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While the O'Reilly Auto Parts salesman said that I don't have to flush coolant system after using Interdynamics 375CS Radiator/Coolant Dye I would still like to have second opinion on that here in stackexchange. Especially since I plan to use it on BMW (X3 2009) which is known to be sensitive car.

Has anyone had experience with this or similar UV dye? Did you flush cooling system preventively also if the fix to the leak was simple enough and did not require to remove any cooling system hoses? Would you avoid pouring this UV dye in BMW or for that matter in any other car?

Obviously $7.99 for UV dye is way cheaper than a $100 pressure system that I think will have to use only once.

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    Everything I'm seeing says it's "compatible" (except with Dex-Cool, for that you have to use 377CS) with all types of antifreeze. This means to me you can just keep it in there after doing the test. I don't think there'll be an issue with your Bimmer. I cannot find anything which says this specifically, so thus the comment and not an answer. May 18 '17 at 22:33
  • @Pᴀᴜʟsᴛᴇʀ2 Yes, I read everything I could find on the UV dye box and bottle and could not find anything explicit about the need to flush or not to flush the system. I also tried to premix small amount of this UV dye and BMW coolant in a water bottle to see if anything "interesting" would happen. The viscosity seemed to remain the same to my naked eye. I guess I will just try it out and try to see what happens in long run.
    – user389238
    May 18 '17 at 22:40
  • I really don't think you're going to have any issues. May 19 '17 at 1:43
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    Has the coolant ever been changed? I would say that on a 2009 car, you probably might want to flush the coolant system and change the coolant anyway soon, so I would do it now instead of worrying about whether to do it. On the other hand, Paulster2's advice is probably good if the coolant is not as old as the car. I don't know about BMW's service intervals, but for Toyota the super long life coolant is changed first after 150 000 km or 10 years if you drive less, then after 90 000 km or 6 years if you drive less.
    – juhist
    May 19 '17 at 13:42
  • One one major no-no in coolant systems is NEVER USE TAP water. Spend a few bucks and purchase distilled water to mix with your antifreeze. Tap water creates corrosion in the coolant system.
    – zipzit
    May 21 '17 at 8:39
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There is a distinction between fluorescent coolant dye and UV (Ultra Violet light) flourescent coolant dye. Years ago you could just buy a coloured dye and add this to the coolant. It was usually green stuff, sometimes yellow, and could easily be seen, even in difficult places with an ordinary, yes ordinary, torch, if necessary. The UV dyes are not really necessary for coolant leaks, although they might make a leak easier to see....BUT.....you will need a UV torch to see a leak to best effect. UV dyes are often used for air- con leaks, although as kits or bottles they can be expensive.

Think about it for a moment. Most leaking fluids can be easily found if the leak is from a hose which might be split or life expended and due to be changed..........leaking fluid is fairly easy to detected, sometimes external to the vehicle, perhaps on the floor. Sometimes the air-con might leak into the vehicle, as might engine coolant from a heater hose.......HOWEVER......leaks internal to a vehicles passenger compartment are pretty rare. Leaks are most often found under the bonnet / hood.

Where fluorescent dyes come into their own is in detecting pin-prick holes in a radiator matrix, (often difficult to spot), or at, say, a radiator cap seal.

Any trace colourant in the water / coolant of an engine coolant system will usually suffice. I have previously used a food colourant liquid as a dye and it has worked well. All that is required afterwards, and presumably once the leak has been detected, is to drain and flush, then after any necessary repairs all that is required is to refill the system with coolant.

You will not cause any damage to an engine, or its seals or gaskets with a simple coloured trace element in a cooling system to detect a leak.

Washing up liquid can be used, (remember how we always use it in water to detect a leak in a tyre inner tube, be it bicycle or car). All you do is to watch for the bubbles. If you do use washing up liquid you have to remember that it is VERY concentrated in the form in which we usually buy it........so you will need VERY LITTLE of it. You will need to flush it out with cold water afterwards. If you use too much of it you might need to flush the coolant system a couple of times.

Experiment with washing-up liquid in a washing up bowl at the kitchen sink and you will see how little of it is necessary to create enough bubbles. The more you use, as a squirt, say when washing-up crockery, the more bubbles you have.........and the more you will use.......The more you use the more the manufacturers will love you as they can sell you more bottles. I digress slightly here, but you get the point.

Washing-up liquid reduces the surface tension of the water, when washing-up, and so helps to cut through grease. If used in a coolant system to detect leaks, the surface tension component is not important since you will only be looking for the bubbles.

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I did not flush the cooling system after using that UV dye.

After a year I had to check again condition of coolant and it did not have any rust deposits and was "just as liquid" as a new 50:50 coolant-water mixture.

One reason why it may be a good idea to flush cooling system after using these UV dyes is because it may confuse next mechanic working on the car. As an example - my "BMW blue" coolant became green after using UV dye. If next mechanic working on car relies on coolant color to figure out compatible coolant to refill he may incorrectly fill in the wrong one (which may or may not be a big problem). A good idea would be to leave a note that explains that coolant is discolored because of UV dye and would mention the actual color before UV dye was introduced.

Also, next time to find coolant leak I am considering to use pressure tester instead of UV dye, because there are leaks that manifest themselves only under pressurized conditions. In these cases UV dye method is not that practical.

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