What happens when you cool long-unchanged coolant? – An experiment with Super LLC past its replacement period

Currently, many vehicles use Super Long Life Coolant (hereafter, Super LLC) as engine coolant.

Similar to the previous Long Life Coolant (hereafter LLC), the main component is ethylene glycol, a polyhydric alcohol (ethylene glycol is a divalent alcohol because it has two hydroxyl groups in the molecule).

While conventional LLC required replacement every 2 years, Super LLC has benefited from advances in additive technology (rust inhibitors, anti-foaming agents, and others), resulting in extremely long replacement cycles – initial replacement at 7 years or 160,000 kilometers, or 11 years or 200,000 kilometers (whichever comes first).

Ethylene glycol is an environmentally harmful substance that becomes toxic when metabolized in living organisms. Proper waste disposal is paramount, but this extended replacement cycle can be seen as manufacturers’ efforts to reduce environmental impact.

In the era when cast iron cylinder blocks were mainstream, neglecting coolant replacement would cause the coolant to turn “rust red,” making inadequate maintenance visibly obvious. However, with today’s predominantly all-aluminum alloy engines, there is almost no visible color change even if internal corrosion progresses due to neglected coolant replacement.

A 19-year-old Toyota vehicle came into our shop.

The Super LLC had been replaced at least once 8 years ago, with earlier history unknown.

The manufacturer’s replacement specification is initial replacement at 7 years or 160,000 kilometers (subsequent replacements at 4 years or 80,000 kilometers, whichever comes first). (I believe the shorter interval for subsequent replacements is due to the water passage structure that makes complete replacement difficult).

TOYOTA(トヨタ) Super LLC 2L 08889-01005

The bottle on the left is new Super LLC (pre-diluted to 50%), the bottle on the right is Super LLC that hasn’t been changed for 8 years.

There is almost no difference in color or transparency.

I placed these two samples in the freezer overnight, simulating Hokkaido’s winter sub-zero temperatures. Consider it around -20°C.

While the new Super LLC remained completely liquid, the 8-year unchanged Super LLC froze into a sherbet-like state. In severely cold regions, this could potentially damage water passage components.

We can predict that rust prevention and anti-foaming performance have also deteriorated, so especially with Super LLC, it would be incorrect to determine replacement timing based on appearance alone.