308 fuel tank question

Keith,

First things first -- did you check the connectors on the sending unit? It's on the top of the left (driver's side) tank. When I started cleaning up my '78 engine bay, the connectors on top of the tank were filthy. May not be the answer, but that's always a good place to start, cleaning the electrical connections.

Regards,
Steve

 

Anything is possible but I'm betting the fuel level sender is dodgy (British term for AFU). The only way you'll know is to either run the car till it quits (put a can of gas in the trunk). An alternative is to syphon/drain the tanks and measure what you get (kind of a messy job, you'll need some empty gas cans for this).

 

The sensor is in the LH tank.......you may want to check vent lines and cross piping below car....I suspect electrical problem..

Should hold 8 gallons each, 16 total.....

 

red light in gauge when low???

 

since we're on the subject, my needle never moves but the reserve lite, mine's green, works ok. any thoughts on that?

 

Although, as a component, there is only one "fuel level sending unit", it actually incorporates two senders -- a variable resistor (for the analog needle gauge) and a switch (for the warning light). The variable resistor portion of your sending unit must be bad, but the switch part is still OK.

Keith -- I'd have to say that a blockage would be very unlikely, and it's much more probable that your analog gauge (or, more likely, the variable resistor sender portion) is a bit wacko. This is a little risky (until you get some confidence), but I like BigTex's implied suggestion to just keep going until the reserve warning light comes "on". Also, by resetting your trip odometer at each fill up you can use miles travelled as a secondary gas gauge if you have a good feel for your approx miles/gallon.

 

I have them in boats that break the wire......works good from F to about halfway, then drops to Empty!

We have a three gallon reserve tank to carry us on home........

A little harder in the Fcar..

 

If it is the sender you have to buy the whole arm/sender unit together from Ferrari since it comes together...However, it is the exact same sender unit on a Fiat (I think 124). Mine was shot so I simply bought the Fiat sender/arm and swapped out the sender onto the GT4 arm. I think it was about $100.

 

Mine seems to be acting up recently too

It is fine above 3/4 tank, then jumps areound a lot and goes to Zero when below 3/4 tank----

 

Sorry to hear you guys are having trouble. It's a relatively easy fix.

Get 'er done!

 

Thanks for the post - same for me with my 1976 308

 

There was a thread on this a while back. Both my cars are the same, I believe all of them are, and I believe its designed that way.

In a long hard corner, fuel will transfer from the inside tank, to the outside tank. If the car gets below half its capacity, it will starve for fuel in a long hard corner once that tank runs dry. Its also good insurance to keep from running out of fuel.

 

I think if there was a problem with the variable resistor (wire wound around an arc, which the float arm sweeps across, to varry the resistance and thus, the level in the tank), wouldn't the gauge not work at all (say, if there was a break in the wire)?

I get the feeling that maybe your sending unit arm was cut short at one time for whatever reason and when it's at full drop, it's actually quite high up in the tank, giving a false low reading.
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As I look at my photos, I can see another possible problem; if those contact wipers are bent incorrectly, as the lever sweeps down, it may loose contact half-way through the sweep, shutting off the signal to the gauge, causing it to read "empty"...

Those contacts can be accessed (at least on my sending unit), because I popped off the little cover so I could photograph the inside.

 

You can try to "repair" it. Usually the contacts are worn or corroded. Don't get your hopes up.

If the car sat for a while with a partial tank and a little moisture in it sometimes there is a problem at that fuel level.

 

Kieth,

Nice pictures. This is the first time I have seen this assembly. Is that variable resistor assembly actually in the tank....with fumes...and oxygen?

Curious...

-Rjay

 

Yep......

The same with my speedboat..

There are other designs of level sensors out there, I researched them as the bow tank in my boat takes huge abuse...breaks them about every other season.....

But with a car it's easier to just put a new one in and move on.....

 

Actually that housing would be submerged below the fuel until the tank was empty.....no fumes......

 

I had the same problem with my 63 Corvette. Removed sensor, tested with a Fluke volt / ohm meter found resistance readings were not smooth through arm travel to simulate fuel level. Found moving contact was lifting off resistor wires causing the resistance to go open. Windings looked good. Had another used sending unit from another C-2 Corvette that I saved for parts. Took spring load contact from saved unit, replaced the original sending unit spring load contact (one that sweeps the resistive windings), no problems for 5 years now. Adjustments were needed. Fuel level arm needed to be adjusted (bent) to show correct level. I had an extra saved fuel tank for bench work adjustments and testing. With empty tank, put measured half capacity exactly in with sending unit connected to tank wire harness, checked level on gauge in car. Made adjustments two times then was correct. Installed in car. Perfect!

 

The gauge in my 79 GTB seems to work fine except I only get about 10 gallons in it when filling from 1/4 full. When I drained the tanks from 3/8 full I got 12 Gallons out. From what I have read here I am assuming that this is normal.

 

good to see I'm not alone here with fuel guage problems. Mine has very similar operating characteristics as others here. Shows full after a trip to the gas station but about 20 miles later reads zero. I haven't had the guts to run it far enough to see if the red warning light comes on, I've used my trip odometer to judge when I should refill. The next few days should bring some rain so I may take a look at the sender then...

 

I thought the fumes were in the tank ESPECIALLY when it was "empty" a little bit of gas is worse.

So, I find this really interesting. I hear all this stuff about fumes and sparks yet this assembly has survived not only Ferrari but many other tanks.

Could this be the source of the exploding 308??? - Yea, probably not.

Anyway, a wirewound resistor is like taking two wires and poking them together as you slide up and down the track. I would think there would be micro-arcing. Wirewounds are pretty much forbidden in explosion proof electronics design (ex-proof), how come it works here? I dunno.

-Rjay

 

Gasoline vapors will ignite when their concentration in air is between 1.4% and 7.6% if a significant heat source is present.

Apparently there are at least 2 things keeping the fuel tanks on most motor vehicles from exploding.

1. The air/fuel mixture is usually too rich to burn (over 7.6%) unless you drive around with the gas cap off.

2. Fuel level senders are crappy ignition sources.

There is one notable example of a fuel tank/electrical explosion that comes to mind. TWA Flight 800 was attributed to a wiring problem inside the empty center fuel tank of a 747.

 

I will never be convinced that that fuel tank, or anything else on that aircraft had any factor in the loss of it.