I've got this old superduty with a 7.3 and 5 speed, 2wd and the gas gauge needle waves all over the place. The front tank stays on full till almost empty and the back tank won't settle down at all. Is this a grounding issue or ?

Also I can't get the heater blower to blow out the vents, seems to be stuck on defrost. I can switch between vent and defrost and the heat will barely blow out and you can hear the blower running.

 

My guess is the sending units are bad. The electrical contacts that make the signal could be whats causing it to just erratically but when it gets near empty it can read properly again. Im assuming from experience.

As for the vents. If they are like my truck (88 F-150) they are cable controlled and its possible that something came apart and you cant switch the flapper door. I believe you can get in under the dash behind the glove box but I am not positive on how yours is set up.

 

Mine did the same with the vents. It is a broken or cracked vacuum line leading to it. There is probably a white and a black vacuum line coming out of the blower area in the engine compartment on the passenger side. Thats where you need to start checking. The heater selector is vacuum controlled and will revert to defrost with a loss of vacuum.

 

i have the exact same prob, my rear tank gauge works fine, but my front tank gauge needle bounces around and gets stuck in the full-most position

 

the bouncing gage is usually the sending unit in the tank. there is a TSB on this. Ford had lots of problems some even on new trucks back then. the float would bounce a lot during shipping and wear the resistive film on the sending unit. both my tanks show way past full for the first 150 miles then bounce around until 1/8 tank then show correctly from 1/8 to empty. usually about 3 gallon left in each tank when dead on empty. I just run to both show empty then fill up. usually takes 31 to 35 gallons. do a search on this forum. Mine has been bad for about 17 years.