This is a long shot: Take a jumper wire from the engine block to a good chassis ground. Sometimes the engine/drivetrain is so well (electrically) insulated that various items won't work if the bonding strap was left off. You'll oftentimes find that the bonding strap is connected to the head and may have been overlooked during an r&r.
With the thermostat.
Oil pressure sending unit, or depending on the year it might be the computer as well. If it runs ok and doesnt make a whole bunch of noise and bang and rattle then its fine to drive to the shop to get it fixed.
belt is stretched buy new one( serp. or single alt.).
If it's a heavy gauge (around 8 AWG IIRC), that's the charge wire to the alternator BATT post. The larger cable (about 4 AWG) goes to the starter.
my gas gauge is not working what could be the problem
after u drop the tank and replace the fuel sending unit your gas gauge should start working correctly. try jcwhitney.com they seem to have the cheapest price on the unit and pump combo.
fuel gauge not working
The actual gauge involves pulling parts of the dash and the instrument cluster. At that point it is a simple matter of plug it in and put it back together. If the float in the tank need replacement you'll have to drain the gas tank and take it out that way. Haynes auto repair manuals have great references for more precise steps.
most likely the gauge
17mm of the feeler gauge for intake and exhaust valves
If the gauge isn't working, then the sending unit has failed.Easy enough replacement and doesn't cost much.See "Related Questions" below for more
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