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?

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    Perhaps some context would be helpful here. Are you recovering from a blown head gasket or something similar where you know you had some quantity of oil in your cooling system, or are you considering this as routine/preventative maintenance?
    – mac
    Commented Oct 31, 2013 at 19:19
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    – amphibient
    Commented Oct 31, 2013 at 19:24

3 Answers 3


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.


Never ever do such things as adding dish soap to the coolant system to flush. Any soap in the system may start eating the engine blocks and could cause a catastrophic failure. Just flush the whole system a couple times with distilled water or bottled water if you really want to flush the system and use a 50000 mile warranty coolant.


Cooling systems do have contradictory requirements; Take for example the coolant pump seal. In order to last, the seal faces require some lubrication, otherwise, the usual anti-corrosión compounds present in every coolant will eventually precipitate on the seal faces (a hot place), dry out and start to erode the faces. Adding a water pump lubricant (that essentially is a soluble oil, similar to the one used on lathes to cool the cutting tool) solves the lubrication of the seal, but contributes to add an oily coating along the cooling system, reducing the heat transfer.

In another topic, the contributor (Dodgethesteamroller) posted an excellent procedure for cleaning and flushing the system with Citric acid. His first recommendation makes sense: before the citric acid, one must get rid of any oily film, to allow the acid to reach the scaled Surface inside the block.

Contrary to the unjustified worry of mac above, dishwasher detergents simply cannot damage the metals used in the engine (otherwise the detergent dould immediately ruin aluminum cooking pans, cast iron skillets and the like!). Dishwasher detergents are not very alkaline or corrosive enough when disolved in water, and thus are safe to remove any oily film, allowing the citric acid to attack the alkaline scale deposits. I have performed the procedure suggested by Dodgethesteamroller and he is right when he said that after the citric acid is flushed with water, I saw a quantity of scale granules comming out of the block (from a 2002 Dodge Stratus R/T Sedan 2.4L Turbo, that always have used the proper 5-year HOAT coolant from MOPAR,even changed before the complete period). The problem now, is that small quantities of scale keep loosening and comming out even after 6 water flushes. I used the citric acid that comes in the "Prestone Heavy Duty Flush", and put the Neutralizer (Sodium Carbonate) after water flushing the citric acid. (The Prestone HD flush is packed inside a cardboard "can". The upper part contains 255 grams of citric acid, while the lower third contains the sodium carbonate neutralizer. That's the part where I disagree with Dodgethesteamroller, because he said "use TWO pound of citric acid..." and I had difficulty trying to disolve the little over one half pound into one gallon of hot water! Even using an electric kitchen mixer, about 5% of the citric acid poder settled without getting dissolved. (I reason that in order to dissolve two full pounds, one would need about five gallons of water, which is an amount too big to fill a car system!

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