What caused this?

On my daily walk today approached an intersection which was engulfed in huge plumes of smoke. So bizarre. Suddenly I see a school bus driving past & the back of the bus was spewing out the smoke. No kids in the bus. Only the driver who seemed to be not in the least concerned. Just kept driving. Guessing he/she was trying to get back to home base.

What would cause this? (I don’t drive so I don’t know these things.)

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Iderno - new freshly barbequed food home delivery service maybe?

The age of the vehicle. Could be that the engine has an oil leak and those drops are burning, causing the smoke you are seeing. Could be a coolant leak doing the same. Or even a weird mixture of fuel and oil that has too much of one or the other and the excess is being burned. Any of that can be from a crack somewhere in the mechanical parts, seals, or hoses of the engine or from faults in the electronic parts that regulate the mixture and combustion within the engine.

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.... or it might just be something like this I suppose

Thank you so much. Was it crazy that the driver kept going? I was sure he/she was going to pull off to the side of the road.

School buses are usually diesel (less refined so it’s more oily), so it’s probably got a fuel mixture problem in the engine and having subpar combustion. That typically happens with age or poor maintenance.

Not weird. If it was a personal vehicle, possibly they know the problem exists but also know they can't afford the repair. Or they feel the repair is not worth the expense, given the age or condition of the engine. Ride it till it dies. If it's burning oil, you just need to check the level often to make sure you don't get too low. You can see people adding oil in parking lots all the time just cuz it's cheaper to kick the can down the road than it is to fix it. Like taking Aleve for headaches instead of paying 250k+ to remove the brain tumor---not that I know estimates for brain tumor removal. Could be a lot cheaper with the help of friends like Chyp, my guess.

If it's part of a commercial fleet or whatever, then the company probably thinks the same. Drive it till it falls apart cuz I don't want to spend the money rebuilding the engine. Or whatever.