71 Duster Tank Fill Issues

[MrTwister]

On our ‘71 duster we’ve been trying to diagnose an issue where the fuel gauge reads full and after driving for a short while it’s already down to 1/2 or 1/4. We thought it might be the sending unit or gauge but when going to fill it up it will only take about 4-5 gallons before gas comes up the filler neck and the gauge reads full again.

From what we’ve seen so far this is not the original tank, the replacement tank didn’t have the 4 vents that hook up to the vapor separator in the trunk and those tubes were cut. Our original plan was to just replace the tank again but wanted to make sure we wouldn’t have the same issue if it was a venting / pressure problem since the filler doesn’t look like it has a vent tube in it. Any thoughts for best plan forward?

 

[halifaxhops]

Make sure the sending unit is working properly first, the aftermarket ones have alot of issues.

 

[MrTwister]

We replaced that first but it didn’t make much difference from the one that was on there when we got the car.

 

[fatoldguy]

Make sure you have a good ground.

 

[Demonracer]

If you have the original sending unit, contact Wolf & Co. They do a great job of refurbing senders.

Wolf & Co.
36470 Biltmore Pl #8
Willoughby, OH 44094
I have nothing to do with them, a friend sent his sender to them to have it refurbed & it works as good as it did when it was new.

 

[67Dart273]

The vent system has nothing to do with fill. The fill venting is done by the baffle in the fill tube.

First check the gauge is accurate, one way or the other. You can separate the tail harness at the left kick panel, pick up the sender wire and check it with ohmeter. But you need to do that with a halfway known amount of fuel

Check gauge by either getting a rheostat which you can set with your ohmeter, or some fixed resistors. Hook these from gauge sender wire to ground, and observe reading:

 

[67Dart273]

By the way what ARE you doing for an operational tank vent?

 

[MrTwister]

By the way what ARE you doing for an operational tank vent?

No idea I’m looking for ideas on what to check.

 

[MrTwister]

The vent system has nothing to do with fill. The fill venting is done by the baffle in the fill tube.

First check the gauge is accurate, one way or the other. You can separate the tail harness at the left kick panel, pick up the sender wire and check it with ohmeter. But you need to do that with a halfway known amount of fuel

Check gauge by either getting a rheostat which you can set with your ohmeter, or some fixed resistors. Hook these from gauge sender wire to ground, and observe reading:

View attachment 1715658000

Will do. Without knowing what the previous owner did I have considered that the sending unit isn’t compatible with the Auto Meter fuel gauge installed. I read in their documentation that they need to be matched or the readings will be off.

 

[Mike69cuda]

What you are seeing is how aftermarket fuel senders work. The oem senders have the wire wound sender shaped to match the shape of the tank. They are called non linear senders.

If you look at an abody tank, there is a big chunk out of the top of the tank for the spare tire well. This means that you don’t need to use a lot of gas to get the fuel level to drop a lot. This is why your gauge drops rapidly with an aftermarket sender. The oem sender compensates for this.

The aftermarket senders are linear, they are made for a tank that does not have a chunk taken out. They do exactly what you are seeing. You don't need to use much gas to get the gauge to drop a lot.

You have three choices:

1. Live with it

2. Find an oem (non linear) sender.

3. Buy a device called a Meter Match. You can use it to calibrate your sender to you gauge to get close to the right readings.

There are bunches of threads on this here on the forum.

 

[67Dart273]

Will do. Without knowing what the previous owner did I have considered that the sending unit isn’t compatible with the Auto Meter fuel gauge installed. I read in their documentation that they need to be matched or the readings will be off.

This is likely the answer. You may have to pull the sender and find out. So far as vent, as I see it here are your choices.

1...First is to research and find out what the tank "is." The evap system changed through the years, sometimes every year, and some of the later tanks have a built-in vapor separator. Also look under there and identify any extra lines coming out and going someplace

2...Some advocate finding a vented cap but I dislike that idea. Good way to "unpaint" the side of the car.

3....If you can fabricate, and the vent seems not "there" consider modifying the filler tube to operate as a 69/ earlier. This had a 1/4" vent welded in near the top of the tube, which made a "U" up close to the top of the fender, then followed down the main tube and "poked" through the floor gasket, ending--open--inside the rear frame rail. Simple as heck and they worked well