Volvo Penta DS4-260

[Mezz]

Hello
I need help or advice......
I work chiptuning on the boats,so i have new customer who have Volvo Penta 2 x DS4-260 with 260HP from 2006.year and the problem is :
When i remap ECU Modul both, one of engine is overheat on 3600rpm (left motor,he comes 90 degrees and over) and the temperature of right motor is normal 85 degrees,but on the other RPM left motor is normal temperature,just when you comes on 3600rpm he is overheat,so i talk with customer and he go to regullar servise and still left motor is overheating,then i try to replace ECU modul from left motor i put ecu on the right and from the right i put on the left,but still the same problem left motor again overheat,so i think that is not problem in my remap i think that is problem in the motor(left)....
So please does anybody have any advice or solution what can be a problem?
Because when customer was on the regullar servise he told to mehanic man about problem and he look and change everything what he must in the regullar service and clean and so on .....

Many thanks....

Bruno....