78stan

So in my 1978 corvette, I ran out of gas. Thought I had enough to get to a station... turns out I didnt. Called AAA, they guy shows up finally, puts a long funnel into the gas tank and pours me some gas. I get to a 711 and fill up (that was the nearest gas station). After I filled up $40 worth, the very next day I run out of gas again... the gauge was showing half full!

Now I am thinking teh floater got stuck... so I topped off at Chevron after another AAA service and even bought the Techron additive. Today when I drive the car (its been two days) it shows FULL at rest, but moves somewhat to 3/4 while driving.



Anyone have any idea on how to solve this? Thanks in advance. This is a 1978 Vette, 350 small block with Holly Carburetor (I am having the Carb rebuild and the jets set to save on gas).

PhotoVette1

Replace the gauge. The felt around the needle gets old and causes weird readings.

MelWff

Check the ground in the rear where it connects to the frame for rust.

78stan

I notice a few wires grounded on that rail above the exhaust, but there is one that is not connected at all... could this be causing the fluctuating? I am looking behind the bumper driver's side rear.

Could bad gas cause this issue, it never was a problem until I bought gas at 711.

Here is the video...
http://dstancic.com/vette/IMG_0561.MOV

78stan

Just had the carb rebuilt, was told it was dirty and the power rod was out of whack. Set the points down to from 6.2 to 6... trying to save gas. I notice a slight difference in performance. The gas needle is at the 3/4 mark now... will keep you posted if anything that was done today affects the gas gauge.

0Willcox Corvette

Sorry Guys.. none of the above.

A fuel gauge without a ground will go to 3 o'clock. A gauge without a ohms input will go to full.. and a gauge with a grounded out ohms wire will go to empty.

If the gauge is sticking on 1/2 a tank I'll be the liner in the gas tank has fuel behind it and is hitting the float.

If you wish to learn more about how the fuel gauges work in the car I have a page here that will show you. http://willcoxcorvette.com/repairand...lp.php?hID=277 This page will show you how your dash unit can read in certain conditions. It also covers how to diagnosis your problem too.

But.. I do think you have a liner issue in the fuel tank and when fuel gets behind it it can push inward toward the float and... well in most cases it will usually stop the float around the 1/2 tank mark.

IMHO,

Willcox

PhotoVette1

Not always true.

I've had to replace a fuel gauge that went bad. Once it was out, I could blow gently on the needle and have it move.

0Willcox Corvette

I'm not sure what in post you are saying is not always true?

In a few days the fuel gauge video will be completed which will show all functions of a fuel gauge when different circumstances are present.

In this instance the OP has a gauge that sticks when it hits 1/2 a tank. This is indicative of a fuel tank bladder swelling outward from gas behind it and pushing outward until it hits the sender arm. It could be a defective sending unit too.

The quick test to isolate the issue from the dash or the tank:

1) Remove the ohms wire from the sender and turn on the key on. The gauge should go to full.

2) Ground out ohms wire and the gauge should go to empty.. thus by-passing the 1/2 tank stuck mark and verifying the dash unit is operational.

3) If the gauge sticks at 1/2 tank without the sender connected then by all means look at the gauge. Which would be very unusual.


A fuel gauge without a ground will go to 3 o'clock.
A fuel gauge without a ohms input will go to full..
A fuel gauge with a grounded out ohms wire will go to empty.
A fuel gauge with a blown resistor will function up to 1/8" past the 1/4 tank mark.

Your blowing on a gauge without a sender attached it would fall under a gauge with no power or ground and yes if you blow hard enough you can make it move. Why? There is no resistance in the gauge except for the resistance light weight oil. In this case you could move the needle anywhere you desire and it would not affect the function of the gauge. As soon as you power it up it will be expecting an ohms input and go to that setting. If the ohms input is missing it will peg full, if the ohms is grounded out it will peg empty. I'm not aware of felt on a gauge from 1953-1982. The spindle is contained in a plastic housing that is filled with oil. Now what can happen is the dampening oil will dry out or dissipate and when this happens you'll get an over-responsive fuel gauge (sways too much).



It will be hard to tell if the issue is the bladder or the sender. It's a 78 so your just going to have to dig in. I'd do the test above and eliminate the dash unit.. then remove the sender and bench test it. It should put out 0-90 ohms with 0 being empty and 90 full.

And then.. with the sender out, look inside the tank and check out the badder.

I for one hope it is the dash unit.. it is easier for me to replace one of these vs. taking out the tank.

Willcox

78stan

Thanks to all who have contributed to this issue of mine... running short on patience and wanting to get this fixed, I took it over to a Corvette Shop... they diagnosed it and informed me it was the sending unit... the inspection fee was 100, the part was a 110 and couple of hours of labor at 75 an hour... all in all I walked out of the Corvette Shop with a working gas gauge for $400

Been having a slew of issues, Power Steering going out and Brake caliper needed replacing... that took me a couple of days as the brake caliper was a hunt and the power steering pressure hose was also a hunt as it froze on the pump. Carb needed to be cleaned and power rod replaced.

With all that being said, I wish I worked on it my self and saved all the cash I spent but because I took two days off work to work on the power steering and brakes, weekends are sacred to my wife... I had to fork over the 4 bills!

PhotoVette1

Be that as it may, the needle can become so loose that any bump or acceleration causes it to move around leading to the belief that the sending unit is bad or there's a loose ground. I'm not guessing here.

0Willcox Corvette

Photo.. what happens is the oil inside dissipates.. the spindle inside looses resistance and the gauge becomes over-active. When this happens to a fuel gauge it will sway with every slosh of the fuel in the back.

Roger and I explored the idea of putting a capacitor on the gauge as a way to stabilize the movement. I played with this for a while but found it was probably more work then the average person would want to though to fix vs. replacing the gauge.

On another note: The fuel gauge video will be up tonight.

78stan.. glad you got her fixed..

PhotoVette1

Thanks for explaining, I knew it had to be something like that. For the longest time I thought it was the sending unit. Finally took it out and cleaned it up--and it made no difference. Then I changed the gauge and and have been happy ever since.

0Willcox Corvette

hunt4cleanair

Ernie

Question than a statement:

After a year of working from underneath my 78 and a minimum amount of fuel in the tank, I added about ten gallons and took it to the office to stretch its legs and check for leaks of any significance. Well, I noted that the fuel gauge came up slightly, out of the read but failed to read to what should have been the half way mark. I suspect the needle on the sending unit is not reading properly and moving along of its arch. You say?

The statement is your write-up is quite thorough having written on this topic myself, (years ago). It takes the novice through the troubleshooting from gauge to tank, breaking the system down into its segments to troubleshoot each system while also describing the tools, how to use them and what to look for. Nice work!

0Willcox Corvette

Thanks Tom.. Wow! I can't believe how fast time has passed. It's been over two years since I did a revision. (I still need you hire you to proof read for me )

On your issue... my gut is pushing me toward the sending unit issue.

I would pull the ohms wire off the sender and see what the reading is on the gauge with the key on. It should go to full just above the top of the face rivet head! If the gauge pegs full then ground that same ohms wire out and it should go to empty. If the gauge functions in this manner you have accomplished four things quickly..

1) It tells you the dash unit is working.
2) It confirms power and ground on the gauge.
3) It rules out a blown resistor on the back of the gauge and
4) It confirms the ohms wire from the tank to the dash unit is not the issue.

If the gauge will only read to just above 1/4 tank.. then your issue would be a blown resistor on the back.. but this isn't the problem because with 1/2 a tank you should be reading just above 1/4 tank.

So.. what is left if you pass the first test... Sender issue.

Another plug for sender: You described that the needle moved slightly. Again this points toward sending unit and the reason is the slight movement. )Missing ohms, pegged full, grounded ohms pegged empty, blown resistor 1/4 tank, and missing gauge ground pegged empty. No symptom possible allows for this issue at the dash unit... only the sending unit).

So with that I would pull the ohms wire and take an ohms reading off the sending unit. I would also verify the ground on the sending unit.. (that video is coming)..

I don't have the 77-82 fuel gauge video finished but if you go to the one I posted for the 68-76 cars you'll see the exact gauge function. This function is exactly the same for 77-82 cars. (The issue with the 77-82 is the hidden element of surprise and that is the printed circuit board).

Oh.. and I fixed the pdf file tonight for you.

On the OP's post.. I steered away from the sending unit based on the long issue with the bladders in these cars and the fact that 1/2 tank stuck needle is usually the trigger to look at the bladder.

Ernie

FatCat

What would it mean of the gauge bounces about a 1/4 of a tank. Example! When I fill it full, it bounces between 3/4 and full as I drive the bounces relit ice to how much file is in the tank. When I have used a 1/4 tank it bounces between 1/2 to 3/4 and so on. Where should I look to solve this problem?
Thanks!

0Willcox Corvette

If the gauge works fine when you are not driving then I suspect the dampening oil is gone from the gauge and you needle is moving from fuel sway in the tank. When the oil is gone the gauge becomes over-responsive. If you watch the video you'll see I move the needle with the potentiometer and the needle cruise to the correct position.

It could be something with the sending unit too.. so don't rule that out. The only way I know to find out is to insert a potentiometer in place of the sending unit and see how the gauge responds. If it is fast to the spot then the gauge is the issue. If it is slow and smooth take a look at the sender.

Now you can use a flasher to do this test... you do this by making two test wires to plug on the flasher. Run one to ground and the other to the ohms wire at the sender. Then turn the key on and you should have just above 1/2 a tank on the gauge. Now.. watching the gauge unground the flasher and then ground it again and see how fast the gauge goes back. If it jumps back quickly then it is the gauge.

FatCat

After I posted this, I thought that I remembered it not moving while sitting. New car for me. I went to check that and I was correct. When I first start the car, it didn't move I put it in gear and just bumped it and sure enough it started to jump and slowed slowly as if you could picture it moving with the fuel.

FatCat

BTW it jumps quickly not a swaying motion at all. If it is the dampening oil does that mean I need a new gauge and where in the world would I find. New one? Hahaha sorry I am new to all of this.

c3dreamer03

Wilcox: What size capacitor were your using? Did you place it across what you are referring to as the ohms (signal?) wire and ground?