740 Wagon 1990 Gas Tank Oddity

[GSmith]

According to the owner's manual, my car should hold about 15 gallons (US) of fuel. In the three weeks I've owned it, I've filled it from empty several time and it's only taken 7.3 to 7.5 gallons.

Now, first instinct is to say that the fuel gauge (or the float or the sender unit) is bad, BUT....

When the gauge reads empty, the car will start acting like it's running out of gas (by that I mean that taking a turn when low on gas causes the car to stutter like the gas has shifted away from the in-tank pump.

So, seeing as the gauge reads full when I top it off, the car acts like it's near empty when the gauge is at the end of the red empty zone, the car is not burning fuel abnormally (averaging 20mg combined city/highway), and I smell no gas leaks I am left with only two conclusions:

a) At some time in the car's history, the tank needed to be replaced and to save money they put in a smaller tank (I'm not really behind this one, but you never know).

my other guess...

b) Someone replaced the pump/intake/float/sender unit (which I believe is accessible in the wagon area floor on the top of the tank) with a unit from another Volvo that, while technically compatible, was designed for a shallower tank. This means that my pump and float are only 1/2 way as far down into the tank as they should be.

I'm going to put the car up on ramps Sunday, so the questions are:

a) How can I be sure the gas tank is the right size/make by looking at it.

b) Are the pumps/intakes/floats/sending units from other Volvos compatible with my 740 and how would I know if they put the wrong one in? Is there a specific part number that I should be looking for on my unit?

If anyone else has had any similar experiences, please let me know. I'd like to use the other 7.5 gallons of my tank. Filling up every 140 miles when you drive 45 - 50 miles every day is getting annoying.

Thanks!
G

[billofdurham]

There were two tanks made for the 740 - 80 litre for multi-link suspension and 60 litre for rigid axle. Yours should be the 60 litre.

The attached shows both and there are obvious differences.

740 fuel tanks.pdf

(415.25 KiB) Downloaded 369 times

The sender units for the 740 are standard across the range. I would suspect that someone has replaced the pump/sender unit and has bent the float arm reinstalling it. Easy to do if not taking care.

You do access the in-tank pump/sender unit through a hatch in the luggage space floor.

Bill.

[billofdurham]

I have just posted the resistance specs for the 960 fuel level sensor and how to test it and the information applies to the 740 so I shall add it to this thread for you to check yours and for future reference.

Disconnect the battery negative lead, having found your radio code first.
Remove the access hatch to expose the top of the sender unit.
Disconnect the connector and connect an ohmmeter between the white/grey and brown wires.
For models after 1993 with a 60 or 80 litre tank the resistance should be between 2 and 130 ohms.
For pre-1993 models with a 60 litre tank the resistance should be between 0 and 280 ohms.
For pre-1993 models with an 80 litre tank the resistance should be between 0 and 367.5 ohms.

If any other reading is obtained the sensor is faulty and should be replaced.

Bill.