Why does it jump time?

if it just stopped with no apparent problems then there are several indicators that can be checked
jump time ---not likely unless the timing belt /chain broke
check to see if the cam shaft is turning
run fault codes and look for cam/crank position sensor, immobilizer or ignition control module

nowhere. it has a timing chain that should last forever as long as you keep your oil in good shape. doesn't matter if you have a 4 cylinder or a v6, they both use chains.

no it can not jump timing it has a chain. have your tried to turn the motor by hand. meaning asocket and rachet on the harmonic balancer bolt

Not common unless the car has not been maintained. Without regular oil changes, the chain eventually stretches, the sprockets wear, and then the chain may jump a tooth. It is more common with overhead cams, because the chains are much longer and rely on tensioners that wear out.

1. Have your timing chain, and timing sprockets checked. The timing chain probably snapped or jumped timing if you have broken teeth on the sprockets.
2. If the timing checks out, have the cylinders compression tested and check for bent valves, etc.
3. Check the firing order and spark plug orientation:

• 2.4 liter DOHC L4 VIN "T"
• 3.1 liter V-6 VIN "M"

I’m happy to help further over the phone at https://www.6ya.com/expert/jeremy_69f3cc28d95bf514

It's likely that the timing chain is on a warped pulley rendering a jumping effect, which will make it jump to another side.

If the timing belt (chain ) broke or jumped the car would not have restarted after your son shut the engine off. (The motor would have stopped on it's own.) Timing belts are an issue with most modern cars and if the motor is what is referred to as an interference engine there will be valve damage. The other possibility is that some components from the butterfly valves in intake manifold were sucked into the cylinder head causing damage to the cylinders , pistons and valves. No matter what the cause its going to cost a lot to repair.

Checked timing chain how? Sounds to me like it jumped a tooth on the timing chain.

Yes, that model is an interference motor. Most of the pontiac grang am's and grand prix's are. Sorry, I'm sure that was the answer you didn't want to hear.

Is the motor still running???? All depends where the piston an valve are when it jumps off?????