...Launched out this past weekend with both fuel tanks full....My SOP is to switch tanks at the half mark, which I did...Gauge went back to Full, which it always does.....

After about 30 minutes, I noticed the gauge at about the 1/2 full mark, telling me: 1) gauge malfunction, 2) fuel system leak, or 3) engine was now getting about 1-2 mpg.....

Stopped to check for leaks - all OK

As a precaution, I switched back to original tank, and it now indicated it was Full. A couple of thumps on the tanks seemed to confirm this......

So, what's up with that?....bypassing in the fuel selector?.....hung-up check valve somewhere? Where's the logical place to start looking? (P30/454 with standard dual tank system, if there is such a thing as "standard")

As always, Thanks for the input.

I am going to assume that you have one gauge, which is connected to the correct sending unit by the selector switch.



If that is the case, go to the back of the switch and determine which wire is which. Disconnect the two sending unit wires and the gauge wire. With ignition on, connect first one then the other sending unit to the gauge wire. If all is well, the switch is at fault. If nothing is well, the gauge is at fault. If only one is well, it is the sending unit or its wiring.

But I would shoot some electrical cleaner into the switch first and work it madly. And check for clean tight connections.

Two things 1; at 3:36 EST this site is dragging along at sub phoneline speeds. Me or something else? 2; When on left tank,{rear], the gauge works, on right tank,[for] it reads all the way full. I run down to 1/4 rear then switch to for until we find cheep gas. [LOL]. Where should I be looking to figure out how to make the for tank register on the gauge?

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Originally posted by Danny Zeeff:
Two things 1; at 3:36 EST this site is dragging along at sub phoneline speeds. Me or something else?

Not just you, Danny.

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2; When on left tank,{rear], the gauge works, on right tank,[for] it reads all the way full. I run down to 1/4 rear then switch to for until we find cheep gas. [LOL]. Where should I be looking to figure out how to make the for tank register on the gauge?

Gotta fake it here, since our Barth has only one tank. Had several SOBs with two, though, so maybe memory will serve.

Ideally, both tanks have sending units which are connected to the tank selector switch. Go to the back of the switch and have a look. It should be as described above. It is common for the connection on the top of the sending unit to need cleaning. I like a multiple tooth lock washer under the lug.

I had one with no sender on one tank, so I burned it down first, then relied on the gauge to tell me how the remaining tank was doing.

OK Danny, I called dibs on Bill's attention first, so I'm gonna jump back in here...

Bill, I've got myself confused again....maybe I didn't describe the malfunction properly:

I do not have an issue with what the fuel gauge is telling me when it's switched to either tank...the gauge indicates the correct amount.

But when I switch to the right tank, things run as normal, but at the same time, fuel is actually being pumped from the right tank into the left tank, i.e.: if the left tank is full and I'm running on the right tank, the left tank will actually overflow and vent fuel.....
(It won't do that in a reverse scenario)

Isn't this more than a gauge sender issue?.....

OK Danny, your turn again.......

Does your valve look like this?



There is also a more deluxe valve that looks similar, but incorporates some smaller nipples for changing the return vents, as well as the supply.

Barth is 2 mi away...

As they say, "I'll get back ta ya on that"....

Let's say that it is, is there something I should do or check for while I'm under there?

If you do have the Pollak valve, one of the ports is to the engine and two are from the tanks. With no power applied to the terminal, fuel goes from one tank to the engine. When power is applied, fuel goes from the other tank to the engine. Case is ground.

You could use a short hose for testing. Disconnect hoses from the valve. You might have to back blow them or plug them. Put your hose on and blow test both positions. You can use a hot wire or someone in the cockpit. If you use a hot wire, be careful if there is spilled fuel. Or remove the valve and do it in the shop.

Do you have electric fuel pump or pumps?

I'm gonna suspect the valve, but today is a sick day so I am brain-impaired.

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but today is a sick day so I am brain-impaired.

Stay well Bill, at least for a few more minutes until we get this resolved

No electric pumps - just cam-driven mechanical.

quote:

But when I switch to the right tank, things run as normal, but at the same time, fuel is actually being pumped from the right tank into the left tank, i.e.: if the left tank is full and I'm running on the right tank, the left tank will actually overflow and vent fuel.....
(It won't do that in a reverse scenario)
.....

Have you actually observed that to happen? I think what's more likely is that the fuel was always being taken from that same tank - the right tank it sounds like. There's no way fuel could get from on tank or the other unless the changeover valve is stuck halfway open, which is unlikely.

It IS likely that the valve isn't operating properly. The dash switch is a 2-pole - one pole to switch sending units to the gage, the other to operate the changeover valve. The valve may be bad, or the connections may need cleaning.

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Have you actually observed that to happen?

Here's all I REALLY know:

1. This past weekend, the RIGHT tank gauge went down abnormally fast.

2. When I switched to LEFT, the gauge indicated more fuel than it had indicated about two hours earlier.

3. A top-off of the LEFT tank indicated that I had gotten about 20-22mpg with my 454 - Not likely.......(Even MMO isn't THAT good!)

4. Earlier this year, after topping off both tanks on the PA Turnpike and starting out on the RIGHT tank, a motorist passed & held up a "You're Leaking Gas!" sign to me. The LEFT tank was venting out the cap/filler neck area. I originally flipped it off as just temp expansion, but now that I have this new scenario, I wonder if it was indeed fuel transfer.....

That's quite odd - your engine is no doubt carbureted (some with throttle body injection or port injection return fuel to one of the tanks). Being an Airplane Guy, I'm sure you've encountered that. My old F-35 Bonanza had a pressurized carb that returned fuel to one of the mains...

But I've never encountered a vehicle chassis that had an inter-tank transfer pump, nor can I think of any way fuel could be transferred from one tank to another the way those tanks are plumbed...my van also has two tanks with a changeover valve.

My 87 P30 454 has a return line from the mechanical fuel pump. I wonder if there are 2 valves 1 inbound and 1 return and the return valve has failed. Just a thought.

I suggest a visual inspection of lines & wiring. I once installed a second fuel tank on a Chev pickup I used for trailer tow. Rather than doing a total re-plumbing job, I installed an electric fuel pump with a manual switch to transfer fuel from the aux. to the main tank when the gauge got low. Sounds like that could be the case here.

OTOH, I had a GMC motor home with dual tanks that were plumbed together so the fuel level stayed the same in each. Worked fine under way, but the filler pipe only led to one tank, & the transfer lines were small. Made for long waits at the gas pump while fuel drained across.

A few years ago Eric from Maine had the same problem, at the time he had a 1977 Barth. I tried to fined the info on search but didn't have any results so maybe if Eric sees this he'll respond.

Ralph