Someone mentioned it is a good idea to wash the coolant system with dish soap to remove residue oil. I was wondering if that is true or can it have any side effects?
I'm assuming that this person was advocating putting a small amount of dish soap in the water used to do a coolant system flush.
I would not do this to my car as routine/preventative maintenance, I would simply use water for a flush. My concern would be that any dish soap residue remaining in the system may adversely affect the chemistry of the coolant, potentially impacting its ability to prevent corrosion or cool the car. I doubt the chemists are checking to make sure their coolant is unaffected by dish soap.
There are purpose-made coolant system flush solutions marketed by coolant manufacturers. Presumably these are safe with the coolant chemistry, but unless I had reason to suspect above-normal levels of contamination/corrosion in the coolant system, I would simply use water rather than spend money on multiple gallons of special flush fluid that I would then have to dispose of properly.
Plain water from a garden hose is a common flush recommendation. Castrol, for example recommends water.