Stopping a Slow Cooling System Leak

Car developed a leak. Car overheated and I discovered this problem.

The first issue was that the thermostat was broken. Probably for a while. This caused coolant to leak when the thermo closed, and water pushed out past the gasket. This showed up as some drips.

I replaced the thermostat, and overheating stopped.

Second issue was a slower leak that consumed water at a little over a half gallon (2L) per month. I went through the diagnosis slowly, because I was learning it. (This was before the overheat.)

Initially, I was replacing lost coolant with distilled water. It seemed to work, and the water level held up so long, I didn’t really notice anything, but I got in a gallon and started to worry a little.

After the overheat, I switched to using 50/50, and went through the gallon pretty quickly (like 2 months). Then I switched to coolant that needed to be diluted, so I diluted it, but tried to mix it thick, to compensate for the fact I put water into the radiator before. (If I had a fluid tester, I would have tested the coolant from the radiator after driving around a while.)

I looked for the usual signs of a leak: smoke out the tailpipe, oil in the coolant, or drips underneath the car. None presented.

I bought replacement hoses, but didn’t replace them because it was a lot of work, and I was too lazy to make the effort. I also didn’t have a pan to catch the coolant.

I replaced the radiator cap. I read that an old cap might not hold enough pressure, and water would steam off. At this time, the coolant was a mixture of old coolant, distilled water, a bottle of drinking water, and the new 50/50 coolant.

I also bought a tester for coolant. I tested, and it was a bit watery, so I added more coolant into the blend.

I noticed that the cap was collecting a bit of gunk that looked like scale or rust. I assumed the fresh chemicals were loosening this stuff. I also noticed that the rate of fluid loss was slowing down.

I continued to add the diluted coolant. I developed a theory that the loosened rust and scaled were acting like “stop leak” and flowing into the pinhole leak.

I checked the plugs, and all were OK, but one had a bit more crud on it than the others. There was no water on any of them. So, possibly not a head gasket leak.

By April, I had gone through a little over two gallons of coolant, so the entire system had leaked out somewhere. It could have been into the exhaust, and is fouling up the stuff in there.

The leak was almost completely stopped, as well. I read about sodium silicate, or water glass. It’s a silicate that hardens when it heats up. They add a small amount to coolant to help plug up leaks. At least they used to – I bought some old coolant that said they had it.

I read more about stop leaks. I learned about using ginger and black pepper. I learned about these new microfiber based products, as well as silicone, water glass, and copper particles.

I decided that using a bottle of stop leak was a bad idea. Reasoning:

  • It can “gunk up” the system by sticking to all the surfaces, and reducing the transfer of heat to the coolant.
  • Some are expensive.
  • Some require a radiator flush, and some require running water to distribute silicone into the system. In other words, it’s work.
  • All the sludge and gunk in the is bits of radiator metal, engine metal, and gasket metal. Therefore, aside from the fact it’s corroded, it’s compatible with the system.
  • Organic materials like pepper and ginger are just fibrous hydrocarbons. They will rot and degrade over time, and if they leak from the coolant system into the engine or exhaust system, they just burn up.

I found a product from Prestone called Radiator Complete Care, which sounded like a bottle of some additives that are already in regular Prestone (from what I read in reviews). So I added a few ounces of this a couple times, for a total of around 5 ounces. I don’t know if it had an effect, but the patent is here.

I bought the Complete Care hoping it had sodium silicate in it, but no.

Despite that, the rate of fluid loss continued to be very low, to the point of imperceptible.

I decided to use the old hack of pouring some black pepper into the radiator to plug up leaks. Given the range of products that stop leaks, I decided that black pepper was cheapest, and also most controllable – I could add half a teaspoon, which is around 1/4 of the recommended 1 tablespoon. This seemed to work, and I can’t tell if I’m losing fluid at this time.

Guesses About What Happened

Neglect led to corrosion.

Refilling with water probably continued corrosion.

Also, a broken thermostat led to greater losses in fluid, and overheating.

Refilling with antifreeze (and also mixing to try and achieve a 50/50 blend) helped fix the problem. I’m not sure of the mechanism, but I suspect that debris in the system, possibly bits of corrosion, were filling the leak. The additional corrosion inhibitors probably helped. The small amounts of sodium silicate in the coolant probably helped.

Adding more corrosion inhibitors via the Prestone Complete Care product may have helped.

Adding a half teaspoon of black pepper may have helped.

I hope to replace the hoses later this year, but want to make sure I can recover this radiator fluid and reuse it. The stuff is pretty expensive, and it’s toxic as well. I will probably skip the flush.

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Copyright 2023 and John Kawakami