Fuel gauge not accurate after filling up.

[Dysco]

Hi guys, quick question, 79 fuel gauge has been spot on for all these years. At my last fill up it registered as 1/2 tank, but I put 10 gallons in (16 gal tank) which immediately put a question mark in my head.
I checked as I put on the next 90 miles, the gauge went down between full and 3/4. When I filled it again it took 7 gallons, so it should be just above the 1/2 mark.
I only use non ethanol 91 octane. I did put 4 oz. of marvel mystery oil in that first tank, but i have never heard of it causing any issues like that.

Suggestions?

 

[73c20jim]

Bad ground on sender???

 

[Shorty81]

Bad ground on sender???

X2

 

[Dysco]

Wouldn't a bad ground cause it to either fail, ie not work like being on E or F all the time, or become very erratic?

 

[Shorty81]

Mine would never climb over 1/8, then I found bad ground on sender. Might be you have a bad sending unit.

 

[73c20jim]

Wouldn't a bad ground cause it to either fail, ie not work like being on E or F all the time, or become very erratic?

Loose wire. Intermittent contact.

 

[Dysco]

Loose wire. Intermittent contact.

Again, wouldn't intermittent electric cause a fluctuating needle, not one that's completely steady, but somehow off by several gallons?
I would think that would rule out electrical?

 

[75gmck25]

The sending unit ground wire is connected to a post that is part of the sending unit. It might be a spade connector or it might be crimped on directly. You may be able to see the wire and verify it is on the post.

Then follow the wire where it comes down and through a grommet in the frame, and its grounded inside the frame. Its very easy to remove the ground screw, clean the frame with a wire brush, and then reinstall the ground wire.

While you are under there, also follow the sending unit wire along the frame and up to the front of the truck. On your older truck the primary tank wire (passenger side) with a single tank will run up the frame rail to the engine compartment, then across behind the engine and into the passenger compartment near the fuse box. The entire wire should be checked for damage, especially near the exhaust manifold area.

If you have dual tanks the path of the sending unit wire is much longer and more complicated. Do you have dual tanks?

Bruce

 

[Dysco]

The sending unit ground wire is connected to a post that is part of the sending unit. It might be a spade connector or it might be crimped on directly. You may be able to see the wire and verify it is on the post.

Then follow the wire where it comes down and through a grommet in the frame, and its grounded inside the frame. Its very easy to remove the ground screw, clean the frame with a wire brush, and then reinstall the ground wire.

While you are under there, also follow the sending unit wire along the frame and up to the front of the truck. On your older truck the primary tank wire (passenger side) with a single tank will run up the frame rail to the engine compartment, then across behind the engine and into the passenger compartment near the fuse box. The entire wire should be checked for damage, especially near the exhaust manifold area.

If you have dual tanks the path of the sending unit wire is much longer and more complicated. Do you have dual tanks?

Bruce

Single tank. Thanks for the details.

 

[MisterB]

Slightly off topic, but you run a square on 91 octane? Seems unnecessary, and expensive.

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[73c20jim]

Extra octane that is NOT needed is not your friend.

 

[Dysco]

I'm running ethanol free fuel, that it's 91 octane isn't the point. Im ok with the extra cost, i dont dont daily drive it.

 

[Doppleganger]

Do those have the plastic floats on the sender? I've had them (on similar setups) develop a pinhole and fill with fuel. Took about a week to empty it was so slight.....then I covered it in "Goop" (as no one made new floats) and it worked perfect again.

My truck is currently this way on the right tank - my guess is it sat so long the float is stuck. Not messing with it as new FI tanks and senders are on the Spring menu.

 

[QBuff02]

Slightly off topic, but you run a square on 91 octane? Seems unnecessary, and expensive.

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I run mine on Sunoco Standard 110. lol

 

[MisterB]

I'm running ethanol free fuel, that it's 91 octane isn't the point. Im ok with the extra cost, i dont dont daily drive it.

You can't get ethanol free in 87 octane?

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