MIKER

The service manual diagnostics specifies the following flow volumes:
Aux. fuel feed to the jet pump, prior to jet pump orifice, is supposed to be 300 ml per 10 seconds of pump run time.
Aux. fuel return to the left tank, from the jet pump, is supposed to be 333ml/8 seconds.

Being that less than 300 ml per 10 seconds is going INTO the right tank and 333 ml per second is going OUT of the right tank, this tells me that more fuel is being evacuated from the right tank than coming in. Thus no designed attempt to balance.
By design, the system is always trying to evacuate the right tank as long as the electric pump is running.
However, excess fuel pumped to the left tank does spill back into the right tank via the cross-over fill tube until <1/2 of the systems fuel capacity remains. From the systems half full point onward the right tank remains virtually empty.

LoneStarFRC

Of course this particular discussion centers on the fuel flow/transfer from/to the two tanks and does not account for the fuel flow rate to the engine (from the left tank only) which is occurring simultaneously. Presumably the engine is drinking fuel at a rate significant enough to impact on any calculations here.

BTW, seems to me that the fuel crossover/fill, which is located near the very top of both tanks would be too high for a "1/2" tank drop before any fuel spillover stops. Unless of course you're really throwing the car around from high G maneuvers.

MIKER

I’m still learning but this is the way I see it, fuel consumption by the engine is small compared to the jet pumps flow capacity so I hadn’t considered actually calculating it. However, for the sake of discussion let’s take a look:
Jet pump flow rate is 333 ml/8 seconds as per service manual. That is equivalent to 150,000 ml/hr or ~40 gallons/hr
For a generic example:
Engine Fuel consumption at an average of 25 mpg, while traveling at 50 mph is 2 gallons per hour (7570 ml/hr). Within that hour the jet pump has transferred the equivalent of 150,000 ml (40 gallons) of fuel (nearly 20 times the engine’s rate of fuel consumption)

For an extreme example:
Engine fuel consumption at an average of 5 mpg, while traveling at 50 mph is 10 gallons per hour (37850 ml/hr). During this hour the jet pump has transferred the equivalent of 150,000 ml. (still nearly 4 times the engine’s rate of fuel consumption)

I’m not sure what your saying. Perhaps I was unclear in my explanation. Sorry.
By ½ tank, I mean that the left tank is full and the right tank is empty. Both tanks make up one unit of capacity when speaking of a “a full tank of gas” in a C5 or C6 Corvette. Stating, “I filled my Corvette with gas” is the same as saying that both of the fuel tanks have been filled!?!
¾ tank is equal to a full left tank and ½ full right tank.
I believe that these examples are correct while the fuel pump is running. I’m still unclear whether or not the tanks equalize while the car/fuel pump is not running. I don’t see how they could equalize but perhaps they do. I’m still curious about that.

Feel free to dissect my reply. I would rather be proven wrong than to be responsible for posting bad information which eventually makes its way into the Corvette Forum archives. Know what I mean.

cruisemon

The right tank does indeed pump to keep the left tank full. At some point the right tank is going to be empty and the left tank full. The fuel level will indicate half full. If the car is driven only occasionally and the right tank sending unit is not immersed in fuel for extended time period, crud/deposits build up on the sender and leads to a well documented problem - the indicated level is apparently okay and then suddenly drops to zero followed by the engine stopping, often at inconvenient places.

Techron and/or Seafoam will often remove the deposits. Put the additive in when the level is nearly zero and THEN fill the tank to get the additive over to the right side tank.

Charlie

LoneStarFRC

I guess the one thing I was trying to satisfy, in my mind at least, was whether the unequal levels resulting from one empty and one full, is/was a deliberate decision that GM decided to see as acceptable, in light of the fact that in a vehicle (particularly a sports car) this can cause a weight shift to one side. Maybe not significant enough to worry about perhaps?

cruisemon

It's not like my wide white country lard @$$ driving and no one in the other seat doesn't create a side-to-side weight bias.

That said, it apparently was a deliberate decision and one that is somewhat hard to understand. The setup could just as easily pump left to right, but there must be a reason for pumping right to left.

Charlie

LoneStarFRC

Actually the system does pump from left to right. That's what initiates the siphon pump's action in the right tank. It's a system design whereby the left tank is constantly re-supplying itself from the right, while supplying the engine at the same time.

As I see it, if the amount of fuel leaving the left tank had been designed to approximate the fuel being transferred back by the right tank, then both tanks would empty themselves (approximately) at the same rate and keep an even fuel distribution right-to-left.

Clear as mud, huh?

mneblett

Question: I was under the impression that the primary fill of the right tank was through the large diameter cross-tank hose, i.e., during refueling, once the left tank is full, the level is high enough to pass fuel over to the right tank through the cross-over hose. My further understanding is that the main fuel pump feeds the jet pump (not the right tank, too), and that the feed to the jet pump returns to the left tank with the fuel it picks up from the jet pump siphon -- in other words, the jet pump feed from the left tank never really "enters" the right tank, because it never leaves the jet pump piping.

Thus, the right tank level can (and should by design) go down faster than the left tank. (FWIW, this would be desireable in theory, too, as it would minimize the "free surface" weight effect during cornering, as you would only have one tank, not both, with any significant weight shift to the outside of the turn.).

Am I missing something here? Happy to be corrected (learning new stuff is always a good thing )

mneblett

I'm not sure this is correct -- if the left-right flow rates were about the same, then the jet pump wouldn't be functioning properly.

The idea of a jet pump is to apply Bernolli's law by using a small volume, high velocity fluid stream to impart motion to a relatively stationary fluid, thereby causing a much larger volume, low velocity fluid stream to be delivered from the jet pump. Accordingly, in the C5 application, the flow being returned from the right tank can be expected to be significantly larger than the incoming flow from the left tank.

If the return flow was only about the same as the incoming flow, you would just be wasting pump energy, because there's no reason to send X amount of fuel out from the left tank if you're only getting essentially the same X amount back to the left tank (that would be the same as the pump pumping fuel directly back into its own tank).

LoneStarFRC

You are correct I think with regard to the inter-connecting fill hose you mentioned.

The siphon jet pump is physically located inside the right tank, so in that sense the elec. pump (left tank) does send fuel under pressure directly to the right tank for purposes of "activating" it, thereby sending the fuel in the right tank over to the left.
It's an interesting and effective design.

A couple years ago I had a failure of the check valve which is incorporated into the output pipe coming out of the left tank. The pipe has a "Y" and one leg goes to the right tank which, as I said, "activates" the siphon jet pump located inside. More than anything, it's this that caused me to examine the whole fuel system set-up. It was fairly easy to diagnose my problem I was having but a bit of a PITA to change out that "rear fuel feed pipe" without dropping the left tank. The pipe was about $70 or so, so the cost (through a forum Chev. vendor) wasn't too bad.

Robert

MIKER

mneBlett, your understandings of how the tanks get filled and the operation of the jet pump are correct.

LoneStarFRC,
Are you now convinced that the system was NOT designed to maintain balanced fuel levels in the tanks?
When the check valve failed, what were the symptoms? What year model is your Corvette?

LoneStarFRC

Given the difference in flow rates, etc that would appear to be the case. Symptoms were hard starting (usually only on 2nd or sometimes 3rd attempt) resulting from the system's inability to maintain fuel pressure. Once the engine is running, everything was fine, since the check valve plays no role. That'll put a lot more wear-and-tear on a starter.
It's a '99.

mneblett

Just to be clear, I wasn't saying the fuel pump doesn't push fluid to the right tank -- it has to, to "power" the jet pump. I was trying to say (apparently ineffectively!) that the fuel pump feed from the left tank doesn't add to the volume of the right tank, as it stays in the jet pump piping until it is returned into the left tank with the right tank fuel it picks up.

LoneStarFRC

MIKER

Since cleaning the jet pump orifice I have run one tank of fuel through the car, down to 1/8th of a tank. All is working well so far.
I'm pretty confident that it is fixed this time.

mxw1

I want to thank everyone on the forum for your help in finding this problem my friend was having.

It turns out it was a plugged orifice in the jet pump assembly. He could have just removed the right sensor and looked for the plugged orifice, but instead took it to Chevy Dealership where they replaced the right sensor/ jet pump assembly. I believe the part cost $500. He is keeping the old one for when it happens again.

I guess he had a similar problem with the gas gauge a few years ago when the car was under warranty; they replaced the sensor AND gas tank then!

No amount of Techtron was gonna help this time.

Thanks,

Mike

MIKER

mxw1, thanks for letting us know what the problem turned out to be. I hate to hear that they replaced a $500 part due to a tiny piece of debris clogging that orifice.
I have ran several tanks of fuel through my car since unclogging the jet pump orifice without any problem.
Forum member sakins.02vert informed me that he recently solved the same problem by unclogging the jet pump orifice on his Vette. He was certainly pleased to have performed the repair for free.
I hope this information saves a lot of people from spending money for parts they don't need.

Mike

hank_rearden1

I am definately tagging this thread...

I have had this same problem for a while. Upon fill up, fuel gauge doesn't read more than 7/8 full after filling up and the chech gauges light comes on (along with fuel giuage reading zero) after about 40 minutes of driving.

I have tried techron several time continously at fill ups. Still no change.

Thanks.

92mr2turbo

Hello all, I have a 00 FRC. Is my siphon pump supposed to have 2 sock/filters attached to it? Im getting the gauge going to zero event. I drained my right side tank and pulled the assembly out and the white piece which i assume is the jet pump has 2 sock filters on it? Pics will be up shortly. Im getting a slew of codes p0463,p0461,p1463. I ran out of gas this time to see if could only pump 9 or 10 gallons and sure enough thats all that will go in. My car has 174k miles btw

Mike

joevet

My 98 vet has same problems, 7/8 to 15/16 reading when I fill tank(s). 40-60 miles down the road gauge goes to zero. Continue to drive another 150+ plus miles without stopping,gauge still reads zero. If I shut car off rebotting occurs and gauge is fine. Never happens when tanks are only 1/2 or less full. also sometimes if I make a very hard turn gauge will start working. Dealer printed a GM bulletin ID:788111 #01-06-04-022 (mar16,2001) for 1997-2000 vets. It refers to a julian date code label found on the right and left sender assembly covers. If the RH date is less than '01301' replace the complete assembly with a new fuel sender kit. If the LH date is less the '32791' replace the existing fuel sender sensor and stainer. 32791 is not a julian date so I am lost. What will labor and material cost to do this at a dealer??