A self-proclaimed vigilante has begun a campaign against boy racers in Blenheim.
At least four cars have had their wide mouth exhaust pipes filled with expanding building foam.
In an anonymous letter to the local newspaper, the "Phantom Expander" says he or she will continue sabotaging cars until the boy racers get the message.
The owner of Murray's Mufflers in Blenheim, Andy Murray, who has repaired several vehicles with blocked exhausts, says the foam could potentially cause big problems for the car's engine.
He said the vehicles targetted do not all belong to boy racers - one was a police officer's private vehicle.
Some car owners have received anonymous notes saying they will be the next targets in the sabotage campaign.
Marlborough Express editor Lance Dodd, says the campaign appears to be continuing, and his newspaper has been alerted to notes received by car owners saying their cars may be targeted next.
The Government and police are moving to crack down on boy racers after weekend violence in Christchurch.