Clue: Endless sign of problem with engines arising – before they do? (4)
Answer: CONK
Explanation: Definition: do
Reverse of KNOC[k] (engine noise indicating a problem) – I think I've only ever seen this in the phrase 'conk out', but Chambers seems to think it can appear on its own. There doesn't seem to be a definition here, except by implication: perhaps the clue could have said '…before they do *this*'.