7.3 Radiator, Cap, Pressure Test, Fan, Clutch, Controls, and Shroud

Radiators, tanks, and what else lives in the core

Light-diesel radiators are usually aluminum cores with plastic tanks crimped on. Inspect tanks for cracks at the crimp, at hose necks, and at the automatic transmission cooler fittings. Many Duramax and 6.7 L Cummins / Power Stroke trucks also put an engine-oil or transmission cooler in or beside the radiator. Pink ATF in the coolant, or coolant in the transmission, is a cooler leak — flushing only the radiator will not save the transmission.

Look for oil-fouled fins after a cooler failure, bent fins from a brush guard, bugs packed in the A/C condenser stacked in front, and internal restriction from stop-leak, silicate dropout, or mixed OAT/HOAT sludge. A radiator that is hot on one side and cool on the other with the engine overheating is a clogged core, not a missing thermostat. Some 6.7 L Power Stroke years add a secondary radiator for the EGR loop — a perfect primary radiator does not cool an EGR circuit that has a plugged secondary core.

Degas bottle versus overflow. Ford Super Duty pattern uses a pressurized degas (expansion) tank; the cap lives on the bottle, not on the radiator. Ram and GM may use a radiator cap, a pressurized bottle, or both depending on year. A remote non-pressurized overflow jug is not a cap location. Installing a 16 psi cap on an unpressurized bottle does nothing.

Cap rating, boiling point, and why the number matters

A cooling system is a pressure vessel. The cap’s pressure valve holds the specified rating — commonly 15–18 psi (about 103–124 kPa) on light diesels — then vents to the bottle. A vacuum valve in the same cap lets coolant return on cool-down so hoses do not collapse.

Rule of thumb: boiling point rises about 3°F (about 1.7°C) per psi of cap pressure. A 50/50 ethylene glycol mix already boils near 223°F (106°C) at 0 psig at sea level. A good 16 psi cap pushes that into the neighborhood of 270°F before it boils. Lose the cap, run an 8 psi cap in a 16 psi system, or run at altitude, and the same truck “overheats” with a perfect water pump because the coolant is boiling at the EGR cooler and the pump is cavitating.

Failed vacuum valve: lower hose collapses after shutdown or on deceleration, flow dies, heater gurgles. Failed pressure valve: boil-over in traffic, overflow, and aeration that looks like a head gasket on a sloppy visual. Always pressure-test the cap on a cap tester at the stamped rating. Do not assume a new-looking cap holds.

Altitude lowers boiling point. A Super Duty that tows over mountain passes needs a cap that actually holds rating more than a truck that never leaves sea-level commuting.

Pressure-test the system — and read what the test is telling you

Cold-test the cooling system with a pump tester at the cap rating, never above it. Watch for 10–15 minutes.

• External drip: hose, clamp, radiator tank, heater core (carpet, fog on windshield, sweet smell in the cab), oil-cooler lines, EGR-cooler external gaskets, water-pump weep, housing.
• Slow pressure drop, no drip: internal leak — EGR cooler, oil cooler, heater core into the HVAC case, or combustion leak that only opens when the engine runs.
• Pressure rise with the engine running (tester still installed, using the correct running-test procedure): combustion gas is entering the coolant, or the coolant is already boiling. Distinguish those two. If the engine is not yet over temperature and pressure climbs past the cap rating, think head gasket, cracked head, cracked block, or leaking injector sleeve/cup (diesel-specific; 6.0 L Power Stroke cups are the famous case, but any sleeveed injector gallery can do it).

A block tester (chemical CO2 tester) in the degas neck is a cooling-system finding you make during this work. Follow the kit: typically a blue fluid that shifts yellow/green with carbon dioxide. False positives happen if tailpipe exhaust washes across an open bottle. A positive test with a stable external system means combustion in the coolant — not a fan clutch, not a 13 psi cap, not a shutter motor.

Electrolysis / stray current: aluminum radiators and heater cores pit from the inside when current uses the coolant as a path. Measure DC voltage from a probe in the coolant to battery negative (engine running, accessories on). Service information sets the limit; many shops treat about 0.10–0.30 V as the concern band. High voltage plus pitting means find the ground strap, poor engine-to-chassis ground, or aftermarket plow lights that were power-fed without a proper ground. Replacing the radiator three times without a voltage check is how those jobs come back.

After any radiator, cap, or hose repair, retest, then verify the fan and airflow path. A tight system that still overheats in traffic is no longer a leak story.

Fans, hubs, clutches, and controls

Light diesels use several fan strategies. Name the one on the VIN before you spin anything.

Viscous clutch bench sense: with the engine off and cold, the fan should turn with some drag, not like a bicycle wheel and not like a welded hub. After a heat-soak idle, it should be noticeably harder to turn. A cardboard-in-front-of-the-radiator stall (used with care, watch temperatures) should make a good clutch pull air hard; a failed clutch will let ECT climb while the fan still spins lazily.

Electronic viscous (Ford 6.7 pattern): do not stop at a hand spin. Check for silicone spray on the clutch face (fluid loss), connector condition, and scan-tool commanded engagement. Unplug tests and duty-cycle commands are in service information. A clutch that never receives PWM will act like a failed viscous unit even if the silicone is still inside.

Electric fans: prove power, ground, relay or PWM, and the request. ECT high or A/C high-side pressure should command the fan. Dual-fan EcoDiesel layouts can run one fan and not the other. A fan that runs at full speed whenever the key is on is a shorted relay or a PCM driver — it will not overheat, but it will overcool and annoy.

Fan hub: the bearing behind the clutch. Groan that changes with engine speed, wobble at the fan, or contact with the shroud is a hub, not a belt. On 6.7 Power Stroke the fan hub often comes with the water-pump job — a new pump with the old sloppy hub puts the belt out of plane (Chapter 7.1) and the fan into the shroud.

Shrouds, air dams, and active grille shutters

The fan shroud exists so the fan pulls air through the core, not from the engine bay. A missing, cracked, or misaligned shroud causes overheat at idle and in traffic with relatively normal temperatures at highway speed, because ram air still hits the core on the road. The opposite pattern — overheat at speed, acceptable in the shop stall — is often airflow blocked in front: bugs, a winter cover left on, a collapsed A/C condenser, or active grille shutters stuck closed.

Active grille shutters (AGS) appear on Ford Super Duty, Ram 1500 EcoDiesel, and GM 1500 3.0 L Duramax among others. They close at cruise for aero and faster warm-up, and they must open under load. A shutter motor DTC, snow packed in the grille, or a linkage that sits closed will overheat a diesel on a grade with a “perfect” viscous clutch. Scan the shutter position; do not assume the grille is open because you can see some slats.

Air dams and lower valence pieces matter on HD trucks with large gaps. After a collision or a brush-guard install, confirm the fan still sits in the shroud opening and that shutters are not blocked by aftermarket lighting.

Putting the cooling-system test in order

When an A9 item describes an overheat, work the path you can defend:

1. Prove actual temperature (ECT/CHT, IR, gauge) — that was Section 7.2.
2. Confirm level, cap hold, and system pressure without exceeding rating.
3. Separate external leak, internal EGR/oil cooler, combustion in coolant, and electrolysis.
4. Confirm flow (pump, thermostat, hoses) before you blame the radiator.
5. Confirm air through the core: fan type, clutch lock, electric command, shroud, shutters.

A Super Duty that overheats in a drive-through with a free-spinning electronic clutch is a fan job. A Cummins that overheats on a grade with shutters closed is an airflow job. A Duramax that pushes the tester past rating with a color-changing block test is a combustion-leak job. Those three trucks can share the same customer sentence — “it runs hot” — and they are not the same repair.