Diesel fuel was injected into the inlet air port of a Perkins 4-236 NADI diesel engine using a Stanadyne 5 micron fuel injector directed onto the back of the inlet valve so as to give the best port fuel injection vaporisation. The fuel was timed to be injected when the inlet valve was open and the exhaust valve closed. Up to 20% of the maximum power fuel flow was injected into the inlet port and the effect is to reduce the diffusion burning phase of diesel combustion at maximum power and hence to reduce soot emissions. The results show that an older relatively high emitting diesel engine can be retrofitted with this technology to produce large soot emission reductions with soot reduced to the level of modern low emission engines. Fumigation also decreases the ignition delay, which at constant fuel injection timing reduces the NOx emissions. The disadvantage of the technique is the increase in specific fuel consumption, hydrocarbon and CO emissions with an associated increase in the VOF proportion of the particulates.