7.4 Coolant Testing and Water Pump Service
Key Takeaways
Coolant color is not an approval or chemistry specification.
A refractometer must be calibrated for the coolant type and supplier chart.
Coolant-probe voltage alone cannot identify a failed ground strap or prove stray-current corrosion.
Note
Coolant Testing and Water Pump Service uses stated teaching examples. Actual pin assignments, dimensions, thresholds and procedures must come from the identified engine, component and diagnostic tool.
Coolant Chemistry, Refractometer Testing, Electrolysis & Vacuum Bleeding (Task D.8)
Commercial fleets operate three primary ethylene glycol coolant families: IAT (conventional green silicate/phosphate), OAT (silicate-free Organic Acid Technology such as orange Dex-Cool or heavy-duty extended-life red/pink carboxylate coolants), and HOAT (Hybrid OAT combining organic acids with low silicate or nitrite additives, typically yellow, gold, or turquoise). Never mix incompatible coolant chemistries: mixing products without verifying compatibility can dilute inhibitor performance or create deposits; gel formation is not an inevitable result of every IAT/OAT mixture that plugs radiator tubes, heater cores, and oil coolers while stripping corrosion protection.
| Coolant Test Parameter | Test Instrument & Method | Normal / Target Specification | Fault Reading & Diagnostic Consequence |
|---|---|---|---|
| Standard Freeze/Boil Concentration | Optical refractometer (temperature-compensated drop test) | 50% glycol / 50% distilled water (-34°F / -37°C freeze point; 226°F boil at 0 psi, use the coolant supplier's pressure/boiling chart, such as 265°F at 15 psi for the cited Prestone product) | <40% glycol risks freezing, boil-over, and water pump cavitation |
| Maximum Arctic Concentration | Optical refractometer | 60%–65% glycol / 40%–35% water (-62°F / -52°C max freeze protection) | >68%–70% glycol reverses freeze point (100% glycol freezes at +8°F!) and cuts heat transfer |
Coolant Acidity / Alkalinity (pH) | Digital pH meter or calibrated coolant pH test strips | pH 7.5 to 10.5 (typically 8.0–9.5 for inhibited ethylene glycol) | pH < 7.0 (acidic) corrodes iron/aluminum; pH > 11.0 dissolves aluminum oxide film |
| Galvanic & Stray-Current Electrolysis | DMM set to DC Volts: black lead on (-) battery post, red probe submerged in coolant (not touching metal) | <0.15 V DC (150 mV) normal; 0.30 V DC (300 mV) absolute max | >0.30–0.40 V DC eats pinholes through aluminum radiator and heater cores within weeks |
| Venturi Vacuum Cooling System Refill | Shop-air Venturi vacuum refiller attached to radiator/degas neck | Pulls 24–26 in. Hg (80–88 kPa) vacuum; must hold steady for 2–5 mins | Eliminates air locks in tall cab heaters and elevated LPG vaporizers/regulators |
Important
Distinguishing Stray-Current Electrolysis from Galvanic Corrosion: When a truck repeatedly blows pinhole leaks in aluminum heater cores or radiators, perform a DMM coolant voltage test with the black probe on the negative battery post and the red probe tip dipped directly into the liquid coolant (avoiding metal contact). Observe the reading with the key off, while cranking, and at 1,500 RPM with high-amperage loads (headlamps, blower motor on high, A/C clutch, liftgate/PTO solenoids) switched ON. If voltage jumps above 0.30 V DC only when electrical loads are energized, a ground fault is a candidate; confirm the relevant ground voltage drops and repeat the test under the OEM procedure before attributing corrosion to stray current (stray-current electrolysis). If >0.30 V DC persists even with the negative battery cable disconnected, a galvanic reading is possible; a probe in coolant can itself create an electrochemical voltage, so this reading alone does not diagnose coolant condition (galvanic corrosion).
Always mix concentrated coolant with distilled or deionized water (never tap water rich in calcium and chlorides that form insulating hard-water scale on hot cylinder head walls). After flushing, use a Venturi vacuum cooling system refiller (24–26 in. Hg) to eliminate trapped air pockets—especially critical on commercial trucks with roof/rear auxiliary heaters or elevated LPG liquid-to-vapor converters, where an air pocket will freeze the LPG regulator solid with frost while the engine overheats!
Water Pump, Housing & Hose Inspection and Replacement (Task D.9)
Centrifugal water pumps circulate thousands of gallons per hour through the block and heads:
- Shaft Bearing and Weep Hole Inspection: With the belt removed, rock the water pump pulley to check for radial bearing play and spin by hand to feel for roughness. Inspect the weep (vent) hole underneath the pump nose housing: a light, dry chalky residue or minor evaporation stain around the weep hole is normal from initial ceramic-carbon mechanical face-seal seating and does not warrant replacement. However, active liquid coolant dripping or wet streaks running down the front cover confirm seal failure and require water pump replacement.
- Impeller Cavitation and Slippage: If an engine overheats at highway speed despite a verified clean radiator and tested thermostat, remove the water pump and inspect the impeller vanes for cavitation erosion (vapor bubbles imploding on the low-pressure suction side of the vanes, eating away the blades) or a composite plastic impeller that cracks and slips on the steel shaft once hot.
- Hose Electrochemical Degradation (ECD) and Lower Hose Collapse: Radiator and heater hoses deteriorate from the inside out due to Electrochemical Degradation (ECD), where electrochemical reactions can damage the inner hose material and create micro-cracks in the inner EPDM tube near the metal hose fittings, shedding rubber particles while the outside of the hose looks intact. Squeeze hoses firmly near the ends to check for spongy softness, and verify that the lower (suction) radiator hose retains its internal support spring or rigid molded wall so water-pump suction cannot collapse it flat at high RPM.
Worked Example: Diagnostic Scenario — Highway Overheating & Repeat Heater Core Pinholes
A Class 5 commercial dump truck equipped with a 7.3L V8 gasoline engine comes into the shop with two complaints: it overheats (238°F) only when hauling a heavy load up highway grades, and it has burned through two aluminum cab heater cores in four months. At idle, cooling fan engagement and cab heat are normal. Testing the coolant with an optical refractometer reveals an over-concentrated 75% ethylene glycol / 25% water mixture (somebody topped off repeatedly with 100% undiluted antifreeze, slashing specific heat capacity and raising the freeze point). A DMM electrolysis test with the blower motor and headlamps ON measures 0.54 V DC in the coolant; inspecting the chassis reveals a broken braided ground strap between the right cylinder head and the cab firewall. Finally, revving the engine to 2,800 RPM in the bay reveals the softened lower radiator hose sucking flat under water-pump suction. Replacing the broken cab ground strap (dropping coolant voltage to 0.06 V DC), replacing the collapsed lower radiator hose, flushing the system, and vacuum-refilling with a 50/50 mix of OEM coolant and distilled water resolves both complaints.
A commercial fleet truck has suffered three pinhole leaks in its aluminum heater core and radiator within six months. During diagnosis, the technician checks the ethylene glycol coolant concentration and performs a stray-current electrolysis test with a digital multimeter (DMM). Which procedure and interpretation are accurate?
Mix 85% pure ethylene glycol with 15% tap water for maximum heat transfer, and verify that DMM voltage between the positive battery post and the coolant is at least 1.50 V DC
Use a floating-ball hydrometer with hot coolant to verify 100% pure ethylene glycol (which protects to -80°F), and ground the radiator core directly to the positive starter stud
Verify that coolant pH is below 5.5 so mild acidity keeps aluminum tubes clean, and check that coolant voltage reads between 0.80 V and 1.20 V DC with accessories turned on
Use an optical refractometer to verify a 50/50 coolant-to-distilled-water concentration (-34°F protection; using the coolant and engine supplier's approved concentration), verify pH is 7.5–10.5, and measure <0.15–0.30 V DC with the DMM black lead on the negative battery post and red probe submerged in the coolant while cycling electrical loads
Sections you finish are checked off in the contents.