TAXI owners are risking engine damage by filling their tanks with paraffin to pass exhaust tests. Many are adding the cleaner-burning fuel to diesel as a radical new method of escaping prosecution and keeping their vehicles on the road. Cars belching smoke are examined by the Environmental Protection Department, and owners face $450 fines if a vehicle is found to breach emission limits. Repeated breaches can cost owners their registration. The department says proper maintenance keeps black smoke at bay, but a leading taxi union says lazy owners are pouring paraffin into their tanks before inspections to dilute the diesel and combat the problem. Northwest District Taxi Drivers' Association spokesman Wong Wing-chung said the growing practice would cheat the industry in the long term, because paraffin damaged diesel engines. Paraffin lacked the viscosity of diesel and its use led to higher engine wear from excessive friction. Its lower flashpoint also caused mis-timing he said. 'Apart from laziness, another problem is that many drivers don't have the time or money to get their engines fixed and properly cleaned up for the test. 'The department is always finding ways to stop minor cheating, and that is leading to more extreme efforts to cheat, like this.' A more common method was to adjust engine intake before the test, reducing diesel in the fuel-air mixture being injected into the engine. But this greatly reduced engine power, making the culprits easy to spot, he said. Principal Environmental Protection Officer Raymond Leung Pak-ming said paraffin abuse was becoming popular because it proved difficult to detect. 'The only real way we could confirm it would be to take a sample from every tank and have a chemical analysis done, which would be a slow process,' he said. The department checked more than 35,000 vehicles last year.