Injector nozzles of direct injection diesel engines (DI) of in-service vehicles showed one or more holes blocked and significant flow reduction even in nozzles with all the holes open. Coked nozzles from field gave measurable increase in smoke, carbonmonoxide (CO) emissions and specific fuel consumption (BSFC).Tests of 500 hour duration on a naturally aspirated DI diesel engine revealed hardly any significant nozzle coking particularly with the fuels containing total cycle oil (TCO). In some cases, an increase in flow rates in nozzles was observed for in-service vehicles as well as in 500 hour engine tests. In the long duration tests, statistically significant increase only in CO and reduction in NOx emissions were observed with a straight run diesel fuel and another fuel containing 20% TCO. Change in power, BSFC and HC emissions during 500 hour engine operation were not significant with any of the test fuels. Cetane number in the range of 45-48 was observed to give minimum smoke and gaseous emissions. Smoke and CO emissions generally, decreased with decrease in fuel viscosity.