1986 S10 Blazer. About year ago, water temp would go to 250, then slowly to 190. Both sensors changed, same result. All freeze plugs changed. Computer showed temp holding 190. Took dash out, tested independently. Dash ok. Reinstalled dash.added external temp gauge. Gas gauge acting erratic. Installed new Delphi pump with sender. gauge was reasonably. Now it goes from full to empty in about 90 miles. Removing ground or or attaching ground to purple sender wire both cause gauge to go to empty. Don’t want to drop tank again. Pump is working fine. Problem is gas gauge is not working. Suggestions as to what to do next?
Maybe the fuel gauge float attached to the fuel pump is binding.It picks up sediments at the bottom of the tank over time.
Maybe the instrument cluster has failed.
I agree that sounds like an instrument cluster issue; probably the least DIY part of a vehicle!
I’ve gotten really good help from https://www.instrumentclusterstore.com/Chevrolet_instrument_clusters.htm in the past. Give them a call, they’ll help you get it sorted out.