Learn Italian: Il motore è sul punto di esplodere.

They are very similar, but essere sul punto di is slightly more dramatic. While stare per simply means 'to be about to' do something, essere sul punto di translates literally to 'to be on the point of.' It emphasizes that the action is extremely imminent or at a critical threshold, like an engine on the verge of blowing up.

Why do we use the preposition di before esplodere?

In Italian, when you link a noun like punto (point) to an infinitive verb, you usually connect them with the preposition di. You can think of it as the English 'of' in the phrase 'on the point of exploding.'

How would I say 'The engine was about to explode' in the past?

You simply conjugate the verb essere into the imperfect tense, creating Il motore era sul punto di esplodere. The imperfect is the correct choice here because you are describing an ongoing background state in the past before the actual explosion happened.

Since esplodere is used in the infinitive here, what is its past participle if I want to say it actually exploded?

The verb esplodere is irregular. Its past participle is esploso. If the engine actually blew up, you would use the auxiliary verb essere and say Il motore è esploso.