Your oil gauge in your 91 cavalier stays up at max when the engine is running When you stop bounces back and forth until start to move then it goes What problem?

The most likely problem is a faulty oil pressure sending unit. This unit is responsible for sending a signal to the gauge, indicating oil pressure. If it's malfunctioning and stuck in the "high pressure" position, it will read max even when the pressure isn't actually that high. The bouncing when you stop and start moving might be due to vibrations affecting the faulty unit.

Less likely, but still possible, are issues with:

* Wiring: A short or break in the wiring between the sending unit and the gauge could cause erratic readings.

* Oil gauge itself: While less common, the gauge could be faulty.

* Low oil: While counterintuitive given the high reading, extremely low oil could cause the gauge to read incorrectly due to the pump struggling to build pressure. Check your oil level immediately.

Recommendation: Start by checking your oil level. If that's fine, the sending unit is the most likely culprit and relatively inexpensive and easy to replace. A mechanic can test the oil pressure directly to confirm if there's a real pressure problem before replacing parts unnecessarily.