A novel experimental unit has been designed, allowing the examination of the fouling tendency in all relevant sections of a steam cracking furnace, that is, dry feed preheater (DFP), dilute feed preheater I & II (DFPH I & II), radiant section, and transfer line exchanger in a single experiment, using among others an electrobalance. Large differences in coke deposition have been observed in each of these sections when cracking a wide range gas oil (WRGO) and a naphtha fraction (NF). WRGO results in fouling rates which are 20, 86, 253, and 10 times higher in the DFP, DFPH I, DFPH II, and transfer line heat exchanger sections is 56% lower. The standard deviations are 13, 18, and 7% for DFP, DFPH I, and DFPH II, respectively. Online effluent analysis reveals that significantly less valuable olefins and more Pygas and pyrolysis fuel oil (PFO) are formed during WRGO cracking compared to NF cracking, that is, 16.2 wt % versus 21.9 wt % ethylene, 12.1 wt % versus 14.1 wt % propylene, 24.8 wt % versus 19.6 wt % Pygas, and 14.1 wt % versus 11.3 wt % PFO. This unit can thus provide vital information to develop single step crude to olefin process by examining possible heavy feedstocks.