Customer limps in with the temp gauge pegged.

Hot coolant will erupt from the pressurized system, causing severe scalding.

The radiator cap seals a pressurized cooling system. When the engine is hot and the gauge is pegged, that system is under significant pressure (typically 15-20 PSI). Removing the cap while pressurized turns the radiator and upper hose into an open vessel—coolant will flash-boil and spray violently.

Immediate actions:

1. Stop the tech—physically block the cap if needed.
2. Wait for cooldown—let the engine sit until the temp gauge returns to normal (at least 30 minutes).
3. Relieve pressure safely—turn the cap slowly only after it's cool to the touch, and use a thick rag while turning.

Code reference: This is basic safe work practice under OSHA 1910.151(c) (personal protective equipment) and general industry duty clause requirements for hazard recognition.

Additional risks:

• Steam burns from flash-boiling coolant.
• Chemical burns from hot coolant (ethylene glycol).
• Eye injuries from pressurized spray.

Never remove a pressurized cooling system cap when hot. This is a common but dangerous mistake—always verify the system is depressurized and cool before servicing.

He is about to get severely burned by a geyser of 260°F+ pressurized coolant and steam.

The Physics: The system is pressurized (~15 psi) to raise the boiling point. With the gauge pegged, the coolant is significantly above 212°F. Removing the cap drops system pressure to atmospheric instantly. The superheated coolant flash-boils, expanding ~1,700x in volume, erupting vertically out of the filler neck.

The Injury: Second/third-degree burns to face, neck, arms, and eyes. Coolant (ethylene glycol) is also toxic if ingested/absorbed.

Correct Procedure (OSHA 29 CFR 1910.132 / General Shop Safety):

1. Stop him immediately.
2. Wait: Engine off. Wait minimum 30–60 minutes (longer for aluminum heads/large capacity).
3. Verify: Radiator upper hose must be cool enough to squeeze comfortably (approx. <120°F).
4. Protection: Heavy leather gloves + full face shield (safety glasses insufficient).
5. Release: Drape thick shop towel over cap. Turn first detent only (safety stop). Step back. Wait for hissing (pressure bleed) to stop completely.
6. Remove: Push down, turn second detent, remove cap.

Diagnosis First: While cooling, check: Fan clutch/electric fan operation, belt tension, coolant level in overflow reservoir (cold line), leaks, collapsed lower hose. Pressure test cold system if empty.

Tell the tech: "That cap stays on until the hose is cold. No exceptions. Go get the face shield and leather gloves, then wait."