people out to see you with two hoses filling the car !! Even Jaguar finally dropped this system back in the early 90's and went to a single tank arrangement.

 

thanks for my learning of the day...

 

I agree! Good info, THANKS!

 

Just to make sure we're clear here, there is no connecting tube which will self-level the tanks without the pump running -- the pump needs to be running to "power" the jet pump in the right tank to transfer fuel to the left tank. The jet pump transfer line is the only "cross-over" from right to left (the big hose between the tops of the tanks is too high to be a transfer tube-- it only permits fuel to flow to the right tank during refueling, once the left tank is full).

 

There are 3 connection between the two tanks ....

1) The crossover pipe, which is located at the top of both tanks. As you point out, this connection is used when fueling the vehicle. Once the LEFT tank is full, fuel flows through the crossover to the RIGHT tank until it is full.

2) The feedline from the fuel pump to the siphon pump. This line carries pressurized fuel from the left tank to the right tank siphon pump

3) The return line from the right tank to the left tank. This is the line that transfers fuel from the right tank to the left.

BUT, since there are no "check valves" between these lines, my understanding is that once you shut off the engine, a "reverse" siphon effect occurs and the two tanks will again pretty much equalize the fuel load through the return line that is supposed to feed fuel from right to left. This occurs up until the fuel load is about 50 percent. After that, the siphon effect of the right tank is not strong enough to pull fuel from the left tank.

Bottom line, with the engine OFF the tanks will self level untl the fuel load is about 50 percent. After that, the right tank should ALWAYS be lower in fuel level than the left tank. Check out the description for DTC P1431 to see how this works ... or how I have COMPLETELY mis-understood the fuel system .... which is QUITE possible ...

 

One would think they would "self-level" all the time for equal weight distribution. I would think they would maintain a 50/50 ratio as long as possible to keep equal weight distributed to the rear end?

 

You are correct in your understanding
Here is how GM explains it even though this is for an FFS (2003-04)system they operate the same as pre FFS:

IP:
The pressure regulator is in the fuel tank, not on the fuel rail on the engine. This type of fuel system is called
returnless, or demand. This means that excess fuel is diverted before it leaves the tank, instead of passing through the
fuel rail before being diverted. The result is that hot fuel is not constantly returning from the engine compartment, so the
fuel in the tank stays cooler, improving evaporative emissions.

The majority of the pressurized fuel is directed through the filter (L) and on to the fuel feed pipe (M) to the engine, where it
is injected into the cylinders for combustion.

Some of the pressurized fuel is directed through a feed pipe (N) inside the crossover hose, to a siphon jet pump in the
right tank. The jet pump relies on the venturi effect to use pressurized fuel to draw additional fuel from the tank. The
combined fuel then flows from the right tank to the left tank, through a return pipe (P) inside the crossover hose. The jet
pump is able to move enough fuel to ensure that all of the fuel in the right tank is consumed before the level in the left tank
begins to drop.
TIP:
The return tube in the left tank has an anti-siphon hole (S), so the fuel in the left tank does not siphon back to the
right side when the vehicle is shut down.

Some of the pressurized fuel is directed to a venturi pump in the left tank. This pump uses fuel flow to siphon fuel from the
main tank into the reservoir, to keep the turbine pump supplied with fuel at all times. As the left tank fuel level drops, the
venturi pump scavenges all of the remaining fuel into the reservoir, regardless of the vehicle's attitude.

When the engine is shut off and the turbine pump stops, a reverse flow check valve (W) maintains pressure in the system
to ensure rapid pressure buildup during the next startup cycle.

Operation of Fuel Level Gauge
When the fuel system is operating as designed, starting with both tanks full, the left tank will remain full until the right tank
is depleted. Then the left tank will be emptied.

Each fuel tank has its own sensor (Q and R in the illustrations). Modules are shown in
figure 5
(LH module) and
figure 6
(RH module). The PCM supplies a reference of 5 volts to the two sensors. Each sensor operates across a range from full
(2.5 volts) to empty (0.7 volts). The PCM monitors the fuel level sensor voltages and calculates how much fuel is in the
two tanks. The readout of the IP fuel level gauge is a result of this calculation.


More info on how the fuel system works:
http://www.corvetteactioncenter.com/...el+Tank+System

 

You are correct IF the two tanks were connected at the BOTTOM of the tanks. The issue is thet the "big pipe" between them is at the TOP of the tanks.

The only remaining connection is the very small feed hose between them. That hose goes up and follows along the same path as the "big pipe" ... so while the "siphon" effect will tend to equalize them to some degree, once the right tank gets "low enough" it cannot pull fuel "up and over" from the left tank.

The whole arrangement is kinda like this ... either the Chevy engineers were incredibly smart figuring out how to make two tanks work as one ... or they'd had a MAJOR big time drug party right before they designed the system ......

I'm voting for the

 

jirasvet,

The MAIN difference between the "old" style fuel system (before 2003) and the "regular" system is summed up in the phrase ...

"The return tube in the left tank has an anti-siphon hole (S), so the fuel in the left tank does not siphon back to the right side when the vehicle is shut down"

The "old" system has no such check valve.

GREAT explanation of the "new" system .... first I've seen that makes sense ...

Thank you,

Steve

 

Thanks all!

 

I think I will cast my vote right along with yours - So the system works the way JIRASVET explained in his write up. This leaves for unequal weight distribution in the system. Apparently this has no major impact on handling? Seems kinda screwy to me. I would have thought they would do it like the the F16 fighter was designed. Fuel from one wing fuel cell is pumped to the other wing fuel cell to equalize weight distribution prior to landing. Had one pilot in Korea forget that check and flipped the plane on landing. I realize a Corvette is not an aircraft, but the principle is the same - equalize weight distribution. Seems to me Chevy set themselves up for an "unequal" weight distribution problem from the start. One would think that unequal weight distribution would be a detriment to handling?

 

IrishJoker,

To be fair to Chevy, they MAY have already thought about balance. For example, the battery is on the far right side of the vehicle ... MAYBE the full right tank and a low left tank help to offset that distribution, for example.

Without putting each wheel on a scale, it is hard to say exactly how well balanced the car is .... though front to rear ratio on the Z06 is 51 to 49, pretty darn good ..... Also a full fuel load (18 gallons) is about 120 pounds in weight, so you're talking a 60 pound weight for a full "tank" on one side .... the weight difference between some of the folks on this forum is easily more than 60 ibs. ...... so I guess Chevy figured there would be more performance gain with a fit driver than anything they could do "balancing" the fuel.

 

Depends on the API gravity of the gasoline, which varies by grade and refinery. Usually, regular unleaded gasoline has a gravity of around 58 and a weight per gallon of 6.216 pounds per gallon. Premium gasoline may have a gravity of 54, or 6.350 pounds per gallon.

 

I cannot take credit for write up. It is a GM information I copied and pasted

 

Thank you .. I kinda knew that gasoline was about 6.x pounds per gallon .... but I'm saving your post ... GREAT info .....

Thank you again,

Steve

 

Anytime