QuestionQUESTION: I just bought this car with 58,000 miles after it sat for 12 years. After driving it for a week, I have found that the drivers side gas tank has two issues. First, it does not register on the fuel gauge. It just goes to empty when selected. Second, when putting gas in the tank, it backs up (overflows) easily. Additionally, when driving and then stopping to check that tank it overflows when opening the cap. The return valve does seem to be working when I checked it. Gas is returning properly when each tank is selected. Any info would be appreciated.
ANSWER: Hi Scott,
What do you mean by "seems to be working" I know what "works" is and "Not work" means but there is no such thing as "seems to work".
A fuel tank CAN'T overflow gas unless you have a gas supply going to it. Or you over filled the tank cold and then it got heated. Tell me how you tested the return system.
Howard
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QUESTION: The way that I checked the tanks and the flow of gas during normal operation was to start on the left tank and then push down the flap inside the fill cap and view the return of gas (it looked like a small river flowing). Next, I switched to the right tank and did the same thing. The flow of return gas was now coming out of a small tube the diameter of a drinking straw. So from that observation, I figured the valves that controlled the flow of gas when switching from one tank to the other were operational.
Now after driving the car for a few days (on both tanks), I went to the gas station to fill up. The right tank was at less than 1/4 and the left was unknown because the gauge for that side does not work. I put 8.5 gallons in the right and the left would not take any gas, just click click click when I tried to fill it. When I got home, I checked the caps and found when I opened the left side, gas overflowed out and spilled onto the quarter panel.
So I guess my reasoning for stating "seems to be working" was from the fact that I viewed the return flows changing from one side to the other properly but the left tank still must be getting more gas on the return when operating on the right side tank.
Scott
AnswerThat's good Scott, however to be sure you should have looked at both return lines to see that either of the return valves was not leaking thus the main part of the fuel could be returning to the correct tank but if the valve was not sealing correctly it can return most of the fuel to the correct tank and still return a portion to the wrong tank so it is a good idea to look at both when testing one side to see that none is returning to the wrong tank. The use of a flash light is a safe method.
Yours may be returning correctly but gas expands a lot when heated and the gas coming out of a station is usually cold so filling all the way and then driving around on the other tank can cause an expansion of the fuel to a point that it overflows. Keep in mind that both tanks are pressurized to about 3 PSI and if you open the full tank the normal pressure of 3 PSI will blow fuel out the filler neck. I drive a ser III too and have learned to only fill short of "Full" and to continuously switch tanks. One time it cost me a wrecker tow. (DUH! Expert Jag mechanic)
You now have a secondary problem when fuel is known to over flow. The vent system is uniform between both tanks so it is possible that the over flowing tank has forced raw fuel up to the pressure valve and on to the charcoal canister thus destroying it. This will also cause the pressure in the tanks to exceed the 3 PSI that the regulator valve holds on both tanks. This can cause extensive damage so you need to check the valve and forward vent lines for fuel.
If you will jack the front of the car up and remove the right front wheel, you can see the vent line up high going to a small plastic valve and a short hose from that valve goes to the large canister just behind the head light. Remove the hose from the tank side of the valve and shake it to see if there is any droplets of fuel in it. If there is, the canister may be destroyed. Then remove the plastic valve and use a "Mighty Vac" or any method you have to test the pop-off pressure of the plastic valve. (aprox. 3 PSI) (I operated on mine and got it down to 1.5 PSI)
3 PSI is better to raise the boiling point of heated fuel but causes other problems so I altered mine and am very careful not to fill the tanks too full.
The fuel gauge sending unit is in behind the rear tail light assembly. The wiring and the gauge switching switch can be tested without removing the sending unit. To test the sending unit you need to drain most of the fuel out of that tank and remove the sending unit. A common failure of the sending units is they rust solid and the arm don't move. I have been able to save about 50% of the failed units that I have worked on by carefully forcing the arm up and down with a little WD-40 help.
If you would like a wiring diagram of the fuel return system let me know and I will post it on my web site for you to copy.
Howard
