Maintenance & Reliability · SOP
Fleet Coolant System Flush And Antifreeze Concentration Testing
This procedure covers the complete flush, chemical cleaning, and antifreeze concentration verification for light- and heavy-duty fleet vehicles. Executed at every 60,000 km or 24-month interval — or immediately after any coolant contamination event — to prevent cavitation, electrolysis, and blocked heater cores.
Estimated Time
120 min
Difficulty
Intermediate
Regulation
ASTM D6210 · SAE J1034
Version
1.0
Purpose
To remove accumulated rust scale, silicate gel, solder bloom, and organic debris from the engine cooling system; to restore heat transfer efficiency and prevent water pump seal failure; to verify that the final antifreeze concentration meets the minimum freeze-point and corrosion-inhibition specification for the operating climate zone (typically –37 °C for a 50:50 ethylene-glycol-to-water ratio).
Scope
Applies to all fleet vehicles classified as light duty (GVWR ≤ 4,536 kg) and medium duty (GVWR 4,536–11,793 kg) operating with liquid-cooled internal combustion engines. Excludes electric-drive battery thermal management systems and stationary industrial engine packages. All work must be performed by a certified fleet technician or licensed mechanic.
Safety Precautions
- Lockout/Tagout: Verify the engine is OFF and the key is removed from the ignition before opening the cooling system. Tag the steering wheel with a "DO NOT START" tag. For remote-start equipped vehicles, disconnect the battery negative terminal.
- Wear chemical-resistant nitrile gloves (minimum 8 mil), safety glasses with side shields, and a shop apron. Antifreeze is toxic if ingested and can cause skin irritation. Change gloves immediately if contaminated.
- Hot coolant and steam can cause severe burns. Never open the radiator cap when the engine is above 40 °C. Use a shop towel to slowly release pressure in two stages: first to the detent position, wait 10 seconds, then fully remove.
- Collect all drained coolant and flush solution in a properly labeled, sealable container. Do not pour down drains or onto the ground. Dispose in accordance with provincial/state environmental regulations using a licensed waste hauler.
Required Tools & Materials
- Refractometer (optical or digital, 0–40 % Brix scale, automatic temperature compensation)
- Coolant system flush chemical (alkaline-based, low-foam, compatible with OAT/HOAT/IAT coolants)
- Deionized or distilled water (min. 20 L per vehicle, two rinse cycles)
- Coolant collection basin (20 L capacity, with spout and sealable lid)
- Pneumatic or manual coolant evacuator with shop air hose
- Torque wrench (N·m, range 10–60) for drain plug reinstallation
- Safety gloves (nitrile, chemical-resistant, 8 mil minimum)
- Safety glasses with side shields (CSA Z94.3 or ANSI Z87.1)
- Fleet maintenance software (ServiceGrid or equivalent) for digital log entry
Procedure: Step-by-Step
1
Vehicle Preparation. Park on a level surface, engage the parking brake, and chock both rear wheels. Allow the engine to cool until the coolant temperature measured at the radiator inlet is below 40 °C. Connect a fleet-grade battery maintainer to prevent battery drain during the procedure. Open the hood and install fender covers.
2
Initial Coolant Drain. Position the 20 L collection basin under the radiator drain plug or the lower radiator hose connection. Slowly turn the radiator cap to the first detent position using a shop towel; wait 10 seconds for pressure to vent, then fully remove the cap. Open the drain plug or disconnect the lower hose and allow the coolant to drain completely. Inspect drained fluid for oil sheen, rust particles, or gelled coolant — record observations in ServiceGrid.
3
Flush Chemical Fill. Close the drain plug (torque to manufacturer spec, typically 12–18 N·m) or reconnect the lower hose. Fill the system with a pre-mixed solution of flush chemical and deionized water per the chemical manufacturer's ratio (typically 1:10). Top up with deionized water to the full mark. Reinstall the radiator cap.
4
Circulation Cycle. Start the engine and run at a fast idle (1200–1500 RPM) until the thermostat opens — verified when the upper radiator hose becomes hot to the touch. Continue circulating the flush solution for a minimum of 20 minutes. Monitor coolant temperature on the dash gauge; do not exceed 95 °C. If the gauge approaches 100 °C, reduce RPM or shut down and allow cooling.
5
Flush Drain & Disposal. Shut off the engine and allow the flush solution to cool below 40 °C. Drain the entire flush solution into the collection basin. Transfer waste to a sealable drum labeled "USED COOLANT / FLUSH SOLUTION." Dispose in accordance with local environmental regulations via a licensed waste hauler. Record disposal manifest number in ServiceGrid.
6
Rinse Cycles. Close the drain plug. Fill the system with deionized water only. Start the engine and run at fast idle for 10 minutes to purge residual flush chemical. Drain completely. Repeat this rinse cycle once more for a total of two rinse cycles. On the second drain, verify that the water runs clear and free of discoloration or debris.
7
Final Coolant Mix Preparation. In a clean 20 L container, mix the appropriate antifreeze type (OAT, HOAT, or IAT per vehicle manufacturer spec) with deionized water to achieve the target concentration for your climate zone — typically 50:50 for frost protection to –37 °C. Use the refractometer to verify the concentration before filling. Adjust by adding concentrate or water as needed.
8
System Fill & Bleed. Pour the pre-mixed coolant into the radiator until full. Fill the reservoir to the cold-fill line. Reinstall the radiator cap. Start the engine and run at fast idle for 5 minutes with the heater set to maximum temperature and fan on high to purge air pockets. Check for leaks at all hose connections, the water pump weep hole, and the heater core inlet/outlet. Top off the radiator and reservoir as needed.
9
Final Concentration Verification & Documentation. Shut off the engine and allow it to cool for 30 minutes. Draw a small sample from the radiator using a clean turkey baster or sampling syringe. Place 2–3 drops on the refractometer prism, close the cover, and read the concentration at the scale line. Record the final concentration (%) in ServiceGrid along with vehicle VIN, odometer reading, date, and technician name. Attach any photos of contamination or unusual findings.