Yesterday while driving down the freeway at a constant speed, I noticed my fuel gauge going down rapidly. Then it came back up and proceeded to go up and down at about the speed of second hand on a clock. Does anybody know what is likely to be going on? I've never seen anything like it. I'd hate not to be able to trust the fuel gauge.

I would suspect the sender in the tank or its connection.   At least the sender is still available and easy to change.

Yep,  my local Nissan got me one for my '72.  I don't know about the 280 but surely it's still available.  Google it but there's a way to test the gauge by shorting out the sending unit. I honestly can't remember but I think if you touch the two post together the gauge goes to FULL???   That's probably backwards so please google before doing that. 8^)

Your float may have a small hole allowing fuel in? 

Edited December 30, 20177 yr by siteunseen

So this thing, eh? - http://www.thezstore.com/page/TZS/PROD/11-3271 

It seems from some past posts I just read that it may be possible to replace the sender without dropping the fuel tank, which I don't have the wherewithal to do at home.

6 hours ago, Grubbs240 said:

So this thing, eh? - http://www.thezstore.com/page/TZS/PROD/11-3271 

It seems from some past posts I just read that it may be possible to replace the sender without dropping the fuel tank, which I don't have the wherewithal to do at home.

Yes that's the one, drain or drive the car enough so the gas level is well below the round sensor opening.  It's always good to replace the O ring and locking ring at the same time, http://www.thezstore.com/page/TZS/CTGY/classic17d03

 

Try soaking  the sending unit in a container of straight vinegar before buying a new unit. The unit in my 78 went wonky, so, since I'm a cheapass, I tried an overnight soak. I was amazed at how clean it was afterward and as a bonus, it worked normally again. Free fixes are the best fixes!

Compare the old to the new.  Some rod bending may be necessary.  The nail head connectors were a PITA for me.  I ended up using crimp on spade connectors, then covered them with liquid electrical tape.