Slipping alternator belt
Loose connection
Bad alternator
Computer problem
A gauge does not idle, please rephrase and re-ask the question.
The spark plug gap can cause the RPMs to fluctuate, while idling. The more probable cause of the fluctuating RPMs is a bad camshaft.
The temperature gauge will not work on a 1984 GMC Sierra truck if the sending unit is defective or the wiring has come loose. It is also possible for the gauge itself to fail, but this is less common.
Means you are over heating.
Hot, idling in drive, 20 lbs.
The oil pressure gauge may be fluctuating on a 2006 GMC Sierra if the oil pump is failing or the sensor is defective. This can be the first indicator of a potentially catastrophic failure.
Depending on mileage--could just be a sign of engine wear Remove oil sending unit and replace with mechanical gauge to determine actual pressure
If you are talking about at startup, this is normal. If you are talking about after startup, then either you are low of oil, defective or incorrect filter, have a defective oil pump, oil sending unit, or bad gage.
If you are have fuel gauge problems I would suspect the sending unit in the fuel tank to be at fault. Rarely the gauge is the problem.
To reset the fuel gauge on a 2004 GM Sierra, start by turning the ignition key to the "ON" position without starting the engine. Press the gas pedal three times within five seconds. This action should reset the fuel gauge. If the issue persists, it may indicate a problem with the fuel sending unit or the gauge itself.
check oil pressure with a gauge possible bad pump or engine is tired
You need to check the radiator and be sure it is full. It can be empty and the overflow tank full. If it is full then check and be sure the radiator cooling is working when the engine is at operating temperature.