MY 1990 CHEVY 5,7 my gas gauge fluctuates every time i press on the brakes it goes to full then when i release them it goes back to normal

Without test we would only be guessing ! Your best bet would be to take to a qualified repair center that know's how to test this . Or go on youtube a check out video's for sending unit an gauge testing .

The code P1245 is for the Intake Plenum Switchover Valve.

This probably has nothing to do with the gauge fluctuations. The gauge problem is probably one of three possibilities:

1. Low coolant level in the system. When coolant levels get low, air moves thru the system and when it reaches the temp sensor, will cause fluctuations.
2. Defective coolant sensor. These can fail over time as do all sensors.
3. Defective gauge in the dashboard.

Fuel gauge and temp gauge are separate circuits that have only one thing in common - the supply.

In order to ensure readings aren't affected by the normal battery voltage fluctuations the instruments work at a lower voltage than the battery. Both 5 volts and 10 volts used to be fairly common and in order to achieve this a voltage regulator is included in the circuitry.

In the early days an electromechanical device was used but that was replaced by a semiconductor device a long time ago. The early device used to fail and cause exactly the symptom you describe but the later ones generally do not but I expect your problem is somewhere in that vicinity.

could be the wiring, faulty temp gauge in the dash. or bad temp reader inside the engine. located where the top hose enters the engine.

pressure release on oil pump faulty

i would say its the gauge sender,but the problem is its inside the fuel tank,so it looks like a job for the garage.adrian,,,,,,

I think so too. May be sulfur from the fuel on the resister card that the wiper travels over. Try adding some Chevron Techron fuel injector cleaner to 1/4 tank of gas and see if it gets better.

The float inside the tank needs to be replaced. or one of the wires has a small short in it.

My 2000 chevy s-10 starts fine and I get 14.5 on my batt. gauge.after driving it a bit it drops to 10.5 to 11.0. when I am at a stop light the batt. gauge goes to 10.5 to 11.0....When I go down the road again the batt. gauge goes back to 14.5.What could be the problem?

Yes thermostat slow to open not the worst of overheating problem's Good luck with your fix. Cheer's