Not sure whether the rusts were disturbed by the added coolant and thin out the metal. What leak isn't the radiator but the core plug (people call Water Jacket is a misnomer), it leak one by one after having added coolant. There are still some difficult to reach core plug which have not had been changed yet but seems to not leak for now but who knows if add in coolant it leaks again thats why was not keen to add coolant to it. Various mechanics said some myth about coolant being acidic and some said it would need very thorough flushing before adding coolant.

Acidic PH is usually theories on old unchanged coolant that have broken down in its chemical properties. But new coolant should be alkaline slightly or near neutral I read. But mechanic like to say it's acidic that's why it corroded your metal and call it a day. So don't put coolant if you haven't been using coolant since day one. That's their advice. Or risk the unknowns and leaks.

If SC they follow procedure for sure will ask replace, the problem once after warranty ends a lot of people go outside and some dont even refill with coolant due to owner or used car reason. If no rust or water still clear replacing with coolant still good, but if found inside already with rust water or can see inner radiator or metal pipes got rust that you got risk of leaking anytime unless you ready to spend to change all the rust items and flush clean everything.

BTW, got coolant tester can be bought to test the so call PH. New coolant usually is neutral to alkaline base and is design to reduce the rust formation.

Normal water depending on where, it can be low or neurtal ph, i got rear fish and i test my tap water time to time got few times i tested PH5.5~6.5 which is slightly acidic. If the owner that time fill with the lower PH tap water it can rust very fast.

This post has been edited by littlefire: Aug 13 2025, 09:44 AM

Just sharing some observation from a few cars that are and were in my family

Case 1. 1973 & 1978 Datsuns that never used coolant until mid-90s when I started helping to maintain them. Initially we used tap water only and rust will form within 6-9mths. But we would flush the system and fill them up with tap water the moment this is noticed. When we switched to coolant (d cheap 1 bottle coolant and top up with tap water), d rust went away although we still flush the system every 3-5 yrs. No leaks in those cars so far.

Case 2. 97 Kancil bought used. Car was generally poorly maintained and quite sure it used tap water for a long time. When i took over maintenance, I continued using tap water for a while and observed no leakage. However during d first few cycles of coolant use, while fluid was normal there were bits of rusts during flushing and some minor leaks were starting at joints which eventually lead to replacement of almost every hose and connecting pipe before all was OK. My theory is after using coolant, rust was almost no longer building up causing existing rust along d line to break off. So in places where breaches were previously held up by rust sediments, d holes eventually revealed themselves or got worst due to heat cycles and water pressure. Of course d quality of the pipes (some are just thinly coated steel) do play a part.

That said, continuously using tap water and risking rust build up can be a big issue for cars with cooling lines that have fine passage ways or like force-induction engines which have less tolerance to overheating.

Coolant usually is neutral to alkaline, the problem is we never knew how worse is the inner parts if rust already form. The rust wont heal tho, it will only continue to corrode but with proper coolant it suppose to slow down the corrosion but the thing is a lot of time mechanic after change coolant, owner complain leakage which i think/suspect the water leak area previously already got or already near the breaking point, some case due to severe rust the rust might have block the leak area, after flushing & replace with new coolant the rust which block the leak area already wash off thus making it easier to leak.

any different for water cooling system with radiator cap (majority i see 1.1bar)
or without radiator cap (no idea pressure spec. haynes book write 1,4bar)

so ngam i was trying enjin oils until found a satisfy one last week.

various brand, various grade of full syn /semi syn even monograde make water temperature raise above 95'c then enjin die off.
altho can restart back, pedal become sluggish & whole bonnet macam oven.
ex-owner told me the water temp will stay between 75-85'c in aircond max mode during summer hot. i rasa it made radiator fan run high speed la.

now using 10w40, water temp stay below 100'c aircond off
never die off, never heat like oven. radiator fan run when needed only

the radiator cap pressure is to reduce the coolant from going back to spare tank during spirited or high rpm driving. If the cap pressure is too low from the original car settings, coolant flow back to spare tank -> overflow -> less coolant in radiator -> overheat
Thus why most race car upgrade the radiator cap pressure with more higher bar to reduce the coolant from flowing out.

This post has been edited by littlefire: Aug 13 2025, 03:28 PM