2005 chevy impala gas gauge up and down?

First eliminate the fuel system by hooking up a fuel gauge and set the gauge where you can read it while you are driving. I once found a problem on a ford pu that was missing on acceleration because the fuel pressure regulator was going bad and on acceleration the fuel rail was getting twice as much pressure that required causing a flooding problem causing the plugs to misfire.

boost pressure is related to exhaust temperature . so if you are not under load then the accelerator position will only allow the fuel that is required and so the boost will be reacting accordingly. If you suddenly accelerate and the boost goes up then the turbo is doing the job. If it stays the same then look for boost leaks in hoses and air cooler. waste gate settings or faulty turbo.

Boost will change with rpm. Depending on what transmission you have, rpm may be pretty low at 70, thus low boost.

The boost gauge is a vacuum gauge, probably connected by a hose. At this age, hoses fail. Check all hoses, they are all connected to the same intake, so one failure will take the whole system down.

the pedal sensor is part of the gas peda assembly. The whole thing needs to be replaced if there is a problem with the sensor.

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You should check your fuses. When you replaced your battery it could have caused a spike in your electrical system and blew some fuses.

This is a rare one and can be tricky. Since your rpm gauge rises back up when you increase your acceleration slightly but you feel no jerking of acceleration then this tells me that a speed/rpm senser either has a bad connecton or the sensor itself is going bad when the engine reaches an optimum heat while driveing on the highway. Since you feel no acceleration when you slightly press the accelerator down but yet the rpm gauge is increasing back to normal, the throttle body sensor may be compensating for the failed speed sensor/failed sensor connector. Please get your uplander diagnosed properly first to confim this.

sending unit is in the gas tank. you must take down the gas tank and remove the fuel pump/ sending unit assembly

Sounds like you have a vacuum leak somewhere causing the engine to flood.Check all hoses coming fron intake manifold.

check with GM on a recall