In a two-machine series production system consisting of an upstream machine and a downstream machine, buffer and spare parts inventory planning must be carried out in such a way as to prevent production line stoppages during maintenance activities on the upstream machine. Therefore, the upstream machine begins to accumulate buffer with a delay and at an optimal time, so that when overhaul activity is needed, the downstream machine can use the buffer inventory to avoid production stoppage. Furthermore, in real-world production, the manufacturing process is imperfect and may go out of control due to disruptions. Hence, detecting changes in the upstream machine is performed through successive inspections, and after detecting a change, a restoration action using a spare part returns the upstream machine to an in-control state. Therefore, this research presents an integrated approach for quality control, maintenance, buffer inventory, and spare parts in a two-machine series production system to minimize costs during the production cycle. Finally, a numerical example from the automotive industry is solved using MATLAB software and a particle swarm optimization meta heuristic algorithm. Additionally, to evaluate the effectiveness of the model, a comparative study is conducted to investigate the effect of delaying the start of buffer accumulation. The results show that the expected total cost in the proposed model improves by an average of 19.41% compared to a similar model without a delay in starting buffer accumulation.