Fuel gauge problem

[Clhaney]

The fuel gauge in my 71 MC is not working correctly. I topped tank off but gauge only shows 5/8 full. I replaced sending unit with no success. Mine is not an SS car so I am told there is no replacement gauge made for it. Any advice on finding problem?

[FrankOC]

Have you checked the sending unit at the plug in the rear of the car?  If the tank is full, you should see 90 ohms.

[cny first gen 71]

What was it doing before you changed sending unit? If there's any change then you could be moving in the right direction.  The aftermarket units sometimes need to be bent a little up or down. I'm going through that right now on mine.

[Dtret]

Welcome Clyde. Great group here. You’ll have fun and learn stuff too. I agree with what was said. I know it’s a part but you may have to drop the tank again. 

[Scott S.]

Welcome to the fray, Clyde...

Odds are that it's not the gauge, but they are available for non SS cars. I've got one in my '70 350 cid car.

Like Frank mentioned, you may not be getting enough power to the sender. That could be as simple as a bad power feed wire. 

Good luck. 

Scott.

[cny first gen 71]

Just remember there isn't a full 12 volts to the sender, I fried one testing one by putting 12 volts to it. I sent it back just saying it didn't work they didn't question it gave me a new one. But Frank is correct  90 ohms when in full position 0 when all the way down (empty)

[Clhaney]

Before I changed sending unit, the gauge showed I had about half a tank. After driving the car within a few miles the gauge dropped to a quarter of a tank and a few blocks after that it dropped to empty. Then a few blocks later the gauge went back to about a half a tank. Before I got back to my shop, it did the same thing again. 

[cny first gen 71]

Did you test the old sending unit or just replace it? Do you still have it to test or just toss it? I always test the new one before I put it in but it didn't help me as I do have to bend the arm down some as I've learned since then. I will test it with tank empty then half then full all the way to make sure its right this time, after pulling the tank 3 times already. 

[stangeba]

For the past several years my my 71 fuel gauge has been moving slowly, up then down, about 1/6 to 1/8 difference when driving but is still when stopped at a red light.

When the tank is full, no movement but more movement at 1/2 tank.

I swapped the entire instrument cluster and wiring from a 71 350 4bbl Monte with the SS gauge package, over 30 years ago.

At this point I am not going to tear into it but I may inspect the connections and grounds at the trunk area.

It's always the ground, right!!!!!!!!  🤣

[cny first gen 71]

Sounds the float is just moving up and down some with the sloshing around of the gas. I have seen that on some of the older models of cars.

[Michael Boyte]

Steve, that was the first thing that I thought as well 

 I remember going around a curve and the fuel needle dropping to empty, straighten up and it's back on the half tank mark 😂 

[Scott S.]

Parking/driving on a hill will do that too...

[cny first gen 71]

2 hours ago, Michael Boyte said:

Steve, that was the first thing that I thought as well 

 I remember going around a curve and the fuel needle dropping to empty, straighten up and it's back on the half tank mark 😂 

May have been going to fast around that curve 😳 🤣

[Scott S.]

Anyone ever notice how much more gas seems to be in the tank just as you step into it leaving a light ??? 

[jft69z]

1 hour ago, Scott S. said:

Anyone ever notice how much more gas seems to be in the tank just as you step into it leaving a light ??? 

Or when you have to remove the tank.

[Rob Peters]

2 hours ago, Scott S. said:

Anyone ever notice how much more gas seems to be in the tank just as you step into it leaving a light ??? 

Not so much with my Monte.... It always seems to go down too fast with my 350 2brl because its primaries are about as large as the secondaries on a 4brl.

rob

[Scott S.]

I guess the 2bbls don't exert enough G force to have an effect on gas..... 

[cny first gen 71]

I actually get better mileage with the q-jet than I did with the 2 barrel unless I put my foot into it then the secondaries are like flushing the toilet 🤣 of course I hardly ever do that 😅 

[Rob Peters]

1 hour ago, cny first gen 71 said:

I actually get better mileage with the q-jet than I did with the 2 barrel unless I put my foot into it then the secondaries are like flushing the toilet 🤣 of course I hardly ever do that 😅 

Steve, that is what I was referring to.  That is why a 2brl gets such crappy mileage.  

rob

[Scott S.]

It was a reference to how the gas moves in the tank when one drops the clutch at 3000 rpm.

[Scott S.]

22 hours ago, cny first gen 71 said:

I actually get better mileage with the q-jet than I did with the 2 barrel unless I put my foot into it then the secondaries are like flushing the toilet 🤣 of course I hardly ever do that 😅 

Yeah, the Q Jets do have huge secondaries. 

My Edelbrock, and I imagine Holleys have fairly equal venturies. 

[Whons]

Yeah 350 engines with a 2 barrel will suck more gas than a 4bbl. Every car in the mid 60's and 70's that I have owned the gas guages have always floated around after dropping from full.

[stangeba]

I am the original owner and the fuel gauge didn't do this for the first 50 years or so and I haven't changed anything in years.

Probably a poor ground, it's always the poor ground that causes strange problems. 

Too cold now, I will think about it this spring..............

[Michael Boyte]

It's always a good idea to check all of the grounds when having electrical issues. !  Most of the time you will find that to be part of the issue. 

[Scott S.]

My '04 has the same issue, and I've owned it pretty much since new.

First 1/4+  tank stays at full, second 1/3 runs through the scale, then the last 1/4+ stays around 1/4 until it drops and the light comes on.