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A thermo-mechanical fatigue analysis was conducted based on a coupled Finite Element Analysis (FEA) - Computational Fluid Dynamics (CFD) method on the crack failure of the exhaust manifold for an inline 4-cylinder turbo-charged diesel engine under the durability test. In the this analysis, the temperature-dependent material properties were obtained from measurements and the model was calibrated with comparison of the predicted exhaust manifold temperatures with the on-engine measurements under the same engine load condition. Temperature and stress/strain distributions in the exhaust manifold were predicted with the calibrated model. Analysis results showed that the cracks took place at locations with high plastic deformations, suggesting that the cause of the failure be thermo-mechanical fatigue (TMF). Using the equivalent plastic strain (PEEQ) as the indicator for thermal mechanical fatigue, three exhaust manifold design revisions were carried out by CAE analysis.