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Fluctuating gas gauge

Shawn

Shawn

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My 79 K5 has horrible fluctuating issue with the fuel gauge. It fluctuates and sweeps a good half of the gauge. When I fill it, its not bad. This is when its standing still with no fuel sloshing around. The grounds seem be ok. Should I be looking at replacing my fuel gauge? I think the previous owner replaced the sending unit and got the same results. Anyone else had this issue?

Tried to search on this and not much besides this thread which doesn't have too much info.

Gas guage fluctuates

Does anyone know what would cause the gas gauge needle to fluctuate back and forth? I filled up the tank and the gauge works, it just fluctuated (spelling?) between full and 3/4 when I drove 150 miles round trip yesterday.. It fluctuates all the time.. plus I hear a clicking noise under the...
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All damping is done in the gauge. Working properly they should take a significant amount of time to move regardless of what the sender is "telling" it. Disconnecting the gauge wire or ground for the tank should have the needle slowly move one way or the other to it's "max" positions past full and empty, not jump instantly.
 
dyeager535 said:
All damping is done in the gauge. Working properly they should take a significant amount of time to move regardless of what the sender is "telling" it. Disconnecting the gauge wire or ground for the tank should have the needle slowly move one way or the other to it's "max" positions past full and empty, not jump instantly.
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Wanted to report back on this. You were right! Replaced the fuel gauge with a used one and its solid as a rock. I wanted to buy something new but LMC was all out of stock for the 80 and older looking gauges and you couldn't even order one. This also fixed my oil pressure gauge reading too high. I just put the gauges in without a bezel to test. I bought some Evapo-Rust to clean the rust off the gauge faces. Hope it works...
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My temp gauge is still reading low so it might be a sender issue. Here it is at 170 degrees. I figure it should be higher than that.
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With key in "run" disconnect the wire from the sender. Gauge should go past max one direction. Now connect that wire directly to ground, should go past max the opposite direction. If the gauge reacts properly, everything is good except the sender or the conection at the sender. If it doesn't react properly, the/another problem is somewhere between the end of that wire and the gauge itself.

Make sure the contacts are good, and that the sender isn't using teflon tape to seal. Really not supposed to use (or need) anything on those senders to seal them. Obviously if its a conductive sealant that would be different.

This all assumes the engine isn't actually running that cold due to wrong/missing thermostat, etc. Senders (or gauges) can be "off", generally I use the gauge as a reference. I know X needle position is normal, and I know the gauge and senders work, so if the reading is abnormal I know something is up.

Edit: I was tired when I wrote that lol. A gauge that is reading off, but moves "normally" is going to be tough to fix. Could be sender, could be gauge. I've seen charts online that show what the resistance reading should be of the sender at X temperature, but the senders have changed over time, so it may be a bit difficult to find exactly what temperature is what resistance for your specific sending unit.
 
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Yes that is a little low, mine stays in the second segment, since I got the correct sender. Before it always read low like yours. Is your sender 1/2" pipe or 3/8ths" pipe. I was unable to find a correct 3/8ths pipe to work with my 77 gauge. The correct sender is 1/2" and can be turned down in lathe and threaded for 3/8th" pipe.
Modified old style sender, correct part number, 2 incorrect senders, eng temp at t stat and gauge reading.

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dyeager535 said:
With key in "run" disconnect the wire from the sender. Gauge should go past max one direction. Now connect that wire directly to ground, should go past max the opposite direction. If the gauge reacts properly, everything is good except the sender or the conection at the sender. If it doesn't react properly, the/another problem is somewhere between the end of that wire and the gauge itself.

Make sure the contacts are good, and that the sender isn't using teflon tape to seal. Really not supposed to use (or need) anything on those senders to seal them. Obviously if its a conductive sealant that would be different.

This all assumes the engine isn't actually running that cold due to wrong/missing thermostat, etc. Senders (or gauges) can be "off", generally I use the gauge as a reference. I know X needle position is normal, and I know the gauge and senders work, so if the reading is abnormal I know something is up.
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Thanks! I will try that and see what happens.

Wes Harden said:
Yes that is a little low, mine stays in the second segment, since I got the correct sender. Before it always read low like yours. Is your sender 7/8ths" pipe or 3/8ths" pipe. I was unable to find a correct 3/8ths pipe to work with my 77 gauge. The correct sender is 7/8th" and can be turned down in lathe and threaded for 3/8th" pipe.
Modified old style sender, correct part number, 2 incorrect senders, eng temp at t stat and gauge reading.
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Thanks for the info. I was able to find some on Rock Auto. I ordered TS43T last week which is the bigger one. I might have to order the one you got if you are having luck with it.
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What size does the cylinder head need. This was my problem the cylinder head wanted 3/8ths pipe
 
Wes Harden said:
What size does the cylinder head need. This was my problem the cylinder head wanted 3/8ths pipe
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That is a good question and I have not looked closely at that sensor since its kind of buried with all the emission tubes and stuff. The 350 engine was replaced with a GM crate 454 and its been a legal CA swap so all the emissions crap is there. I assume its the bigger thread size but not 100%.
 
I am not sure but I think sometime in the early 80's gm went to 3/8 pipe thread in the heads.

Wow ca emission legal big block in a K5 thats amazing since gm never made one.
 
Wes Harden said:
I am not sure but I think sometime in the early 80's gm went to 3/8 pipe thread in the heads.

Wow ca emission legal big block in a K5 thats amazing since gm never made one.
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Could be the smaller size. I will check.

It use to be registered in California and I have a book of papers with all the work they did including the new engine so it would pass emissions. Not a a cheap swap back then. Not pretty under the hood with all that stuff but I'm keeping it that way for now. Needs a good cleaning. Runs good but I think its got the peanut port heads so not a monster 454 in terms of power. Colorado emissions had no issues with it when it got sniffed last month.
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wow been a minute since I have seen a cruise control transducer, does it work ?
 
Wes Harden said:
wow been a minute since I have seen a cruise control transducer, does it work ?
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Good question! I have yet to test it. I assume I just push the cruise button on the turn signal an pray that sucker doesn't stay on when I hit the brakes. :)
 
Had one got wide open throttle on me, had to shut the ignition off to stop the car. crappy manufacturer
 
Yikes! It's nice its still all intact including the AC. Rare to find these days. Keep getting random strangers that want to buy it.
 
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