Whats up guys and gals? Havent been on in a while but ive made soo much progress on my '86 Z28. It now has a mildly built 400 sbc (509 block casting). The new 1/4 mile personal best is a 13.5 at 104mph!

Now onto my issue. I went down to a exhaust shop and had him make me a custom 2.5" Y-pipe with a 4" cutout and mate it up to my factory 2.25" "catback". He did a great job and everything but after I left and topped off the gas tank to make it home, the gauge needle jumped from full tank down to 1/4 tank before I even left the gas station parking lot and stayed there for most of the drive home (40ish miles). After about 15 miles it moved up to half tank, then after another 10 miles or so it moved to 3/4 tank. Now a day later its back to 1/4 tank but I know it's really close to being full. I'm not sure what happened but my fuel gauge is important when I get 13mpg on the highway lol.

Its a factory vin H car with the (now impossible to find) carbureted fuel sending unit that is 35 years old. Can anyone help shed some light on what could've happened? I just had the gas tank out like a month ago to lengthen the in-tank return line down to the bottom of the tank in preparation for my Holley Sniper Stealth EFI and am not looking forward to taking it back out.

Here are a few pics from when the exhaust was finished

 

Mine does the same thing, just an old sending unit. Full tank - then at what should be between 3/4 and 7/8 of a tank it'll drop to 1/4, then once it's below 3/4 of a tank it'll pop back up and read normally again down to 1/4 tank, at which point I fill it up.

Old electronics, they get finicky in their old age.

 

Is there anything I can do to fix it? I won't be driving the car much anymore if there isnt a way to make it work properly again. I need it to read accurately 100% of the time without failure

 

Drop the tank and replace the sending unit.

 

They don't make replacement carbureted sending units, I've looked far and wide for new replacement but had no luck. I can only find ones for TPI/TBI cars which I dont think will plug into the connector on my car

 

Are you using the factory fuel pump? The sending unit is integrated into the fuel pump, i.e., the sending unit is the same but the fuel pump is different. If you're not using the pump then you should still be able to make the sending unit work.

 

'Would be nice to get some confirmation of this, as my sending unit has gone bad in my carbed '83...

 

I had a mechanical fuel pump on the side of the engine block from factory. There is nothing in the tank except the pickup screen and fuel level float. Im currently using a Holley Blue replica pump crudely mounted in the rear to push fuel up to the front

 

Need more info

 

Ok perfect. Since the connector is outside the tank you could buy the TPI/TBI unit, hook it all up, and just crimp on the connector for the carbed unit (if they end up being different). Your pusher pump will pull through the tbi unit just fine. I know this because my Holley Blue pump has been doing that for over a decade now.

 

There isnt a crimp connector or anything. The sock just pushes onto the hard line that goes to the bottom of the tank and friction holds it there. Will the FI sending unit plug into the body connector for the gas tank? Are they the same connectors?

Got any pics of how you did yours?

 

And forgot to ask but does the level float in your FI sending unit work like it should? I'll be using an in-line Walboro 255 pump for the EFI unit soon instead of the Holley Blue

 

the plug for the sending unit is what I meant, it’s really simple to switch out an electrical connector.

When it wants to work my sending unit works, if that answers your question lol.

 

No pics of mine, I just intercepted the fuel line outside the tank and connected the Holley pump up.