How will a car act if the valve stem seals are bad?

If the valve stem seals are bad the intake valves will "suck" oil into the cylinders. One common indicator of worn valve stem seals is to take your foot off the accelerator for several seconds when going downhill, then watch for smoke when you step on the accelerator again. If you see a puff of BLUISH smoke, you probably have bad valve stem seals. The catalytic converter can become oil fouled so that would be another indicator that you might have valve stem seal problems.