7.2 Thermostats, Water Pump, Coolant Type, Flush, and Bleed

Verify temperature before you condemn parts

Area D work on A9 starts with proving coolant temperature, not with throwing a thermostat at a gauge that “looks a little high.” Use three views:

1. Scan-tool engine coolant temperature (ECT) — and cylinder head temperature (CHT) on engines that have both, including many 6.7 L Power Stroke trucks.
2. Infrared or probe temperature on the thermostat housing, radiator inlet hose, and radiator outlet hose.
3. Instrument-cluster temperature gauge and any low-coolant lamp.

If scan data shows 230°F while the gauge sits at “normal,” you have a sender, cluster, or wiring problem — not a cooling system that passed. If the gauge and ECT agree at 250°F and the radiator inlet is cool, think stuck thermostat or no flow. If both tanks are hot and the engine is hot, think fan, shroud, airflow, or internal restriction — those belong with radiator and fan diagnosis, but you still had to verify temperature first.

Temperature sensors, switches, and level sensors fail in ways that mimic overheating. An ECT that is pulled high by a short will command fans and derate when the engine is actually cold. An ECT that is open can keep fans off. A low-coolant switch in a pressurized degas bottle (Ford Super Duty pattern) can false-alarm from aeration after a sloppy refill, or stay dark while the bottle is empty if the float or reed is stuck. Older systems still use a temperature switch to trigger a warning lamp or a fan. Compare commanded fan state, ECT, and a real temperature before you replace a water pump.

A healthy diesel after a full warm-up typically shows a stable ECT in the thermostat’s control band, a hot inlet hose, a cooler outlet hose (radiator doing work), and a heater core that puts heat in the cab with the blower on. Record those numbers. A9 items will describe exactly this kind of split: hot engine, cold radiator, or cold engine, no heat, ECT stuck low.

Diesel thermostat temperatures — hotter is often correct

Many gasoline engines use thermostats that begin to open near 180–195°F (82–91°C). Light-duty diesels often run higher opening temperatures — commonly in the 190–203°F (88–95°C) range, and some applications sit even higher in the fully open region — because EGR, the diesel oxidation catalyst (DOC), the diesel particulate filter (DPF), and selective catalytic reduction (SCR) need exhaust heat. A “160°F towing thermostat” that a gasoline hot-rodder might install is the wrong parts-store instinct here. An engine that never reaches the specified temperature will soot the DPF, run rich in aftertreatment logic, and produce a no-heat complaint that looks like a heater core.

Stuck closed: rapid overheat, heater may be hot, radiator tanks stay relatively cool, upper hose not fully hot. Stuck open or missing: long warm-up, low ECT on the scan tool, poor cab heat, possible aftertreatment efficiency codes after a cold commute.

6.7 L Power Stroke trucks often use dual thermostats in one housing. Replace them as a pair when the procedure says to. A single new stat in a dual housing leaves a parallel path that can still overcool or still block flow.

Bypass circuits let coolant circulate through the engine, heater, and often the EGR cooler while the thermostat is closed. A plugged bypass, a missing bypass valve, a warped housing, or a reused single-use seal that leaks into a dead-end cavity will create hot spots or a heater that never gets flow. Inspect the housing, bypass, and seals whenever the stats come out. Plastic housings crack; aluminum housings warp; O-rings take a set. Reverse-flow or unusual housing designs (some TDI and EcoDiesel layouts) are VIN-specific — install the gasket the way the old one sat, not “shiny side out” from memory.

Map-controlled and electronic thermostats

Several modern light diesels — notably VW/Audi TDI, some BMW diesels in light vehicles, and certain GM 3.0 L Duramax (LM2/LZ0) applications — use a map-controlled (electronically assisted) thermostat. A conventional wax pellet still moves the valve, but the engine control module (ECM) can energize a heater in the stat to open it earlier or hold a different temperature based on load, aftertreatment temperature, and warm-up strategy.

Diagnosis is not only “does it open in a pan of water.” A bench-heated wax pellet can look fine while the heater circuit, connector, or ECM command is dead. Use scan data: requested versus actual coolant temperature, thermostat heater duty, and related DTCs. An electronic stat that stays closed when the ECM is commanding heat-open will overheat under load. One that is commanded closed for warm-up but is mechanically stuck open will look like a conventional stuck-open stat plus a control-side code.

If you replace a map-controlled stat with a plain wax stat that has no heater terminal, you may lose the ECM’s ability to pull temperature down under high load or to force open for aftertreatment. Use the specified part, seat the seal dry or wet as the procedure states, and bleed afterward — a dry start on a new housing is a good way to toast a fresh gasket.

Coolant type — OAT, HOAT, phosphate, and the mixing rule

Color is not chemistry. Orange is not automatically Dex-Cool, and yellow is not automatically Ford. Read the owner’s manual and service information for that VIN.

Never mix families as a convenience top-off. Mixing OAT and HOAT, or dumping universal “any color” coolant into a Duramax OAT fill, can precipitate inhibitors, foul the water-pump seal, clog the heater core and EGR cooler, and void the only corrosion package the aluminum head and radiator had. If the system was topped off with the wrong product, flush to the specified coolant — do not add a second bottle of the “correct” color and call it balanced.

Light-duty pickup diesels are not Class 8 wet-sleeve engines that live on nitrite supplemental coolant additives (SCA) unless the maker says so. Do not add heavy-duty SCA “for cavitation” to a Dex-Cool Duramax or a Motorcraft-filled Super Duty. Use the specified prediluted 50/50 or mix concentrate with the water quality the maker calls for (often distilled or deionized). Hard tap water with phosphated coolant is a scale machine in an EGR cooler.

Flush, refill, and diesel bleed paths

A drain at the radiator petcock does not empty a diesel. Coolant hides in the block, EGR cooler, heater core (and rear heat on crew cabs and vans), engine oil cooler, and on some 6.7 L Power Stroke years a secondary cooling loop for EGR. Follow the flush sequence in service information. Chemical flushes can strip leftover silicate or attack aluminum if they are left in too long — time them.

After an oil-cooler failure (Duramax is famous for chocolate-milk coolant), the cooling system must be flushed until the water runs clear. Oil film left in an EGR cooler or heater core will overheat the truck with a “new” radiator.

Refill with the specified coolant type and concentration. Prediluted 50/50 is the usual field fill. A 70% glycol mix heats poorly and can still cavitate a pump; a 20% mix freezes and boils early.

Air is the diesel-specific killer. High points include the heater core (especially dual HVAC), the EGR cooler, and some turbo pedestal or fuel-cooler lines. Trapped air in the EGR cooler yields localized boiling, cooler cracking, and a later internal leak. Trapped air in the heater core yields ice-cold air at the vents with a “normal” ECT because flow is bypassing the core.

Preferred bleed methods:

• Vacuum fill at the degas bottle or radiator neck until the specified vacuum holds, then draw in coolant. If vacuum will not hold, you still have a leak.
• OEM bleeder screws on EGR coolers or housings — open them until a solid stream appears, then torque.
• Heat-cycle procedure: heater on full hot, engine at specified RPM, bottle cap off or on per SI (many pressurized degas bottles stay closed). Watch the level through several warm-up and cool-down cycles.

Do not “burp” a modern Super Duty by driving with the degas cap off so the system sucks dirt and more air. If the level keeps dropping with no external drip, move to internal leak diagnosis — not another gallon of random coolant.

Water pumps, housings, hoses, and the weep hole vs EGR cooler

Weep hole: a small drain on the pump body. A wet weep hole, pink/orange crust, or dried tracks from that hole mean the shaft seal is leaking. That is an external pump leak. Replace the pump. Do not seal the weep hole; the next leak path is the bearing into the timing cover or belt.

EGR cooler internal leak: coolant level drops, no puddle, possibly white sweet exhaust, misfire, or steam from the tailpipe. Oil may still look normal (unlike a blown oil cooler). A pressure test may slowly drop, or you may find coolant in the intake or exhaust after a soak. This is not a weep-hole story. Confirm with pressure, block tester if combustion is also in the coolant, and cooler isolation as SI allows.

Other pump failures: impeller erosion from aeration or wrong chemistry, plastic impeller slip on the shaft (engine overheats, pump pulley still spins), bearing growl that was misheard as a belt, and wrong rotation pumps after a catalog error. Some Ford diesels historically used reverse-rotation pumps; a clockwise impeller in a counterclockwise housing moves almost no coolant.

Gear-driven Duramax pumps: inspect the drive gear, housing bore, and gasket. Belt work will not restore flow if the gear is chewed. Belt-driven 6.7 Cummins and 6.7 Power Stroke pumps: inspect the associated idler on that span when the pump comes out — a collapsing idler bearing is why the belt walked into the new pump pulley.

Hoses: replace oil-softened lower hoses, check constant-tension clamps, and look for electrostatic pinholes on the lower radiator hose. A collapsed hose on deceleration (failed cap vacuum valve) looks like a pump complaint. After pump or thermostat service, pressure-test before you release the truck, then verify ECT, heater output, and no weepage at the new seals.