Testing Engine Coolant: Test Strips, Refractometer or Lab Analysis

A good routine: test in spring before the season, and again if the engine ever overheats, loses coolant or has cooling-system work. Test before you top up, so the sample shows what the engine has really been running on. The engine must be cold, ideally sat overnight.

Open the pressure cap on the heat exchanger or coolant tank slowly, to the first stop, then remove it. Draw the sample there rather than from the recovery bottle, which can hold stale coolant that isn't circulating. Use a clean syringe or baster. For a lab sample, fill the lab's bottle to the line straight from the syringe, cap it, and wipe it clean. Rinse your tools in distilled water afterwards so you don't contaminate the next test.

Pour a little sample into a clear cup and hold it up to light.

• Clear, any color: good start, but keep testing.
• Rusty or muddy: corrosion or sediment inside; plan a flush and change.
• Oily film, sludge or a "chocolate milk" look: possible oil getting in; stop and get a pro (see the closed cooling coolant guide).
• A smell of fuel or exhaust: also a reason for a mechanic.

BoatUS explains the ball tester: squeeze the bulb, draw coolant into the tube, tap off any air bubbles, and count how many balls float; the chart on the tester converts that to protection. A hydrometer works the same way with a float or needle. These are fine for a quick check but tell you nothing about the additives. Practical Sailor adds two cautions: check a hydrometer first in plain water (if it doesn't read correctly, replace it), and know that oil floating in the sample or dirt settling in it will throw the reading off.

Put two or three drops on the prism, close the cover, point it at a bright light and read where the light and dark areas meet on the scale. BoatUS notes refractometers need no batteries and many have extra scales for other fluids. Read the scale that matches your coolant type. Most engines want about a 50/50 mix (typical; confirm in your manual). Wipe the prism clean with distilled water afterwards. Practical Sailor notes that many pocket refractometers have a small calibration screw; checking that distilled water reads zero now and then keeps the tool honest.

Use strips made for your coolant type and within their date. Dip as the package says, wait the stated time, and compare colors in good light. BoatUS says strips show concentration plus pH and alkalinity. Write the readings down; a reading that drifts toward the "change" zone over a couple of years is as useful as a single result.

• Freeze protection or glycol too low: usually water has been added over time. If additives still read good, your manual may allow adjusting with concentrate; if additives are also low, change the coolant.
• Glycol too high: BoatUS explains too much antifreeze cools worse, and very strong mixes can gel. Partly drain and refill with the correct premix, or change it.
• pH or additives in the warning zone: the protection is wearing out. Plan a full change soon (see the closed cooling coolant guide).
• Everything good: log it and test again next season.

Mail-in labs go further than strips. Blackstone, for example, says it checks coolant for pH, freeze point, specific gravity, nitrites and hydrocarbons (oil or fuel contamination), and accepts coolant in its normal sample kit. Caterpillar's S·O·S program tests oil, coolant and fuel, says its labs handle equipment from a variety of manufacturers, marine included. Check with the lab how to order a kit and where to send it. Fill in the form completely: engine make and model, hours, coolant type, when it was last changed, and why you're sampling.

• pH and nitrites (or other additive levels): whether the coolant still protects the metal.
• Freeze point and specific gravity: the mix strength, like your refractometer.
• Hydrocarbons: oil or fuel in the coolant, which points to a leak between systems. Take that to a mechanic right away; common suspects include an oil cooler or head gasket.
• Other items some labs report, such as dissolved solids or corrosion products. Ask the lab what a high number means for your engine.

As with oil analysis, the trend over several samples matters more than one number. Sample at the same point and the same time of year each time.

Change it when your manual's interval comes up, or sooner if testing shows worn-out additives, low pH, the wrong mix that you can't correct, rust or contamination. Typical intervals (confirm in your manual): BoatUS gives about 5 years or 1,000 hours for modern extended-life coolants and about 2 years for older-style coolants, and Northern Lights tells its generator owners to replace the coolant every 2 years. Practical Sailor says change intervals for heavy-duty coolants run from about five years to thousands of hours, the high figures being for land-based engines, and suggests the more conservative 1,000-hour figure for marine engines. The change itself is in the closed cooling coolant guide.

No need to swap it for "winter antifreeze." Practical Sailor points out that if the test shows the glycol content is right, the coolant in a closed system stays in over winter; it already protects against freezing. The pink propylene-glycol antifreeze sold for winterizing water systems is not an engine coolant and doesn't belong in the closed side of the engine.

Westerbeke says a good-quality antifreeze mixed 50/50 with distilled water doesn't need an extra coolant additive, because the antifreeze already contains what's needed to prevent pitting. Some heavy-duty diesels do call for a supplemental additive and specific strips to measure it; add one only if your engine manual says so. Never mix coolant types to "freshen up" old coolant.

Top up what you removed with the correct premix, refit the pressure cap firmly, and wipe up drips. Record the date, engine hours, readings and any lab report in BoatMend so you can see the trend next year.