1990 Buick riveria crank shaft

Could be a number of things, leaky fuel injector to bad timing (it will NOT be a crank shaft). Take it in for a tune up.

Common on the 3800 V6 engine, if it will not run check the crank sensor. Bad cam sensor, the engine will usually run but performance and gas mileage are bad. Often the cam sensor is not bad but the magnet in the end of the cam has fallen out so the sensor does not sense. If you are working in that area of the engine check both. Also, if you had a service manual for your Riviera, section 8 of the service manual tells you how to do diagnostics on both of the above using the onboard diagnostics that are built into the car.